THESIS OUTPUTS Publications:

Patil, M.H., Tanguy, G., Le Floch-Fouéré, C., Jeantet, R., Murphy, E.G., 2021

GENERAL INTRODUCTION

Milk is a premium source of nutrition, but highly perishable in its original liquid form.

As a result, milk and its derivatives are widely incorporated into a wide variety of food and beverage products globally. Therefore, transforming liquid milk into stable powder forms with longer shelf life (up to 3 years) is an essential operation. Moreover, transformation into powder form significantly reduces volume (~70%), leading to savings in storage/transport as well as convenient handling. Spray drying is the industrial gold standard to manufacture dairy powders, as it can handle large volumes (up to 30 t.h -1 ), while also achieving desired powder nutritional and functional properties.

In addition to product physical and nutritional quality, sustainability is a key driver for dairy manufactures, due mainly to concerns relating to climate change. Compared to other dairy and food ingredients, the large amount of water removal required means the manufacture of dairy powders consumes a relatively large amount of energy. Therefore, reducing energy consumption in the drying processes is a priority for industries looking to minimise environmental footprint. Increased energy efficiency also results in lower operational costs and, in turn, higher profits. Therefore, it is in the interest of the industries to optimise the powder manufacturing technology to improve process sustainability by reducing energy foot print.

Recent years have witnessed strong demand for the milk proteins along with consistent growth in demand for cheese. Manufacture of both of these products results in generation of lactose rich side-streams (1 kg of cheese  ~7 to 11 kg of whey), which have inherently lower value than the primary products. Whey may be processed further into whey protein concentrates; however, this again yields high quantities of low-value permeate. Such streams may be sold as animal feed or used to generate lactose, however many manufactures choose to stabilise via spray drying. However, due to low-value and high volume, optimisation of drying is key for processors. It is also important to note that as drying capacity is a finite resource, processing of these streams can limit the manufacture of high value protein-rich milk powders. Therefore, an alternative energy efficient compact technology to process side-streams would be of significant interest to the dairy industry.

The overall goal of this project was to investigate a novel technology to manufacture dairy powders without the spray drying step, in order to markedly reduce energy consumption while maintaining key powder functionality. The current work expanded the principles of a patented superconcentration-granulation process (poudre sans tour (PST) process, EP 3174402, Schuck et al., 2016), i.e. superconcentration using high shear to maintain the product in fluid state, and granulation of the paste into discrete powder particles using a powder which is mixed into the superconcentrate. The focus was on understanding the rheology of the superconcentrated streams and developing insights into the physical, physico-chemical and process parameters influencing the superconcentration-granulation process. As well as the high lactose streams outlined above, other dairy streams were included to determine suitability of the process for such streams. The application of analytical technologies and methods from other disciplines was evaluated.

The first part of the thesis presented: Finally, the key contributions and perspectives of research are compiled in the conclusion and outlook sections.

PART 1 ECONOMIC AND SCIENTIFIC CONTEXTS

CHAPTER 1:

ECONOMIC CONTEXT

Global Dairy trend

Global milk production increased by 157 megatonne (Mt) to 881 Mt in the last decade (at >2% CAGR, 2010(at >2% CAGR, -2019;;[START_REF] Idf ; Iveson | Nucleation, growth and breakage phenomena in agitated wet granulation processes: A review[END_REF]). Cow's milk accounted for the largest share (80% at 714 Mt) followed by buffalo, goat and sheep milk contributing around 15%, 2.2% and 1%

respectively [START_REF] Idf ; Iveson | Nucleation, growth and breakage phenomena in agitated wet granulation processes: A review[END_REF]. India emerged as a crucial contributor with close to 8% growth, which was followed by China (4.1%) and Pakistan (3.8%), whereas only a marginal growth was observed in the main milk producing regions, EU, USA and Oceania in 2019 [START_REF] Idf ; Iveson | Nucleation, growth and breakage phenomena in agitated wet granulation processes: A review[END_REF]. Furthermore, the strong demand for dairy products is revealed from the average per capita consumption pattern at a global scale (Fig. 1.1), which increased to 114.7 kg in 2019, i.e. 9% growth in the last decade (2010-2019), despite the expanding population base (+11.6%). However, a prominent difference exists between the Africa, Asia and Europe regions, wherein the per capita consumption ranged from 42, 87 and 275 kg respectively.

Nonetheless, the rising per capita income trends in the regions of Africa and Asia forecast growth in the consumption of dairy products. World: cow's milk deliveries (A) World: dairy product output, grow (right axis;million inhabitants) between 2010million inhabitants) between and 2019million inhabitants) between (IDF, 2020)).

Moreover, demand for dairy products and ingredients is higher than production in the regions of Africa, South East Asian countries and the Middle East. For instance, the selfsufficiency rate for Asia and Africa in 2019 was 90% and 85% respectively. This deficit seems to be a major factor contributing towards the trade from surplus regions such as Oceania, Europe and North America, which together dominate the global trade (73% of export share). Because of this deficit, close to 9% of total milk production in 2019 was traded globally across borders. For such purpose, milk is processed into stable products (e.g. powders, cheese, butter) which facilitates exports: indeed, the powder form dominates the world trade accounting for close to 69% share in terms of milk equivalent (FAO, 2021). This can be explained by the longer shelf life (1-3 years) and significantly reduced volume, which consequently reduces storage and distribution costs in addition to convenient handling.

Dairy powders market

The global production of skim milk powder (SMP) and whole milk powder (WMP) was 4.8 and 4.7 Mt in 2019, respectively, which remained more or less stable from 2014 to 2019.

In contrast, within the same period, global whey powder production showed strong increase by ~20%, reaching 3.2 Mt in 2019. Whey based ingredients represent a growing market worldwide driven by the growth of nutritional foods and infant formula, and enabled by the growing production of cheese, which dominates the production of the liquid whey (~80%). In addition, permeate powders are emerging as a cost-effective bulking agent (up to 35% lower cost than sweet whey powder) in a growing range of food applications, which is revealed by the record increase in new product launches containing permeate powders at a global scale (Arla Foods Ingredients Group P/S, 2021). This can be explained by the recent release of a

Codex standard for dairy permeates (Cornall, 2017;FAO, 2019), which enables regulatory approval by major importing countries.

Irish and French dairy powders market

The recent trend in the manufacture of dairy powders in France and Ireland is presented in Fig. 1.2. A steady fall in the manufacture of whey powders was observed in

France from 2017 to 2019, whereas in contrast this period showed an increase in the manufacture of whey powders in Ireland. This can be explained by the sharp rise in the manufacture of cheese (+26%) during this period in Ireland (CNIEL, 2020). Overall, dairy powders are of significant importance to the French and Irish dairy economy. In 2019, milk powders (whole and skimmed), infant formula and whey powders accounted for 1069, 914 and 520 million euros, respectively in France (CNIEL, 2020). Moreover, the value of dairy powders exported from Ireland reached 3 billion euros (0.9 Mt) in 2020, which increased by 13% in last five years (personal communication).

Conversion cost (Energy consumption) for the manufacture of dairy powders

The energy consumption for the manufacture of permeate powders (13,860 kJ.kg -1 ) was determined to be 30% higher than that for skim milk powders (10,656 kJ.kg -1 ; (Schuck et al., 2015)), in relation to the lower dry matter of permeate raw material compared to that of skim milk. At the same time, the value of permeate powders (882 €.Mt -1 ) was found to be 73% lower than that of skim milk powder (3328 €.Mt -1 ). Therefore, alternative low-cost technology can be attractive for the industries to reduce their environmental footprint in a profitable manner to process increasing volumes of permeates.

CHAPTER 2: LITERATURE REVIEW ENERGY USAGE IN THE MANUFACTURE OF DAIRY POWDERS: ADVANCES IN CONVENTIONAL PROCESSING AND DISRUPTIVE TECHNOLOGIES

Energy efficiency has been one of the top research priorities driving sustainable initiatives in the previous decade (2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). Manufacture of dairy powders is extremely energy intensive, due mainly to the high specific energy consumption of dehydration processes. The main contributor to this is the final drying step, which within the dairy industry is generally achieved through spray drying. This chapter aims to provide relevant literature background with a focus on energy efficiency in the manufacture of dairy powders. Firstly, a global overview of the complete manufacturing process along with key technological constraints is presented. This is followed by the latest energy-saving developments in the individual technology building blocks of the conventional spray drying process. However, even accounting for recent innovations in conventional processes, drying the consistently growing pool of dairy co-product streams continues to challenge the dairy manufacturers.

Indeed, the need to handle higher volumes of lower value co-products at the expense of high value products is a significant constraint. Therefore, in the last section, spray-drying free processes that target breakthrough reductions in energy usage are discussed with regards to their application to dairy powders manufacture. This includes a reflection on the key features, advantages and disadvantages of novel technologies as compared to conventional spray drying process. This chapter on the energy usage has been published as a review in Drying Technology (Paper 1 ).

1 Published as: Patil, M.H., Tanguy, G., Floch-Fouéré, C.L., Jeantet, R., Murphy, E.G., 2021a. Energy usage in the manufacture of dairy powders: Advances in conventional processing and disruptive technologies. Dry. Technol. 0, 1-19.

The main aims of this chapter were to:

-summarise the recent developments in dairy powders manufacturing process regarding energy efficiency, with emphasis on optimisation of the entire production chain -present latest disruptive innovations in spray-drying free dairy powders manufacturing

Introduction

Drying is a crucial process that transforms highly perishable liquid dairy products ( 85% moisture content [M], water activity [aw] ~1) into powders (  8% water, aw~0.2).

Consequently, considerable increase in shelf life at ambient temperature and significant volume reduction (up to 90%) is achieved [START_REF] Schuck | Dehydrated Dairy Products: Milk Powder: Types and Manufacture[END_REF]. This enables long term preservation (1-3 years), economical transport and simplified material handling.

Nutrition, stability and functionality are key attributes of dairy powders. Spray drying (SD) is a unique technology, including rapid dehydration (<30 seconds) that avoids thermal damage and removes moisture to acceptable levels, ensuring powder stability [START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF]. SD is also responsible for generating specific powder structures which impart functionality to powders (flowability, wettability etc.). For instance, viscosity of the concentrate plays a central role in determining size of powder particles [START_REF] Murphy | Physical stability of infant milk formula made with selectively hydrolysed whey proteins[END_REF](Murphy et al., , 2013) ) while, rapid drying kinetics induces a glassy amorphous structure [START_REF] Palzer | Generation of Product Structures During Drying of Food Products[END_REF]; in addition, various degrees of agglomeration and porosity can be produced depending on the process conditions and dryer configurations [START_REF] Gianfrancesco | Current trends in spray drying research: Understanding the development of particle stickiness enables controlling agglomeration during dehydration[END_REF].

Furthermore, SD can continuously process huge quantities, e.g. production plants with powder production capacities up to 30 tph [START_REF] Burke | The Dairy Industry: Process, Monitoring, Standards, and Quality | IntechOpen[END_REF]. As a consequence, SD is the most widely accepted technology in the dairy industry.

On the other hand, energy footprint for milk powder processing (~8 to 10 MJ.kg -1 product) was found to be the highest among dairy products, which was around 7 times that of packaged liquid milk [START_REF] Flysjö | Greenhouse gas emissions in milk and dairy product chains Improving the carbon footprint of dairy products[END_REF][START_REF] Moejes | Closed-loop spray drying solutions for energy efficient powder production[END_REF]. This was mostly as a result of the energy intensive thermal processes for water removal, which reportedly accounted for greater than 90% of primary energy demand [START_REF] Ramirez | From fluid milk to milk powder: Energy use and energy efficiency in the European dairy industry[END_REF]. Energy-related environmental footprint is major concern in dairy powders manufacturing. SD can be considered as the most energy intense operation which removes, depending on the product, only around 5 to 9% of total moisture but accounts for almost 50% of the total energy consumption [START_REF] Ladha-Sabur | Mapping energy consumption in food manufacturing[END_REF][START_REF] Ramirez | From fluid milk to milk powder: Energy use and energy efficiency in the European dairy industry[END_REF]Schuck et al., 2015). Energy consumption is reflected in operational costs, greenhouse gases emissions and depletion of resources due to use of large amounts of fuels such as natural gas (heating of inlet air for SD, steam generation in boilers, etc.). Therefore, in order to improve sustainability, a continuous focus is required on increasing process energy efficiency and/or, replacing energy intensive operations with suitable alternatives. However, in doing so it is essential that product quality and profitability are not compromised.

Another concern for industry is related to processing of dairy co-product and byproduct streams (e.g. whey, permeate). Market value of dairy powders is often related to its composition. For instance, products with high protein and/or fat content (e.g. skim milk, protein concentrates and/or whole milk) can be many times more expensive than lower protein products (whey, permeate, etc.). While drying co-product streams, the inherently high operational costs of conventional SD based process, can result in little to no profitability.

Other non-drying related factors may also play a role i.e. lower initial dry matter (DM) of permeate compared to skim milk result in 15-30% higher energy costs (Schuck et al., 2015).

Additionally, in such cases, substantial drying capacity is utilised which could be used for more profitable streams. Therefore, low cost, energy efficient, compact technologies are of interest for whey/permeate processing.

This work reviews recent advances in dairy powder production processes, with a particular focus on energy efficient operations. Initially, the main features of conventional SD powder manufacturing processes are outlined along with technical constraints which can limit performance. Approaches to improve efficiency and reduce energy in conventional processes are discussed. Recent innovations and disruptive energy-reducing technologies are presented in the final section.

Overview of dairy powder manufacturing processes

2.2.1 Pre-treatments for optimum process and product performance

Understanding the mechanisms and effects of product-process interactions is paramount for optimising performance of powder manufacturing processes, especially due to the complex nature of macro-nutrient interactions which can occur during the various manufacturing steps. Furthermore, composition is a key factor influencing the processing route employed, for example, manufacture of a lactose-rich powder such as permeate has different challenges (stickiness, mineral fouling etc.) compared to a nutritional powder such as IMF (fat stabilisation, retention of nutritional quality etc.). Therefore, processes are adapted to meet these challenges. However, even within the same category of powder, significant differences can be observed depending on preference of manufacturer, equipment available, customer requirements etc. Therefore, while in the context of this review it is attempted to define a conventional process, this definition is broad by necessity and consists of pre-treatment steps (formulation, heat treatment etc.) followed by a pre-concentration step and subsequent SD (Fig 2 .1).

Pre-treatment operations are generally performed to ensure the desired specifications of powder are met (microbiological quality, composition, physical properties, etc.). In terms of microbiological quality, different heat treatment steps may be applied to give the required level of bacterial reduction. Heat treatment must also be considered as a way to modulate functional properties: it has a vital role in powder manufacturing as it has substantial influence on product physicochemical properties (denaturation of whey proteins, viscosity development, etc.) [START_REF] Murphy | Decoupling Macronutrient Interactions during Heating of Model Infant Milk Formulas[END_REF][START_REF] Sutariya | Influence of milk pre-heating conditions on casein-whey protein interactions and skim milk concentrate viscosity[END_REF], which in turn determine processability and powder functionality. Generally speaking, the higher the intensity of heat treatment (temperature/time combination) applied the longer the shelf life and the greater the modifications induced at a molecular scale [START_REF] Deeth | Heat Treatment of Milk -Overview[END_REF]. For instance, skim milk powders are classified as low (e.g. 72 -80 °C/15 -30 s), medium (e.g. 85 °C/60 s, 90 -105 °C/30 s) and high-heat (e.g. 90 °C/300 s, 120 °C/60 s, 135 °C/30 s) [START_REF] Deeth | Heat Treatment of Milk -Overview[END_REF], based on the severity of heat treatment applied, with low heat products having typical shelf lives of 1-year, high solubility and low levels of Maillard browning, compared to shelf lives of 2 years, lower solubility and higher Maillard browning for high heat products [START_REF] Patel | Methods to determine denaturation and aggregation of proteins in low-, medium-and high-heat skim milk powders[END_REF][START_REF] Stapelfeldt | Effect of heat treatment, water activity and storage temperature on the oxidative stability of whole milk powder[END_REF].

Formulation is another commonly encountered processing step used in a wide range of dairy products. This can range from standardisation of protein in skim milk to more complex recipes, such as infant formula where compliance with macro and micro-nutrient specifications is essential (Masum et al., 2020;[START_REF] Murphy | Influence of protein standardisation media and heat treatment on viscosity and related physicochemical properties of skim milk concentrate[END_REF]Schuck et al., 2016).

Homogenisation is performed primarily to improve emulsion stability, which also influences physical properties of concentrates. In order to overcome processing challenges (e.g. fouling in tower), due to the extremely sticky nature of high lactose products (e.g. whey, permeate), a controlled crystallisation step is introduced prior to SD. More than 70% of lactose is transformed into stable crystalline state which ensures consistent operation and product stability (Písecký, 2005;Schuck et al., 2016). It is clearly evident that pre-treatments influence vital properties of both liquid concentrates and final products, and therefore, play an important role in overall performance of spray drying plant.

Technological constraints for maximizing overall performance

The objectives of an efficient SD plant are to:

 achieve maximum production capacity  ensure product safety and functionality  minimize energy consumption SD capacity is limited by the designed water removal rate. Therefore, for higher powder production capacity, higher solids feed concentrate is desirable. Moreover, maximising pre-concentration using a combination of reverse osmosis (RO) and vacuum evaporation is beneficial from energy consumption perspective (Fig. 2.1). This is because the thermal efficiency of SD is less than 60% [START_REF] Cheng | Methods for Improvement of the Thermal Efficiency during Spray Drying[END_REF], which consumes 10 to 20X more energy than falling-film evaporators (FFE). RO has dramatically lower energy requirements as concentration is obtained without phase change of water (Fig. 2.1).

The performance of membrane concentration is limited to low DM contents, due to deposits, concentration polarisation, rising osmotic pressure and lower permeate flux as concentration increases [START_REF] Meyer | Concentration of Milk and Whey by Membrane Technologies in Alternative Cascade Modes[END_REF]. The end-point of membrane concentration is generally based on a minimum permeate flux to avoid inefficient operation [START_REF] Chen | A pilot scale study on the concentration of milk and whey by forward osmosis[END_REF][START_REF] Meyer | Concentration of Milk and Whey by Membrane Technologies in Alternative Cascade Modes[END_REF]. [START_REF] Schuck | Changes in Functional Properties of Milk Protein Powders: Effects of Vacuum Concentration and Drying[END_REF] reported application of RO for pre-concentration up to 18% and 22% w/w DM for skim milk and whey, respectively, whereas, Hausmann et al.

(2014) stated 30% and 25% w/w DM as limiting concentrations for skim milk and whey, respectively.

Therefore, high-solids drying is largely dependent on the maximum achievable DM content in FFE, which can typically achieve >50% w/w DM (composition dependent).

However, inherent design restrictions limit this maximum value. A consistent product film inside the tubes of FFE is essential to sustain their uniform and adequate wetting, which guarantees heat transfer efficiency and minimal fouling [START_REF] Morison | Minimum Wetting and Distribution Rates in Falling Film Evaporators[END_REF]Tanguy et al., 2015). Film-breakdown at higher concentrations due to limited flowability leads to excessive fouling and local burn-on (Tanguy et al., 2017).

Additionally, efficient spraying is paramount for consistent SD operations and can have significant impact on tower fouling and product quality. Conventional atomizers fail to transform thicker concentrate into fine particles of desired size (10-400 mm) [START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF]. Insufficient drying due to larger droplets leads to increased fouling in the tower, which can lead to blockages and forced shutdown if not controlled, thereby causing huge economic losses. Moreover, product quality is deteriorated (e.g. increased insolubility, caking, etc.) due to burnt particles from evaporator and insufficiently dried particles from SD (Schuck et al., 2005a;[START_REF] Schuck | Dehydrated Dairy Products: Milk Powder: Types and Manufacture[END_REF][START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF]. Productivity and energy efficiency can be elevated by technical solutions which circumvent the need for atomisation. For instance, granulation of highly viscous superconcentrated pastes followed by fluid-bed drying, or accelerated heat and mass transfers due to modified properties (e.g. aeration) of concentrate entering the drying tower can be potentially beneficial, as viscosity is no longer a major limitation.

Furthermore, stickiness, which is a result of product-process interactions, imposes practical limitations on SD performance. While stickiness may be desirable to promote to the critical moisture content has a direct impact on profitability (production capacity, energy) [START_REF] Palzer | Generation of Product Structures During Drying of Food Products[END_REF][START_REF] Schuck | Relative humidity of outlet air: the key parameter to optimize moisture content and water activity of dairy powders[END_REF][START_REF] Schuck | Keeping quality of dairy ingredients[END_REF]. For instance, excessive drying can result in productivity loss and increased energy consumption. An increase in residual moisture set point by 0.2% was reported to be equivalent to increasing total solids from 48% to 54% w/w DM for whole milk powder processing, from energy efficiency point of view [START_REF] Niels | Control and Energy Aspects of Milk Powder Spray-Drying in Relation to Product Quality[END_REF]. Mapping sticky curve (T °C, %M) for products can provide crucial insights into limits of exhaust temperatures, while implementation of dehumidifiers can provide improved control over both production capacity and consistency. In contrast, any technical solutions to make powder stable at higher moisture contents than currently possible, can improve sustainability and profitability.

Heat recovery can be challenging in convective drying processes such as SD. In a typical powder manufacturing plants, evaporators consume 45% of total energy, while SD account for 51% approximately [START_REF] Ladha-Sabur | Mapping energy consumption in food manufacturing[END_REF]. Unlike FFE, where multipleeffects and use of MVR/ TVR technology improve thermal efficiency, enormous amounts of energy are wasted in SD exhaust air streams resulting in poor thermal efficiency [START_REF] Moejes | Energy saving potential of emerging technologies in milk powder production[END_REF].

Pre-concentration using membrane technology for energy savings

Minimizing thermal energy inputs is key for energy efficient processing. Membrane concentration technologies such as RO and Nano-filtration (NF) separate water from product without involving water phase change, thus saving energy required to provide latent heat of vaporization. [START_REF] Chamberland | Integrating reverse osmosis to reduce water and energy consumption in dairy processing: a predictive analysis for Cheddar cheese manufacturing plants[END_REF] reported close to 36% reduction in natural gas consumption due to pre-concentration of cheese whey and milk permeate to 20% and 22% w/w DM, respectively, using RO prior to evaporation.

Maximisation of concentration using membrane technologies (RO) may be attractive; however, operational costs with respect to production run times and cleaning requirements should be considered for deriving optimum performance. [START_REF] Depping | Integrating environmental impact assessment into new product development and processing-technology selection: Milk concentrates as substitutes for milk powders[END_REF] observed increasing the skim milk concentration from 30% to 35% w/w DM using RO, resulted in ~58% reduction in run time due to increased fouling occurrence and cleaning frequency.

There is a trade-off between achieving higher concentration factors and productivity, production runs were reduced from 15 h to 5 h, when DM increased from 25% to 35% during RO [START_REF] Stefansdottir | Impact of shelf life on the trade-off between economic and environmental objectives: A dairy case[END_REF].

In order to overcome the concentration limitations of removal by half, as compared to RO at 15 °C. While the above research points to the potential benefits of cascade filtration processes, it must be noted that a major limitation is the production of side-streams which can be difficult to valorise.

FO operates at considerably lower specific energy in comparison to RO, due to lower pressure requirements. FO was reported as a promising technology for its higher concentration capacity and ability to preserve organoleptic properties [START_REF] Blandin | Forward Osmosis as Concentration Process: Review of Opportunities and Challenges[END_REF]. The disadvantages associated with RO, namelyhigher pressure and lower permeate flux with increasing concentration is overcome in MD, therefore higher concentrations as compared to RO can be achieved [START_REF] Hausmann | Performance assessment of membrane distillation for skim milk and whey processing[END_REF]. In an innovative approach, a combination of RO and MD was implemented to concentrate skim milk from 9% to 50% w/w DM [START_REF] Moejes | Membrane distillation for milk concentration; 29th EFFoST international conference -Athens[END_REF]. In addition to negating the requirement for a falling-film concentration step, MD can operate using waste heat, which can result in substantial energy savings. Low flux, however, limits water removal capacity and is a critical barrier for commercial implementation especially when considering the large capacities of conventional downstream equipment (e.g., FFE, SD).

Although, RO consumes only 10% energy (SEC 20-40 kJ kg -1 ) in comparison to sixstage evaporator with thermocompression (SEC 260-330 kJ kg -1 ), its energy saving potential is comparatively lower when compared to MVR (SEC 36-72 kJ kg -1 ) (Schuck et al., 2015).

Therefore, the efficacy of implementing a preconcentration step using RO may be limited to evaporators where thermocompression is in use.

High-solids drying for increased productivity and energy efficiency

Energy cost for water removal is higher in SD compared to evaporators, therefore entering SD at increased DM concentration is desirable. For example, 2% increase in feed concentrate (~50% w/w DM) can lead to around 6% energy savings [START_REF] De | Energy reduction by high dry matter concentration and drying[END_REF][START_REF] Winchester | Model based analysis of the operation and control of falling film evaporators[END_REF]. High-solids drying is challenged by equipment performance constraints and product quality, due to feed concentrate physical properties, especially viscosity.

Measures to promote high-solids drying comprise of two approaches:

(1) control the viscosity of the feed concentrate and/or

(2) utilise innovative process equipment capable of handling highly viscous concentrates

Viscosity control

Viscosity is a key limiting factor affecting the processability of dairy concentrates [START_REF] Carr | Protein Hydration and Viscosity of Dairy Fluids[END_REF]Schuck et al., 2016). This is because both process performance (e.g., FFE, atomization and SD) and product quality are negatively influenced by increased concentrate viscosity [START_REF] Baldwin | The effect of concentrate viscosity on the properties of spray dried skim milk powder[END_REF][START_REF] Morison | Viscosity and Non-Newtonian Behaviour of Concentrated Milk and Cream[END_REF][START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF]. In practice, viscosity of concentrate is kept below 100 mPa s for effective atomisation and optimum product quality (Schuck et al., 2005b;[START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF]. Composition (protein:lactose ratio, type of protein) has significant role on the viscosity of concentrated systems, which is reflected by different total solids required to reach limiting viscosity: e.g., protein isolate ~20-25% w/w DM, MPC70 ~30% w/w DM, Skim milk ~45-50% w/w DM, Whole-milk ~50-55% w/w DM, IMF ~50-60% w/w DM, Whey ~60% w/w DM [START_REF] Bista | Monitoring Viscosity and Total Solids Content of Milk Protein Concentrate Using an Inline Acoustic Flowmeter at Laboratory Scale[END_REF]Masum et al., 2020;[START_REF] Schuck | Changes in Functional Properties of Milk Protein Powders: Effects of Vacuum Concentration and Drying[END_REF][START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF]. Generally speaking, the higher the protein to DM ratio, the lower the total solids at limiting viscosity.

Viscosity represents the resistance to flow of the system, which is influenced by serum phase viscosity and volume fraction of suspended particles. Morphology of suspended particlessize distribution, shape and aggregationhave additional impact on system viscosity. In milk system, serum phase consists of dissolved lactose, whey and minerals components, while caseins and fat globules form the suspended phase. During concentration, due to depleting water, volume fraction of suspended phase increases, which results in progressively higher viscosity that eventually shows exponential rise when the system approaches jamming [START_REF] Hogan | Soft matter characterisation of whey protein powder systems[END_REF], or, in other words, maximum volume fraction. Pre-treatments, and in particular heat treatment, have a large influence on viscosity.

For instance, volume fraction of whey proteins increases by up to three times upon denaturation, and, interactions with other protein fractions may lead to excessive aggregation, which in turn results in marked increase in viscosity [START_REF] Carr | Protein Hydration and Viscosity of Dairy Fluids[END_REF]Masum et al., 2020). Due to the benefits associated with increasing the DM prior to SD, researchers have examined technical solutions to reduce concentrate viscosity, comprising formulation fine-tuning; adapting heat treatments, and; implementing emerging technologies such as ultrasound, hydrodynamic cavitation, etc. (Table 2.1). [START_REF] Zisu | Application of ultrasound to reduce viscosity and control the rate of age thickening of concentrated skim milk[END_REF] 1 DMdry matter, Dv(90)volume based 90 th percentiles, SDspray dryer, MPCmilk protein concentrate, WPCwhey protein concentrate, IMFinfant milk formula [START_REF] Buggy | Stabilising effect of alpha-lactalbumin on concentrated infant milk formula emulsions heat treated preor post-homogenisation[END_REF][START_REF] Crowley | Impact of αlactalbumin:β-lactoglobulin ratio on the heat stability of model infant milk formula protein systems[END_REF]. Increasing levels of αlactalbumin positively impacted concentrate viscosity by reducing protein-protein interactions and decreasing aggregation of whey proteins upon heat treatment.

Effect of heat treatment

Heat treatment is a vital parameter that has marked influence on product viscosity due to its effect on denaturation of whey proteins and associated aggregation. Arzuaga et al.

(2021) evaluated influence of heat treatment (75 °C or 100 °C for 18 s) and wetmix solids content (50% or 60% w/w DM) on spray dried IMF powder properties. They showed substantial reduction in concentrate (60% w/w DM) viscosity for lower heat treatments, i.e., 50 compared to 96 mPa s (300 s -1 ) for heat treatment at 75 °C x 18 s and 100 °C x 18 s, respectively. This was attributed to lower levels of whey protein denaturation. Pasteurization at 75 °C and 60% w/w DM concentrate resulted in significantly lower energy consumption (up to 17%) during SD, due to higher solids in concentrate. In a similar study, effect of heat treatment (medium heat: 90 °C x 60 s, or severe heat: 120 °C x 270 s) on skim milk concentrates (46% w/w DM) prepared by subsequent FFE was evaluated. Concentrate viscosity was reduced by 33% (from 103 to 69 mPa s at 300 s -1 ) for medium heat as compared to severe heat treatment [START_REF] Murphy | Influence of protein standardisation media and heat treatment on viscosity and related physicochemical properties of skim milk concentrate[END_REF]. Murphy et al. (2013) heat treated model IMF at 60% w/w DM (110 °C for 3 s) using direct steam injection, where the resultant DM was approx. 55% due to steam condensation. Viscosity of steam injected concentrate was markedly lower in comparison to indirect heat-treated concentrate (55% w/w DM), which was attributed to lower denaturation levels of whey protein.

Effect of homogenisation

Homogenization is often included in manufacturing processes as a means of emulsion stabilization, resulting in enhanced physical properties of dairy powders [START_REF] Mesa | High Homogenization Pressures to Improve Food Quality, Functionality and Sustainability[END_REF]Vignolles et al., 2007). Homogenization, which is usually applied in two steps, reduces size of fat globules through means of shear forces and concomitant association of protein at the fat interface results in emulsion stabilization. Typically, fat globule size may be reduced by a factor of 10 and surface area may increase by a factor of 5-15 (Vignolles et al., 2007). This results in modification of concentrate properties which can affect the spraying performance. Location of the homogenizer (pre-heat/ post-heat treatment), homogenization pressure and concentrate DM are key factors that influence viscosity of concentrate. For instance, α-lactalbumin content in IMF powders was found to influence the interaction between homogenization position and heat treatment. Viscosity of model IMF concentrates (55% w/w DM) where α-lactalbumin constituted 12% of total protein was significantly (p < 0.05) lower when homogenized after heat treatment, compared to samples homogenized before heat treatment. In contrast viscosity of equivalent IMF concentrates, where αlactalbumin constituted 48% of total protein, was not affected by homogenization location [START_REF] Buggy | Stabilising effect of alpha-lactalbumin on concentrated infant milk formula emulsions heat treated preor post-homogenisation[END_REF]. Viscosity of whole milk concentrates has been shown to be highly influenced by applied pressure [START_REF] Mercan | Effect of high-pressure homogenisation on viscosity, particle size and microbiological characteristics of skim and whole milk concentrates[END_REF]. Whole milk concentrate (42% w/w DM) viscosity was reduced due to homogenization at pressures up to 25 MPa, whereas homogenizing pressures higher than 25 MPa increased the concentrate viscosity [START_REF] Mercan | Effect of high-pressure homogenisation on viscosity, particle size and microbiological characteristics of skim and whole milk concentrates[END_REF]. In contrast, homogenization pressure (6.9-13.8 MPa) has been shown to have no significant impact on the concentrate viscosity of model IMF emulsions [START_REF] Mccarthy | Optimising emulsion stability during processing of model infant formulae using factorial statistical design[END_REF]. [START_REF] Bodenstab | Process for the preparation of milk powder[END_REF] demonstrated the influence of total solids during homogenization on viscosity of concentrate post-evaporation. A two-stage homogenization at intermediate total solids (30% w/w DM) resulted in lower viscosity of whole milk concentrates, with the potential to subsequently implement high-solids drying [START_REF] Bodenstab | Process for the preparation of milk powder[END_REF]Walmsley et al., 2018). Overall, it is clear that homogenization can significantly affect viscosity, however the exact effect may vary as a function of many factors such as composition, position of homogenization, pressures employed, etc.

Innovative technologies for high solids spray drying

The tendency of whey proteins to form aggregates in dairy systems can result in significant viscosity increases. Cavitation induced by either hydrodynamic or ultrasound technologies, may be employed to reduce the size of aggregates, thereby reducing viscosity [START_REF] Deshpande | Effect of sonication on the viscosity of reconstituted skim milk powder and milk protein concentrate as influenced by solids concentration, temperature and sonication[END_REF][START_REF] Gregersen | Viscosity reduction in concentrated protein solutions by hydrodynamic cavitation[END_REF][START_REF] Zisu | Application of ultrasound to reduce viscosity and control the rate of age thickening of concentrated skim milk[END_REF]. Marked decrease in particle size distribution (D90 size, from 21.17 to 0.45 μm) in milk protein concentrate dispersions (20% w/w DM) was reported due to application of hydrodynamic cavitation (HC) in contrast to conventional high shear mixing [START_REF] Pathania | Applications of hydrodynamic cavitation for instant rehydration of high protein milk powders[END_REF]. The authors suggested viscosity reduction was directly linked to particle size distributions, and cautioned that the effects of HC on viscosity can be impaired in the case of simultaneous denaturation and aggregation of whey proteins. Table 2.1 presents application of innovative technologies to control viscosity.

In another study, [START_REF] Marella | Manufacture of modified milk protein concentrate utilizing injection of carbon dioxide[END_REF] reported the interesting effect of CO2 injection while manufacturing spray-dried MPC80 powders. The viscosity of the CO2 injected ultrafiltration retentate (17% w/w DM) was reduced 10-fold (from 24 to 2 mPa s at 100 s -1 ) compared to the control. The reduced viscosity of CO2 injected retentate can have a positive influence on both subsequent membrane concentration or FFE, although no such studies were performed to the best of authors knowledge.

Another way to overcome the barrier of viscosity is to adopt equipment (evaporators, atomisers, etc.) which are designed to handle highly viscous products. Tanguy et al. (2015) examined effectiveness of a thin-film spinning cone evaporator to increase concentration beyond typical FFE constraints. Pilot scale tests (~150 kg.h -1 capacity, 2 hours operation) validated the ability of new evaporator to reach higher solids concentration for skim milk (from 48% to 57% w/w DM) and for WPC80 (from 26% to 43% w/w DM), while maintaining key attributes of subsequent powders. Atomisation of highly viscous skim milk concentrate (57% w/w DM) was achieved by adapting the nozzle aperture and pressure. Resulting powder properties were satisfactory. However, concerns were raised with respect to the fouling and robustness of the approach for extended processing, which needs to be tested.

Application of thin-film spinning cone evaporator for WPC80 production reported 29% energy savings for water removal. Fouling in evaporators poses considerable challenges with respect to production duration. [START_REF] Morison | Reduction of fouling in falling-film evaporators by design[END_REF] reviewed design aspects of FFE in order to limit fouling (particularly relevant for higher solids processing), which can result in extended production runs (>20 h), thereby reducing cleaning frequencies and increasing productivity.

A decrease in evaporation rate was reported due to poor distribution, lower wetting rates and response of heating systems for milk products with higher solids content [START_REF] Silveira | Thermodynamic and hydrodynamic characterization of the vacuum evaporation process during concentration of dairy products in a falling film evaporator[END_REF].

The limitations were overcome with a newly designed cone-shaped concave distribution system, adapting the inlet flow rates and heat regulation system which improved wettability rate at higher solids. The results demonstrated that thermodynamic characterisation and implementation of suitable instrumentation can improve evaporator process performance. Murphy et al. (2013) studied the effect of direct steam injection in the field of infant formula manufacture. In contrast to typical wet-mix process {wet-mix (20-30% w/w DM)  heat treatment and homogenisation  concentration (55% w/w DM)  SD} (Masum et al., 2020), the new process {wet-mix (60% w/w DM)  SD} used a high shear rotor-stator to efficiently disperse high-solids formulation (60% w/w DM) and a direct steam injection device to heat treat prior to SD. Powders obtained compared favourably to standard powders. on droplet sizes, encountered during higher solids drying [START_REF] Erdmann | Method of controlling the spray droplet size of a spray nozzle apparatus for spray-drying applications, spray drying apparatus and nozzle therefore[END_REF]. [START_REF] Staehle | Performance and Efficiency of Pressure-Swirl and Twin-Fluid Nozzles Spraying Food Liquids with Varying Viscosity[END_REF] proposed a new approach which controls spraying systems to optimise sauter mean diameters (SMD) of spray droplets based on energy density. They found a robust corelationship between the energy density and SMD, which was influenced by viscosity and nozzle geometry. The relationship can potentially simplify scale up and manipulation of process parameters to obtain desired SMD. A process control system was implemented which continuously monitors relevant parameters (e.g. viscosity, concentrate mass flow rate, pressure and density), with variability accounted for by adjusting the geometry of the spray nozzles to deliver consistent droplet size with positive effects on process efficiency and product quality. This is particularly interesting as higher variability may be expected while processing higher solids concentrates by SD [START_REF] Niels | Control and Energy Aspects of Milk Powder Spray-Drying in Relation to Product Quality[END_REF].

Innovative atomisers that can produce controlled droplet sizes at high viscosities are of an interest for dairy industry. van Deventer et al. (2013) reported on an innovative nozzle based on ink-jet technology which is capable of processing viscosities up to 500 mPa s with additional benefit of producing mono-disperse droplets. They reported several benefits such asincreased energy efficiency, lack of fines generation and improved SD process control;

however, scalability remains a major challenge for its adaptation. In another study, simulations based on mono-disperse droplet drying suggested substantial energy savings, mainly attributed to controlled droplet sizes [START_REF] Atuonwu | Reducing energy consumption in spray drying by monodisperse droplet generation: modelling and simulation[END_REF]. Recently, Wittner et al. ( 2020) demonstrated an air-core-liquid-ring atomiser (ACLR), which can process up to 50% higher solids content (45% w/w DM) as compared to pressure atomisers (30% w/w DM)

for maltodextrin. The higher solids feed viscosity was 390 mPa s and close to 30% energy savings were estimated. [START_REF] Sorensen | External mixing pressurized two-fluid nozzle and a spray drying method[END_REF] introduced a novel twin-fluid nozzle design with improved capability to provide swirling motion of gas. The nozzle was reported to operate at industrial capacities producing the desired particle size and demonstrating low span of particles (<2). Combined use of pressurised feed liquid and gas was found to be more energy efficient than conventional nozzles. [START_REF] Wittner | Spray drying of high viscous food concentrates: Investigations on the applicability of an Air-Core-Liquid-Ring (ACLR) nozzle for liquid atomization[END_REF] examined the ability of ACLR nozzle to process highly viscous dairy concentrates (WPC80). Temperature of the concentrated product was modulated to obtain an experimental concentrate with higher viscosity (40°C vs industrial 60°C which resulted in 50% increase in viscosity to 90 mPa s at 1000 s -1 ). The concentrate was spray dried at pilot scale using ACLR nozzle and powder properties were compared to industrial reference. The experimental powders were similar to an industrial reference, in terms of particle size and morphology, which suggests ACLR can be used for efficient atomisation in high-solids drying initiatives. ACLR can overcome viscosity limitations of pressure swirl nozzles, at considerably lower gas to liquid consumption ratios (<0.5) to produce acceptable spray for high viscous concentrates [START_REF] Staehle | Performance and Efficiency of Pressure-Swirl and Twin-Fluid Nozzles Spraying Food Liquids with Varying Viscosity[END_REF]. Therefore, ACLR nozzles have potential for high-solids drying applications.

While the novel atomiser technologies discussed here are interesting due to their ability to process higher viscosities, it should be noted that concomitant increases in DM could be limited due to the exponential relationship between viscosity and DM observed for most dairy products beyond particular solids (e.g. over 45% w/w DM for skim milk) [START_REF] Snoeren | The viscosity of skim-milk concentrate[END_REF]Velez-Ruiz and Barbosa-Canovas, 2000). Therefore, energy savings could still be marginal considering additional capital and operational costs associated with implementation of such approaches.

Another approach is to circumvent conventional challenges associated with pumping and spraying systems at higher solids (higher viscosities), [START_REF] Dessica ; Durand | Method and Equipment for the Continuous Production of a Porous Powdered Product[END_REF] developed an intensified drying process, i.e. accelerated heat and mass transfers by replacing conventional concentrate droplets with a foamed concentrate structure using twin-screw extrusion porosification technology (EPT TM ). The viscosity challenge was overcome by using compatible equipment (twin-screw extrusion) and injecting CO2 to reduce the viscosity in two stages (first by extrusion itself followed by post-extrusion aeration). Extrusion-aeration produced superior functional properties in milk protein concentrates, while simultaneously realising energy savings [START_REF] Bouvier | Physical and rehydration properties of milk protein concentrates: comparison of spray-dried and extrusionporosified powders[END_REF][START_REF] Mchugh | Influence of nitrogen gas injection and agglomeration during spray drying on the physical and bulk handling properties of milk protein concentrate powders[END_REF]. EPT TM has recently been shown to be an effective alternative to atomisation for manufacture of milk protein concentrate (MPC80) [START_REF] Bouvier | Physical and rehydration properties of milk protein concentrates: comparison of spray-dried and extrusionporosified powders[END_REF][START_REF] Mchugh | Influence of nitrogen gas injection and agglomeration during spray drying on the physical and bulk handling properties of milk protein concentrate powders[END_REF]. Excellent rehydration properties of extrusion porosified MPC80 were reported, in comparison to standard spray-dried, due to improved micro-structure. Micromixing, which was achieved by injecting CO2 in the highly viscous concentrate, using high shear twin-screw extrusion, distinguished this technology from investigations which used gas injection into concentrate before atomisation in conventional SD [START_REF] Mchugh | Influence of nitrogen gas injection and agglomeration during spray drying on the physical and bulk handling properties of milk protein concentrate powders[END_REF]. The EPT TM reported 20-40% energy savings as well as increased production capacity as compared to conventional SD, which was attributed to higher solids processing and more efficient heat and mass transfer [START_REF] Mchugh | Influence of nitrogen gas injection and agglomeration during spray drying on the physical and bulk handling properties of milk protein concentrate powders[END_REF]. Interestingly, and in contrast to EPT TM , [START_REF] Brisset | Process and facility for producing a powdered porous product[END_REF] developed a process based on aeration, but without the necessity of highshear pre-treatment and partial CO2 injection, i.e. without twin-screw aeration set-up. The new development requires a suitable pump to feed highly viscous concentrates to an aeration device, followed by drying.

End-point of drying and dehumidification of inlet air: optimisation of process control for consistent operations and powder stability

Stickiness in tower during drying and caking during storage are major challenges in powder manufacturing process. Material properties (hygrocapacity, hygrosensitivity and hygrostability) together with operating conditions (e.g. exhaust air temperature, RH, etc.) and storage conditions (temperature -RH) influence stickiness and caking. In order to achieve powder stability, maximum moisture content is determined by taking into account aw and Tg [START_REF] Palzer | Generation of Product Structures During Drying of Food Products[END_REF][START_REF] Schuck | Keeping quality of dairy ingredients[END_REF]. For instance, 4% moisture in skim milk powders corresponds to an aw < 0.2 measured at 25 °C and a Tg close to 50°C, which is compatible with safe storage at ambient temperature [START_REF] Schuck | Relative humidity of outlet air: the key parameter to optimize moisture content and water activity of dairy powders[END_REF]. Additionally, moisture content control is an important consideration for efficiency of the process; over-drying of powders will result in increased energy and productivity loss. In order to better control moisture content and water activity in final powders, [START_REF] Schuck | Relative humidity of outlet air: the key parameter to optimize moisture content and water activity of dairy powders[END_REF] demonstrated usefulness of monitoring exhaust air relative humidity. They found no relationship between outlet exhaust air temperatures and powder moisture, and concluded monitoring and control of exhaust air RH is equally important as exhaust air temperature. In another study, [START_REF] Gianfrancesco | Achieving food products performance by mastering the drying process[END_REF] reported that the stickiness curve (i.e. the sticky temperaturemoisture content relationship), is of prime importance to determine the exhaust air conditions for maximum efficiency while achieving optimum product quality. High fat dairy powders were characterised to establish stickiness curves using particle gun for variable RH and temperature (60 to 80 °C). A marked increase in stickiness was reported when the RH of the air was higher than a critical point for a particular temperature, and the critical RH value decreased as temperature increased. Stickiness was majorly attributed to lactose content as the temperature was appreciably higher than Tg [START_REF] Paterson | Stickiness curves of high fat dairy powders using the particle gun[END_REF]. A predictive model for optimizing the SD process of nutritional products based on stickiness curve was reported by Camino-Sanchez et al. (2020). The application of the resulting model, which predicts powder moisture for variable dryer operating conditions, together with sticky curves developed for individual products were reported to increase the capacity by 6 to 8%, reduce downtime and ensure consistent powder physical properties. [START_REF] Petersen | Industrial application of model predictive control to a milk powder spray drying plant[END_REF] reported industrial application of model predictive control to optimise SD operations (~7000 kg.h -1 capacity). The model controlled exhaust air conditions (temperature and humidity) to avoid stickiness issues while achieving a powder moisture content close to its maximum limits.

They reported increase in production capacity by up to 4%, while simultaneously reducing energy consumption. Application of process control techniques is of greater importance in high-solids drying approaches, as increased variations in higher-solids feed concentrate can cause more disturbances in SD operations.

Recently, [START_REF] Masum | Effect of lactoseto-maltodextrin ratio on emulsion stability and physicochemical properties of spray-dried infant milk formula powders[END_REF] studied effects of partial replacement of lactose with maltodextrin (up to 30%) in IMF. Inclusion of maltodextrin (Dextrose Equivalent of 20) maintained emulsion stability in wet-mix and led to satisfactory final powder properties. They reported increase in Tg at higher levels of maltodextrin. Glass transition and water activity are important aspects of powder stability. Examining innovative formulation changes (targeted to end-use) that manipulate Tg and aw so that residual moisture could be increased without compromising stability can improve powder manufacturing processes. This can also provide the possibility to manipulate exhaust air conditions to favour higher thermal efficiency. For instance, within the range of nutritional products studied, a product which had higher maltodextrin content and consequently higher Tg (140 °C), was dried at higher target moisture as well as higher humidity in exhaust air, resulting in increased capacity, energy utilisation and profitability (Camino-Sanchez et al., 2020).

Another important parameter affecting the drying performance is ambient air humidity, which can vary considerably with time. Furthermore, ambient air humidity can interact significantly with product composition to reduce capacity in certain products. This is because composition is the key factor in determining stickiness of powders and associated exhaust humidity limits. For instance, an exhaust air humidity of 50 g.kg -1 may be a safe limit for milk; in contrast, for a stickier product such as infant formula, exhaust air humidity is often maintained below 30 g.kg -1 (Dairy Industries International, 2018). Therefore, drying capacity of sticky powders is more sensitive to changes in ambient air conditions. Using the example above, if ambient humidity changes from 5 to 10 g.kg -1 infant formula capacity will be reduced by 20% compared to 11% for skim milk. Implementation of dehumidification, can therefore result in greater benefits for stickier products, with the effect being more pronounced in humid climates e.g. capacity increase of 150% for IMF vs 43% for milk, based on reducing ambient humidity from 22 to 10 g.kg -1 (Dairy Industries International, 2018). A case study on impact of de-humidification of inlet air on energy consumption and a comparison of means of dehumidification (refrigeration or adsorption) was reported for a SD plant in Brazil [START_REF] Madeira | Economical Analysis of the Spray Drying Process by Pre-Dehumidification of the Inlet Air 7[END_REF]. They estimated higher savings in primary energy consumption for adsorption (~68%) over refrigeration systems (33%) when the drying capacity was constant; inlet air temperatures were reduced to account for mass balances.

The improved performance was related to higher drying capacity of adsorption-based systems. The benefits for adsorption-based system is based on the fact that waste heat can be used for regeneration and in contrast to refrigerated systems, the air temperature is increased with removal of moisture from air. In another case study involving a milk powder production plant, implementation of a silica gel desiccant wheel to control the inlet air humidity to approx. 2 g.kg -1 dry air reported an increase in production capacity between 35 to 45%, with an attractive return on investment of less than a year (Dessica, 2020).

Heat recovery from SD exhaust air

Convective dryers (e.g. SD, fluid bed dryers) have lower thermal efficiency than contact dryers/evaporators (e.g. FFE) [START_REF] Masters | Designing Fluid Bed and Spray Dryers for Low Energy Consumption[END_REF]. One way to improve energy savings is enhancing heat recovery. SD is one of the largest energy consumers in powder manufacturing plant, yet a large proportion of this energy is lost when exhaust air is vented to the environment. Several researchers reported the possibility of heat recovery measures with the potential to save up to 20% of energy [START_REF] Foods | Integrating heat recovery from milk powder spray dryer exhausts in the dairy industry[END_REF][START_REF] Golman | Analysis of heat recovery from a spray dryer by recirculation of exhaust air[END_REF][START_REF] Masters | Designing Fluid Bed and Spray Dryers for Low Energy Consumption[END_REF][START_REF] Walmsley | Thermo-economic optimisation of industrial milk spray dryer exhaust to inlet air heat recovery[END_REF]. Conventional operations often do not include heat recovery from exhaust air [START_REF] Walmsley | Improving energy recovery in milk powder production through soft data optimisation[END_REF], as the implementation is challenged by concerns over undesirable condensation spots, microbial re-contamination, higher fine particles loading, fouling of heat exchangers and demand for suitable heat sinks [START_REF] Foods | Integrating heat recovery from milk powder spray dryer exhausts in the dairy industry[END_REF][START_REF] Kockel | Heat recovery from spray dryer exhaust air: An example from an infant formula factory installation[END_REF][START_REF] Moejes | Closed-loop spray drying solutions for energy efficient powder production[END_REF][START_REF] Walmsley | Thermo-economic optimisation of industrial milk spray dryer exhaust to inlet air heat recovery[END_REF]. Moreover, due to characteristic low heat transfer coefficient of air/air heat exchange, heat transfer efficiency is poor.

Atkins et al. ( 2011) investigated feasibility of exhaust heat recovery by analysis of the entire milk powder production chain to identify suitable sinks (e.g. pre-heating inlet air for SD and fluid bed after-dryers, milk reception, cleaning-in-place systems, etc.). They reported energy savings were dependant on the selection of heat sinks based on modelling liquidcoupled heat recovery systems, with appropriate heat sinks resulting in energy savings up to 20%. [START_REF] Walmsley | Improving energy recovery in milk powder production through soft data optimisation[END_REF] applied pinch analysis along with soft data optimisation in order to develop maximum energy recovery networks (MER). They reported heat recovery could be potentially improved by over 30% using pinch methods and integrating the process stream data for the industrial plant. A model to evaluate thermo-economic design of an industrial liquid-coupled loop exhaust heat recovery system was developed [START_REF] Walmsley | Thermo-economic optimisation of industrial milk spray dryer exhaust to inlet air heat recovery[END_REF]. While demonstrating economic feasibility of heat recovery, the model also presented optimum design and operating conditions for heat exchangers that can be applied to other sites. The outcomes from the work of [START_REF] Walmsley | Thermo-economic optimisation of industrial milk spray dryer exhaust to inlet air heat recovery[END_REF] were appreciated by [START_REF] Kockel | Heat recovery from spray dryer exhaust air: An example from an infant formula factory installation[END_REF]. The author reported that a pinch point method was effective in identifying suitable heat sinks. The inhibition due to fine particles leading to excessive fouling was overcome with a bag house filter and adaptation of a novel heat exchanger design (wet-cleanability, low air velocity < 4 m.s -1 , etc.). [START_REF] Kockel | Heat recovery from spray dryer exhaust air: An example from an infant formula factory installation[END_REF] concluded that heat recovery from exhaust air in an infant formula factory was effective in realizing substantial energy savings based on consistent operational performance over 2 years. They recognised the potential of heat pump to further enhance energy savings and improve environmental footprint.

More recently, [START_REF] Moejes | Closed-loop spray drying solutions for energy efficient powder production[END_REF] simulated a closed-loop SD system. The system was based on a mono-disperse droplet drying to reduce fines generation coupled with an exhaust air dehumidification system. The study estimated up to 40% reduction in energy consumption for milk powders (from 8.4 to 4.9 MJ heat kg -1 powder). This was based on utilisation of latent and sensible heat from exhaust air by implementing air dehumidification and heat integration with dryer and concentration operations. While the authors recognised closed-loop drying is not possible in current milk SD set ups, they projected technologies that lead to absence of fines in exhaust air and air dehumidification at elevated temperatures could enable future adoption. Microbial recontamination is a practical threat that needs to be addressed for successful implementation of any recirculation system.

Disruptive technologies for sustainable manufacturing with enhanced or comparable functionality

As mentioned in previous sections, SD is the main utiliser of energy in dairy powder manufacturing. Breakthrough achievements in energy savings could be obtained by alternative powder manufacturing technologies which can replace SD. Only a few investigations have focused on this approach. Technologies based on high shear thin-film evaporators/dryers (Tixotherm ® , Towerless drying (poudre sans tour) -PST, Agitated thinfilm drying -ATFD) were examined for high lactose dairy streams (whey, permeate) and whey protein isolate (J. Písecký, 2005;[START_REF] Qiu | a. Agitated thin-film drying of foods[END_REF]Tanguy et al., 2017). Extrusionaeration was evaluated for superior functional properties of milk protein concentrates, while simultaneously realising energy savings [START_REF] Bouvier | Physical and rehydration properties of milk protein concentrates: comparison of spray-dried and extrusionporosified powders[END_REF][START_REF] Mchugh | Influence of nitrogen gas injection and agglomeration during spray drying on the physical and bulk handling properties of milk protein concentrate powders[END_REF].

Previous studies have reported increasingly shear thinning behaviour of dairy concentrates with increasing DM [START_REF] Bienvenue | Rheological properties of concentrated skim milk: Importance of soluble minerals in the changes in viscosity during storage[END_REF][START_REF] Morison | Viscosity and Non-Newtonian Behaviour of Concentrated Milk and Cream[END_REF][START_REF] Snoeren | The viscosity of skim-milk concentrate[END_REF]Trinh et al., 2007b;Velez-Ruiz and Barbosa-Canovas, 2000), thus high-solids concentrates can be maintained fluid during high shear operations. This can be exploited to overcome viscosity limits observed in conventional evaporators. For instance, high-shear enabled Tixotherm ® and PST evaporation technologies can concentrate whey to 86% and 80% w/w DM, respectively (Písecký, 2005;Tanguy et al., 2017). The product at the outlet of the evaporators, consisting of a turbo concentrator implementing high shear forces at the heat transfer surface, was a thick paste. In the Tixotherm ® process, over-concentration was followed by crystallisation using a screw conveyor and final dehydration by fluidised bed.

Grinding and sieving of the dried product was recommended (Písecký, 2005). In contrast, PST process used reverse granulation after over-concentration (Fig. 2.2), i.e. the paste (80% w/w DM) was fragmented into discrete powder particles by addition (recirculation) of dry powder (97% w/w DM). The resulting discrete non-sticky granules (>88% w/w DM) can be further dried, using a similar thin-film horizontal rotary evaporator or using a fluidised bed, to final DM of 97% w/w DM (Tanguy et al., 2017). Whey and permeate powders produced using both the technologies compared favourably with standard spray dried powders. Energy savings to the tune of 30% and 10%, compared to SD and the global process, respectively, were reported on the basis of water removal (Tanguy et al., 2017). Significant savings in capital costs (up to 40%) are estimated due to highly compact installation, which would also result in lower building maintenance, lower cleaning and wastes associated with managing huge SD towers. Such features make these technologies more attractive for production of low value dairy co-product streams. Little is known about the processability of other dairy streams (high protein, high fat) using high shear superconcentration-granulation based processes, however it may be expected that high protein and/or fat contents could prove to be challenging.

Depending on the output of the dehydration process, equipment is generally classified as a dryer or evaporator; i.e. if dry powder is obtained, the equipment is dryer and if intermediate moisture concentrate or paste is obtained, the equipment is referred to as evaporator. [START_REF] Qiu | a. Agitated thin-film drying of foods[END_REF] investigated similar high shear thin film equipment for obtaining dry powders from whey protein isolate at lab scale (ATFD). They reported ATFD was suitable for materials that crystallised easily and higher molecular weight materials (proteins). In contrast, low molecular weight sugars formed a sticky mass that blocked the equipment. Despite formation of a thick sticky mass which halted operation, authors suggested higher scale ATFD could break the mass into granules due to their ability to induce higher torque. In contrast to ATFD approach, PST used back-mixing, to overcome this sticky regime, facilitating smooth equipment operations. Several studies, reviewed by [START_REF] Bennamoun | Review on fundamental aspect of application of drying process to wastewater sludge[END_REF], have reported highly cohesive phase at intermediate moisture range during drying in agitated dryers [START_REF] Deng | Measurement and simulation of the contact drying of sewage sludge in a Nara-type paddle dryer[END_REF][START_REF] Lowe | Development in the thermal drying of sewage sludge[END_REF]. A recent study from the present group of authors (to be published) confirmed this behaviour in reference to dairy ingredients (whey permeate, demineralised whey, skim milk and fat filled milk). Composition (protein:lactose ratio) was the key factor influencing the onset and extent of the sticky regime. Mapping cohesive behaviour is a useful tool to test feasibility of applying such technologies to other dairy ingredients.

A comparison of recent technologies is presented in Table 2.2. Overall, breakthrough achievements in energy savings and environmental footprint are possible by adopting alternative technologies. The complex nature of dairy products (e.g. compositionmacronutrient interactions, heat sensitivity) and associated evolution of physico-chemical properties during processing is a challenge that needs to be addressed when adopting newer technologies. Characterising dairy products for properties that determine the processability of novel technologies and investigating opportunities that are presented by such adaptations can be a way forward for producing dairy powders with new functionality in a sustainable and profitable manner. For instance, circumventing cohesive behaviour of superconcentrates and designing improved microstructures by micromixing gases (N2, CO2, etc.) could be investigated.

An innovative approach to spray dry liquids at markedly lower air temperatures (<100°C) was introduced recently [START_REF] Beetz | Methods and apparatus for low heat spray drying[END_REF][START_REF] Beetz | Ultrahigh efficiency spray drying apparatus and process[END_REF]. The invention reports induction of localised turbulent zones into the chamber, by injecting secondary streams of air at multiple locations. Consequently, elevated hydrodynamic efficiency (turbulence greater than 10X) and shorter residence times lead to higher productivity at substantially reduced energy consumption and capital costs (compact installation).

Lastly, in addition to the energy benefits associated with disruptive technologies, other considerations must be taken into account. Technological evaluations must include effects on resulting powder properties and energy saving potential while also considering cleaning requirements and length of operation. In this respect, SD remains the standard by which these new technologies will be judged.

Conclusion

Dehydration of milk is a key operation for the dairy industry, allowing for extended preservation of powder in a stable form without refrigeration. Thermal water removal processes involved in dehydration account for almost 90% energy consumption in milk powder manufacturing plants. Therefore, significant optimization can be achieved through focusing on dehydration processes; however, it is a mistake to consider dehydration in isolation. This review paper shows that consideration of the entire production chain is crucial during process optimization due to the cumulative effect of process-product interactions during pre-treatments and the significant effects these can have on dehydration processes, both conventional and disruptive.

Pre-treatments have considerable impact on product properties, whereby they play a major role in the process efficiency of the SD step. Optimization of heat treatments and implementation of technologies to reduce product viscosity at the inlet of the spray dryer can produce favourable operating conditions, which enable higher product concentration and reduced fouling, with associated higher productivity and shorter downtime. Higher solids processing is attractive with respect to economic and environmental perspectives. The bottleneck of atomization, can be overcome by alternative technologies such as extrusionaeration, or depending on the product requirements, utilization of innovative routes of powder production such assuperconcentration and granulation (e.g., Towerless drying technology). While disruptions may be necessary to dramatically reduce energy consumption, energy efficiency of existing equipment may be improved by implementation of measures such as improved heat recovery and minor modifications within existing setups to reduce the energy consumption without impacting product quality. Such approaches can complement other energy saving techniques such as high solids processing e.g., improved process control is vital for high-solids drying to balance risk of fouling with the increased capacity associated with this approach.

Considering the challenge of energy usage from a different perspective, significant improvements may be possible using disruptive technologies. In particular, the challenges associated with product viscosity, which are limiting factors in conventional evaporators, atomizers and spray dryers, can potentially be circumvented. These technologies, however, need to be extensively tested to demonstrate their robustness for industrial production capacities without any compromise on product functionality and stability. In addition, scientific understanding of highly concentrated (>60% DM) dairy streams needs to be expanded upon to support such novel approaches.

In summary, it is clear that much work has been undertaken to develop novel energysaving approaches, in both conventional and disruptive dairy powder processes. However, many of the approaches described are currently at a precommercial stage. The next challenge for the industry is to assimilate the available information before testing and implementing feasible technologies. For academics and process developers, the challenge remains to improve on existing approaches, either incrementally or by completely new methodologies. With that in mind, this paper provides a summary of the latest trends in dairy drying research, which can aid both industrial and academic readers seeking to improve energy efficiency.

CHAPTER 3: AIMS AND STRATEGY OF THE PHD PROJECT

Context of the PhD project

Dehydration is the single most important step that determines energy efficiency of powder manufacturing in addition to functionality of dairy ingredients. As outlined in the review (Chapter 2), typically >90% of water is removed using relatively lower energy processes such as membrane filtration and/or vacuum evaporation. Residual water (<10%) is removed using spray drying which accounts for 50% of the total energy. Therefore, spray drying can be considered to be an extremely energy intensive process. High solids spray drying is a commonly investigated strategy for increased energy efficiency. However, this is challenged by rising viscosity of concentrates which can result in increased fouling, lower ability to flow and spray, all of which contributing to inferior product quality. As an alternative, replacing the spray-drying step by a novel superconcentration-granulation (patented PST, Poudre sans Tour) approach can result in breakthrough reductions in energy consumption, in addition to opening new possibilities in the manufacture of dairy powders.

The PST process for permeate powders comprises of two unique steps:

i. Superconcentration of permeate from 60% to ~80% w/w DM ii. Granulation of superconcentrated paste into discrete particles by recirculation of dried powder. The granules (~88% w/w DM) can be dried using either turbodryers or fluid bed dryers.

The behaviour of dairy streams in superconcentrated states is not well understood, which represents a major knowledge gap and a significant barrier to efficient operation of the new process. Moreover, as the functionality of dairy powders is a key property for manufacturers and end-users, the effect of the novel process on powder physico-chemical and functional properties should be determined. Finally, the energy saving potential of the novel process must be determined. It is, therefore, pertinent to build relevant insights into the dairy streams with respect to the novel process steps, in order to contribute in the scale up, process optimisation and industrial acceptance.

Aims, objectives and strategy

The overall goal of the PhD project was to investigate the novel patented dehydration technology as a potential alternative to energy intensive spray drying for production of dairy powders with enhanced or comparable functional properties, while realizing significant cuts from an environmental and economical point of view (energy, water, waste).

The focus was on the superconcentration-granulation steps, which are new for the dairy industry. The process performance is dependent on these steps; therefore, it is vital to build understanding of the dairy superconcentrates in addition to the granulation behaviour of the superconcentrated pastes.

In the frame of this PhD, the initial schedule was to study the process on multiscale:

first at a lab-scale and then validate the lab-scale results during scale up to pilot plant installation, in addition to thorough characterization of process and product at a manageable scale (~100 kg.h -1 ). To this end, a pilot unit specifically designed was scheduled to be delivered by Teagasc in Nov 2019. However, delivery and commissioning delays due to Covid 19 and beyond the control of the PhD project team shifted the second step in time, and the whole PhD was finally dedicated to the lab scale study. In order to organize the research strategy, the overall aim has been split in the following research questions:

1. What are the limiting factors for the novel superconcentration-granulation process to manufacture dairy powders?

2. How does the composition of the dairy streams influence the superconcentrationgranulation process?

3. What are the specific characteristics of the powders produced by the superconcentration-granulation process compared to conventionally spray-dried powderspowder physical properties, functionality, microstructure, etc.?

4. How do the differences in composition of high lactose dairy side-streams (demineralized whey and whey permeate) affect the granulation behaviour?

5. How to overcome, taking advantage of the granulation behaviour knowledge, the challenge of high recirculation rates? Could this approach be used to manufacture added value formulated products from high lactose dairy streams?

The experimental strategy to address the research questions is presented in Fig. The main aims of this chapter were to:

-develop a lab-scale model and protocol to identify limiting factors of superconcentrationgranulation as a means of powder production.

-provide insights into the functional behaviour of the dairy superconcentrates.

Introduction

Dairy powders are produced by dehydration of highly perishable liquid ingredients, which increases the shelf life by up to 2 years and also reduces volume (> 80%) and weight required for transportation. The transformation of liquid to a stable powder form (water activity, aw < 0.3) requires removal of almost all the water and is generally realized in either two or three operations in series. Initially, the bulk of the water (>90%) is removed by means of the relatively low energy unit operations, membrane filtration and/or vacuum evaporation.

Water removal by membrane filtration is typically achieved through reverse osmosis or nanofiltration and can be utilized to concentrate to between 25 and 30% w/w dry matter (DM). Vacuum evaporation, which can be employed as the sole concentration operation or in combination with membrane filtration, is typically achieved through falling film evaporation (FFE) and transforms the liquid into a viscous concentrate (25-60% w/w DM, depending on composition). Spray drying (SD) then removes the remaining moisture, resulting in dry powders (typically >95% w/w DM). The primary step in SD is atomization of the concentrate into fine droplets in a flow of hot air. In some cases (i.e. 2 or 3 stage drying), additional fluid bed dryers are used to remove the final quantities of water, which increases energy efficiency (Schuck et al., 2015;[START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF].

SD is widely used to manufacture dairy powders because the majority of water is removed rapidly which results in high retention of functionality with minimal nutrient degradation. Furthermore, the technology is mature and is available from a number of equipment suppliers at production capacities ranging from small (kg.h -1 ) to large (tonnes.h -1 ).

However, SD is an energy intensive operation, which can consume up to 10 times more energy than FFE. Despite removing less than 10% water, SD can account for almost 50% of the total energy consumption in a dairy powder plant [START_REF] Ladha-Sabur | Mapping energy consumption in food manufacturing[END_REF].

High solids drying is a promising prospect for energy efficient operation (Murphy et al., 2013;Walmsley et al., 2018), which minimizes the quantity of water to be removed by SD, either through maximization of water removal by FFE, or, reconstitution of dried ingredients to high DM, as can sometimes be employed in the manufacture of formulated products such as infant formula. However, technical constraints limit the high solids drying approach, due to exponentially rising viscosity with increased DM. Firstly, achieving higher DM in the evaporator is limited by poor product distribution, increased fouling and disrupted flow at the evaporator outlet. Secondly, poor atomization of the concentrate at higher total solids (due to higher viscosity) can result in incomplete drying and stickiness issues which can, in turn, impede SD operations and cause poor final product quality (Schuck et al., 2016;Tanguy et al., 2017).

Several investigations have examined approaches for controlling the viscosity issues associated with high solids SD in order to improve plant efficiency (Murphy et al., 2013;Patil et al., 2021;[START_REF] Sutariya | Influence of milk pre-heating conditions on casein-whey protein interactions and skim milk concentrate viscosity[END_REF]Tanguy et al., 2015;Walmsley et al., 2018). However, only a few aspiring attempts have investigated breakthrough technologies which transform viscous concentrates into powders without using SD (Patil et al., 2021). For example, the patented technologies Tixotherm ® (Písecký, 2005) and PST (Poudre sans tour or Towerless powder) process (Schuck et al., 2016) are highly compact installations which do not utilize SD and can result in lower capital and operational costs (energy, maintenance, cleaning, wastes, etc). In the superconcentration-granulation based process, the initial water removal is achieved in energy-efficient multiple effect FFEs i.e. similar to conventional SD process.

For whey and permeate type streams, this step is usually terminated at ~60% DM, where viscosity limits further FFE concentration and/or spray drying. The subsequent superconcentration step, however, is capable of handling much higher concentrate viscosities. The vigorous mechanical agitation during superconcentration results in efficient water removal, taking DM from 60% to 80% w/w in the case of whey and permeate.

Subsequently, the PST process uses a 'back-mixing' operation, where dry powder is recirculated into the superconcentrate, thereby transforming the resulting mixture into discrete granules which are subsequently dried. Back-mixing is widely used to overcome stickiness issues in wastewater sludge drying, where it is used to transform cohesive masses into discrete non-sticky granules [START_REF] Kudra | Sticky Region in Drying-Definition and Identification[END_REF][START_REF] Peeters | Challenges of Drying Sticky Wastewater Sludge[END_REF]. However, as an operation, it is not widely reported in the dairy industry with the exception of Tanguy et al.

(2017) who demonstrated feasibility of the PST process for manufacture of whey permeate powders. Powders produced were comparable to SD powders, however, it was estimated that 32% less energy was used to transform concentrates (60% w/w DM) into powders (97% w/w DM).

As water is removed, transition of products into a highly cohesive (sticky, rubbery) phase with increased resistance to flow eventually becomes a limiting factor in high-shear, thin-film dehydration technologies [START_REF] Bennamoun | Review on fundamental aspect of application of drying process to wastewater sludge[END_REF][START_REF] Peeters | Challenges of Drying Sticky Wastewater Sludge[END_REF][START_REF] Qiu | a. Agitated thin-film drying of foods[END_REF]. In dairy systems, information regarding physical behaviour during superconcentration under high shear conditions is not reported. Furthermore, application of Tixotherm ® and PST is focused on only high lactose streams (whey and permeate) with very few studies published. To the best knowledge of the authors, no studies are published in reference to suitability of these technologies to other dairy productsskim milk, fat-filled milk, etc. It is crucial to build understanding of the dairy superconcentrates behaviour in order to optimize superconcentration-based powder production. Furthermore, it follows that the knowledge of granulation of these systems through recycling of final powders is also not well understood.

The current study examines the superconcentration and granulation approach for production of dairy powders at lab scale. Four dairy powders were selected to cover a range of compositions with respect to lactose, protein and fat content. The objectives in this investigation are two-fold: 1. To identify the limits of superconcentration for dairy products by developing a simple and reliable lab protocol, 2. To produce dairy powders utilizing the novel process at lab scale and compare powder properties with standard spray-dried materials.

This investigation provides insights into the functional behaviour of the different dairy products, as they are transformed from liquid to powders, based on their composition.

Materials and methods

Materials

Whey permeate powder (WPP), demineralized (90%) whey powder (DWP), skimmed milk powder (SMP) and fat-filled milk powder (FFMP) were acquired from local dairy ingredient suppliers. Serrated plate was used for higher DM content concentrates. Samples were pre-sheared at 300 s -1 for 20 s and then equilibrated at rest for another 30 s. Shear rate was increased in steps from 1 to 1000 s -1 and apparent viscosity was reported at a shear rate before sample discharge or slip was observed. The viscosity measurements were limited up to a certain DM content, beyond which it was not practical to perform measurements at high shear due to slip and discharge of the thick pastes from the geometry.

In addition to viscosity measurements of dairy ingredients, agitator power consumption of mixer was measured to characterize resistance to flow as a function of DM content. An energy consumption meter (Energy Logger 4000 FR, Voltcraft, Germany)

attached to the power cable indicated the power consumption, expressed as the current (ampere, A) supplied to operate the mixer at a constant agitation of 500 rpm, for various DM contents. Demineralized water was progressively added in to the dry powder to produce samples with varying DM content and corresponding power consumption was measured.

Novel lab scale superconcentration-granulation process to produce dairy powders

The lab scale process developed and investigated an innovative approach to produce dairy powders, which comprises of superconcentration of dairy concentrates to between 60 and 80 % w/w DM (depending on composition) followed by granulation through addition of dry powders to produce dairy powders as described by Tanguy et al. (2017). In the work of Tanguy et al. (2017), superconcentration, granulation and drying were achieved using three horizontal thin-film, rotary evaporators in series at pilot scale (VOMM, Rozzano, Italy). Trials were performed for the production of permeate powder. For superconcentration, the high shear within the unit allowed for effective concentration despite significant viscosity development at high DM. Granulation was achieved through back-mixing of powder on a 1:1 basis. Finally, drying of the subsequent granules was achieved using a similarly configured rotary evaporator/dryer. Table 4.2 provides details of the experimental conditions followed. Initially, the dairy powders were reconstituted in demineralized water to produce superconcentrated pastes of desired DM contents using a temperature-controlled mixer (TM31, 1.5 kW, Thermomix, Vorwerk, Germany), as described in section 4.2.2. The resulting highly viscous paste was fragmented into discrete granules by addition of respective dry dairy powders. The optimum quantity of powder addition rate (back-mix) for successful granulation was studied by addition of dry powders into paste at three superconcentrate:powder w/w ratios (1:0.8, 1:1 and 1:1.2, respectively). The standardized process of powder addition for 30 seconds at 500 rpm and granulation at 2000 rpm for 10 seconds was followed for all the experiments. The granules obtained were dried immediately as a thin layer (around 5mm) over drying pan in a vacuum oven (OV-12, 1.5 kW, JEIO TECH, Korea) at 45 °C overnight. The dried granules were ground in the mixer at 500 rpm for 30 seconds and 2000 rpm for 5 seconds. The powders obtained were immediately stored in airtight plastic containers.

In the process described above the powder used to granulate had the same DM composition as the superconcentrate. However, it is also possible to use different powders to granulate. Such an approach can result in novel compositions while also potential affecting the granulation process and therefore the overall efficiency of the process. Therefore, to test the effect of different compositions, a common drying aid (Maltodextrin (DE12) and highwater holding capacity (apple fiber) were also used during granulation. Maltodextrin (DE12) and apple fiber were procured from local suppliers. Powders were gradually added into whey permeate (WPP) superconcentrated paste (80% w/w DM), until discrete non-sticky particles were obtained. The moisture content of the powder samples was measured in a rapid Halogen moisture analyzer (Sartorius, Germany). The DM of the concentrates were measured by differential weighing using a rapid microwave moisture analyzer (Smart Trac CEM, Germany). For higher DM superconcentrates and granules, samples were first diluted in a known quantity of deionized water to measure the total solids.

Bulk Density, True Density and Porosity

The loose and tapped bulk densities were measured by a tapped volumeter with a graduated cylinder (Funke Gerber, Germany). The volume occupied by 100 g powder was used to calculate the loose density while the tapped density was calculated using the volume after 100 taps (GEA-Niro, 2006). The true density of powders was measured by Gas

Pycnometer (AccuPyc II 1340, Micrometrics Instrument Corporation, USA). Finally, the porosity was calculated using the tapped density and true density.

Light microscopy and scanning electron microscopy

Crystalline structure of powder was observed using polarized light microscopy (Olympus Corporation, Japan). For the scanning electron microscopy (SEM) measurement, the powder samples were mounted on a double-sided carbon tape and fixed to SEM stubs and then sputter coated with Chromium (Emitech K550X, Ashford, UK). Powder samples were examined with a field emission scanning electron microscope (Zeiss Supra, Carl Zeiss SMT Ltd., Cambridge, UK) at 5000X magnification.

Estimation of energy consumption

Energy consumption corresponding to water removal was calculated for the novel and conventional SD processes. The evaluation was based on the previously established methods to estimate energy usage during dehydration steps (Schuck et al., 2015;Tanguy et al., 2017). Energy consumption ratio (ECR, kJ.kg -1 water removal) was used to determine the energy required during dehydration steps (Table 4.3). The energy calculations were based on conventional inlet and outlet DM contents in SD (Schuck et al., 2015;Tanguy et al., 2017), while DM for superconcentration-granulation process were determined using the data obtained in the present study. The detailed calculations are explained in Tanguy et al. (2017). In this work, the specific energy consumption ratio (ECR) was estimated from a mass balance on the basis of the following data set for permeate:

-The first concentration step from 5.5 to 60% w/w DM, realized in a FFE, was considered to involve an ECR ranging from 75 to 400 kJ•kg -1 of evaporated water depending on the evaporator configuration.

-The ECR of the overconcentration step in a turbo concentrator, from 60 to 80% w/w DM, was estimated at 2875 kJ•kg -1 of evaporated water (manufacturer's data).

-The ECR of the final drying in a turbo dryer, from 88.5 to 97% w/w DM, was estimated at 5400 kJ•kg -1 of evaporated water (manufacturer's data).

Experimental and analytical repetition

Production of powders by means of superconcentration, granulation and drying was performed in triplicate. The characterisation of the superconcentrates (section 4.2.2) was performed in triplicate. The physical and functional properties (section 4.2.4 and section 4.2.5) were analysed in duplicate on each powder.

Results

Behaviour of dairy ingredients at superconcentrated DM content

This study investigated behaviour of dairy systems at DM contents higher than those typically encountered in vacuum concentration. The concentrates were characterized by measuring viscosity and power consumption as a function of DM content (Fig. 4.1). All the dairy ingredients studied, exhibited a strong concentration dependence and an exponential rise (Fig. 4.1a) in apparent viscosity with increasing DM content, which is consistent with previous studies [START_REF] Trinh | Rheological characterisation of age thickening with special reference to milk concentrates[END_REF]Vélez-Ruiz & Barbosa-Cánovas, 1998). SMP and FFMP (medium protein content) demonstrated exponential rise close to 50% and 55% w/w DM, whereas DWP and WPP (low protein content) showed exponential rise at higher DM content (~ 65% w/w DM). These observations are in agreement with previous studies that reported apparent viscosity was strongly affected by protein content [START_REF] Murphy | Decoupling Macronutrient Interactions during Heating of Model Infant Milk Formulas[END_REF]Schuck et al., 2005).

Furthermore, Fig. 4.1b shows power consumption in the temperature controlled highshear mixer as a function of DM content. As the DM content progressively increased, all compositions demonstrated a similar trend i.e. a sharp increase to peak power consumption followed by a sharp decrease. The onset of rise in power consumption and subsequent fall was highly influenced by composition. Onset occurred at lower DM content for SMP and FFMP, around 62% and 66% w/w DM respectively, compared to DWP and WPP where onset was at ~ 80% w/w DM. Among all the compositions studied, SMP showed the highest peak in power consumption indicating higher resistance to flow (higher cohesiveness).

Granulated powder production

Granulated powders were produced using the novel superconcentration-granulation process as described in Table 4.2. Dry powder was added into superconcentrated paste in order to fragment the paste into discrete large particles. Fig. 4.2 depicts the granulation achieved at different back-mix (dry powder addition) ratios for the dairy products. WPP and DWP granulated at a minimum DM of 88% and 87% w/w, which corresponds to dry powder addition rate of 1:1 and 1:0.8 (superconcentrate:powder w/w ratios), respectively. WPP results are consistent with previously reported findings i.e. whey permeate granulation occurred at DMs greater than 88% w/w (Tanguy et al., 2017). SMP granulated into larger particles at a minimum DM content of 78% corresponding to minimum back-mix of 1:1 ratio, whereas, FFMP granulated at a back-mix ratio of 1:0.8 corresponding to the minimum DM content of 77% w/w DM. For the lower back-mix ratio of 1:0.8, WPP and SMP continued to exist in a highly cohesive and sticky mass with high resistance to flow i.e. no granulation was observed (Fig. 4.2).

Differences in granulation behaviour were observed for different compositions. For SMP and FFMP, granules obtained under constant conditions (1:1 back-mix, powder addition for 30 secs at 500 rpm and 10 secs at 2000 rpm) were manually separated using a combination of 2, 1 and 0.63 mm sieves. SMP had 88% mass fraction over 2 mm sieve size, whereas FFMP had 41%. SMP and FFMP had 2% and 21% mass fractions finer than 0.63 mm, respectively. It is evident that SMP offered higher resistance to fragmentation (granulation) into discrete particles as compared to FFMP.

Granulated powder physical and rehydration properties

Table 4.4 provides a comparison of physical and functional properties of the granulated powders. Overall, the density of the granulated powders was higher than standard spray-dried powders. Lower porosity was observed in all the granulated powders, which is consistent with features of high shear granulation, where strong shear forces compact the granules providing comparatively higher density and lower porosities [START_REF] Ji | The structural modification and rehydration behaviours of milk protein isolate powders: The effect of granule growth in the high shear granulation process[END_REF]. Higher particle size was observed for all the granulated powders, which is in agreement with observations made in the previous study on PST-manufactured powders (Tanguy et al., 2017). Granulation improved flowability for all powders. DWP and WPP had comparable rehydration properties, whereas SMP and FFMP exhibited poor rehydration properties (Table 4.4). [START_REF] Person | Influence of the drying step in the steam-jet granulation process of dairy powders[END_REF] reported that rehydration properties of skim milk agglomerates were influenced by the final drying stage and DM content. The solubility of FFMP wet granules at 80% w/w DM was found to be 99.8% indicating that the poor solubility was caused by the drying step.

Granulated SMP and FFMP powders had a higher content of crystallized lactose compared to standard spray-dried powders, which can be seen as bright spots in light microscopy images (Fig. 4.3). The granulated powders had compact sharp structures with very low porosity, whereas the spray-dried powders demonstrated typical spherical porous structures (Fig. 4.4). A similar, highly compacted structure with low porosity was observed in the high shear granulation of milk protein powders [START_REF] Ji | The effects of fluidised bed and high shear mixer granulation processes on water adsorption and flow properties of milk protein isolate powder[END_REF].

Discussion

The primary objective of this study was to investigate limiting factors of the novel superconcentration-granulation approach for producing dairy powders. Four dairy ingredients covering a range of compositions (protein:lactose ratio and fat content) were investigated to understand the influence of composition. A lab scale model was developed to simulate the PST approach using a temperature-controlled mixer; power consumption was measured as a function of DM content to gain insights into the behaviour of superconcentrates. subsequent atomization during SD (Schuck et al., 2016;Tanguy et al., 2017). This can result in product burn-on, increased fouling in the evaporators, incomplete drying and stickiness in the tower, all of which negatively impact powder properties and functionality. Therefore, increased viscosity is a key limiting factor in the conventional dairy powder manufacturing process. From the data presented here it would appear that the viscosity becomes a limiting factor for in SMP production at lower DM content compared to WPP and DWP. This is broadly correlated with protein content (Table 4.1) and is also in agreement with the typical DM contents achieved for these products during FFE, with typical post-evaporation DM contents for skim milk being ~50% w/w DM compared to ~60% w/w DM for whey and permeate. This is an important point because the maximum levels of concentration, in turn, define the conditions for superconcentration. Therefore, in this study, superconcentration is a composition dependent term which is > 50% w/w DM for skim milk, >55% w/w DM for fatfilled milk and > 60% w/w DM for whey and permeate.

In processes which are capable of handling superconcentrated materials, such as PST, the viscosity increase, as seen in Fig. 4.1a, is not a significant factor due to the inherent high shear and the lack of a requirement for atomization. However, the limiting factor for such systems appears to be the transition of the product into a highly cohesive mass at a composition dependent DM content (Fig. 4.5). In addition, the present investigation demonstrates that this highly cohesive phase exists over a composition dependent DM content range (Fig. 4.1b). The details of the cohesive phase were found to shed light not only on the limits of superconcentration but also on subsequent granulation operations. While the onset of the cohesive phase resulted in extreme resistance to flow, as indicated by a sharp rise in the power consumption, which designated the maximum extent of superconcentration, the end of the cohesive phase also gave information on the minimum DM content at which the wet mass transformed into non-sticky discrete granules. A cohesive phase was observed in all the dairy compositions studied, however the onset was highly influenced by the composition (Fig. 4.1). In particular higher protein content resulted in earlier onset (SMP 62% w/w DM, FFMP 65% w/w DM), whereas lower protein and higher lactose contents resulted in delayed onset (DWP and WPP at 80% w/w DM). Thus, lower protein streams can be superconcentrated to higher DM contents compared to higher protein dairy streams.

<78% w/w DM Viscous Concentrate 80% w/w DM to 87% w/w DM Highly Cohesive (sticky) mass >88% w/w DM Non-sticky granules The onset of the cohesive phase is a significant technical challenge, which defines the superconcentration limits for the superconcentration-granulation process. The cohesive phase offers huge resistance to flow and can result in uncontrolled agglomeration which drastically reduces heat and mass transfer rates leading to highly inefficient drying conditions (up to 60% lower heat transfer rates) [START_REF] Kudra | Sticky Region in Drying-Definition and Identification[END_REF]. Furthermore, the highly cohesive mass has the potential to damage the mechanical integrity of rotating parts of the equipment [START_REF] Kudra | Sticky Region in Drying-Definition and Identification[END_REF][START_REF] Peeters | Challenges of Drying Sticky Wastewater Sludge[END_REF]. Therefore, the main requirement of an efficient superconcentration and granulation process should be to avoid the cohesive phase through termination of the first stage at an appropriate concentration, followed by application of an adequate back-mixing ratio. Table 4.5 presents DM content limits for the three distinct regions of behaviour observed in the superconcentrated ingredients studied.

The approach applied here, while novel for dairy ingredients, is analogous to approaches applied in drying of sludge. Technologies similar to PST, i.e. utilizing high shear thin film conductive dehydration process, have reported evolution of similar three distinct phases while drying wastewater sludge, as reviewed by [START_REF] Bennamoun | Review on fundamental aspect of application of drying process to wastewater sludge[END_REF]. The waste water sludge passes through pasty, lumpy and granular phases which are detected by following variations in torque during concentration [START_REF] Bennamoun | Review on fundamental aspect of application of drying process to wastewater sludge[END_REF]Ferrasse et al., 2002;[START_REF] Kudra | Sticky Region in Drying-Definition and Identification[END_REF]. Another study utilized a shear-based lab protocol to map the sticky phase (highly cohesive-rubbery phase) encountered at some intermediate moisture content range during drying of activated sludge, consequently providing valuable information to circumvent or manage the stickiness phenomenon in industrial sludge dryers (horizontal rotary dryers similar to PST process) [START_REF] Peeters | Using a Shear Test-Based Lab Protocol to Map the Sticky Phase of Activated Sludge[END_REF]. Across all dairy compositions in this study, the evolution of cohesive phase was observed at DM approximately between 10 and 20% w/w greater than the concentration at which exponential viscosity increase occurred. The exponential viscosity rise indicates dramatic change from liquid to solid as the system approaches a jamming state [START_REF] Hogan | Soft matter characterisation of whey protein powder systems[END_REF]Ovarlez & Coussot, 2007). Due to the higher water holding capacity of proteins (casein -~4 g/g, whey -~1.5 to 3.5 g/g), the higher protein materials, SMP and FFMP, have less "free" water at a given DM (Liu et al., 2018), which manifests itself as an increase in cohesion at lower DM. Similarly, as DM is further increased, the higher water binding capacity in SMP and FFMP likely results in less available water for interparticle liquid bridging, resulting in a drop in cohesion and granule formation at lower DM [START_REF] Iveson | Growth regime map for liquid-bound granules: further development and experimental validation[END_REF].

Recirculation rate and process efficiency

Achieving maximum possible superconcentration, while avoiding the cohesive phase, is key to the efficient operation of the novel process. It is evident that there exists a critical DM content for successful granulation in each of the compositions which is well related to the cohesive phase mapping (variation in power consumption) represented in Fig. 4.6. Table 4.5 highlights the maximum DM content during superconcentration and minimum DM required for granulation of different dairy streams. The cohesive phase for SMP extended over 16% DM (from 62% to 78% w/w DM), compared to ~8% for DWP and WPP (from 80 to 88% w/w DM). Therefore, the required quantity of dry powder to achieve granulation was markedly higher for SMP which ultimately affects the process performance for higher protein compositions. Recirculation rate increases if the DM content of the superconcentrated paste falls below the maximum possible, as successful granulation is linked to DM content at the end of cohesive phase. Recirculation rate is doubled for a 5% lower concentration of the paste at 80% w/w DM (Tanguy et al., 2017), which drastically reduces the powder outlet flow and can nullify benefits in energy consumption vis-à-vis SD.

Furthermore, compositions like FFMP and SMP, which need removal of more than 20% moisture in final convective drying, could potentially end up consuming similar energy consumptions as that of SD, if not more, as specific energy consumption in final convective drying steps is highest among all drying steps in conventional process (Schuck et al., 2015;[START_REF] Westergaard | Milk Powder Technology: Evaporation and Spray Drying[END_REF].

Although no energy consumption measurements can be performed at this stage, an estimate based on key principles and procedures presented in previous studies (Fig. 4.7)

was undertaken to guide future scoping (Schuck et al., 2015;Tanguy et al., 2017). Table 4.3

A B C

presents the energy estimates based on water removal in alignment with the previous studies. Theoretically, all the dairy powders can be produced at lower operational costs as compared to SD, largely due to the lower energy costs associated with superconcentration as compared to drying (Tanguy et al., 2017). Furthermore, the new process may have additional benefits: very compact installation that should come with lower cleaning solution and effluents volumes; lower capital (up to 40% based on estimates); and simpler operations (Tanguy et al., 2017). These factors in combination with energy estimates in this study, can encourage industries to test the robustness of the new process at a pilot scale. However, it must be stated that the lower DM content of SMP and FFMP superconcentrates results in a less favourable drying energy reduction as compared to DWP and WPP (18-20% reduction for SMP and FFMP; ~32% reduction for DWP and WPP). This finding, in combination with the relatively poor functional attributes associated with the lab-scale SMP and FFMP powders produced, indicate that such a process may have limitations in handling high protein/fat materials.

Influence of final drying stage on phase transitions and final powder structure and functional properties

Producing specific final powder structures (particle size, density etc.) is key to delivering the required functional properties (rehydration, flow properties, stability) in dairy powders [START_REF] Palzer | Generation of Product Structures During Drying of Food Products[END_REF]. Stability and functional properties of granules is highly influenced by the drying stage [START_REF] Person | Influence of the drying step in the steam-jet granulation process of dairy powders[END_REF]. The rapid dehydration associated with SD ensures lactose remains in a stable glassy state for SMP and FFMP. The PST process encounters longer residence time (~30 mins) in oversaturated zone, conditions which can initiate spontaneous crystallization of lactose (Vuataz, 2002). The crystallized lactose is clearly visible in SMP and FFMP powders produced in lab scale. Longer residence time during dehydration of agglomerated skim milk powders resulted in increased amount of crystallized lactose and influenced rehydration properties [START_REF] Person | Influence of the drying step in the steam-jet granulation process of dairy powders[END_REF]. Moreover, high shear granulation produces powders with dense structures and lower porosity as compared to SD [START_REF] Ji | The effects of fluidised bed and high shear mixer granulation processes on water adsorption and flow properties of milk protein isolate powder[END_REF]. Therefore, it is to be expected that powder properties such as rehydration and flowability will differ to standard SD powders, as observed in this study.

Conversely, the crystallization observed in the PST process may be a positive in the case of DWP and WPP production, having the potential to partially replace or enhance the batch crystallization process, bringing associated benefits in process efficiencies. The application of this feature is evidenced in patented Tixotherm ® process (Písecký, 2005),

where the batch crystallization step is completely avoided. Further investigation is necessary to understand impact of superconcentration on lactose crystallization and final powder stability and functionality. For example, the relatively long residence time of lactose crystals in a highly supersaturated state will likely result in the production of a large number of small crystals [START_REF] Parimaladevi | Influence of supersaturation level on the morphology of α-lactose monohydrate crystals[END_REF], which may increase the ease of lactose crystal solubilization upon rehydration.

Prospects and perspective

The investigated process demonstrated, at lab scale, an interesting low-energy, compact method to produce dairy powders. A key feature of the process is the recycling step, whereby powder from the end of the process is used to granulate. However, it is also possible to replace, or partially replace the final product with different powders, thereby effecting the granulation. Utilization of other powderssuch as powders with higher water holding capacity or drying aids can be evaluated to overcome recirculation challenges. Two powders were tested at lab scale for their feasibility, maltodextrin (typical drying aid) and a fruit fiber (high water holding capacity, high fiber ingredient) a coproduct from fruit juice industry. Maltodextrin addition reduced the powder addition rate by 40%, whereas fruit fiber addition reduced by 80%. The addition of such powders thus removes the recirculation requirement and tremendously improves the process efficiency and capacity of new approach. The low-cost technology can be a platform for further investigation of new opportunities with respect to low value dairy streams and food industry co-products such as fibers i.e. mixing streams of different compositions to generate novel ingredients which contribute to the circular economy.

The lab scale model and the protocol to measure cohesive phase development can be a valuable tool for scale up, optimisation and assessing the feasibility of applying the novel process to other dairy and food ingredients. The effectiveness of this tool will be further investigated in a pilot scale installation.

This study was performed using reconstituted powders which was the most viable option from a logistical point of view. The results are encouraging because they tracked the previous study (Tanguy et al., 2017), providing new insights into superconcentration and granulation at a lab scale. Therefore, further studies are being undertaken to test the lab scale protocol using fresh liquid ingredients as a starting material. It should also be stated the drying process utilised here (vacuum oven drying) differed significantly to the pilot/ industrial scale process (as described in section 2.3). The effect of drying on the final product properties will be assessed during large scale (100 kg.h -1 ) pilot plant trials, incorporating the findings achieved using the lab scale protocol.

Conclusion

A reliable protocol was developed to characterize the behaviour of superconcentrates of dairy ingredients. It was shown that measuring power consumption as a function of DM content can be used to identify a cohesive phase, which appeared for each of the ingredient studied. The exact onset and extent of the cohesive phase was, however, composition dependent and points to different behaviour of ingredients as a function of composition.

Mapping of the cohesive phase is particularly useful in superconcentrationgranulation based processes. In the first instance it provides information on the limits of superconcentration achievable for a given composition. This data may be inferred from flow rheology; however, viscosity increases exponentially in such analyses at DM contents between 5-20% lower than onset of the cohesive phase, which in this study was deemed to be the upper limit of superconcentration. Furthermore, the width and the endpoint of cohesive region gives useable information on the extent of powder mixing required to form discrete powder granules, which is the ultimate goal of the process. The data generated can be used to give an indication of the potential energy reduction associated with this innovative process.

Furthermore, granulated powders produced at a lab scale model were characterized and compared with standard spray-dried powders. While the powders were dried in a different manner (vacuum oven drying) to the industrial protocol, the results point to some interesting conclusions. Firstly, similar to observations by Tanguy et al. (2017), DWP and WPP produced powders with excellent rehydration and flow properties, which was in contrast to SMP and FFMP powders which both behaved poorly upon rehydration.

The identification of limits of superconcentration and granulation as a function of DM content and composition give strong indications on the potential in-process behaviour of different dairy streams. In particular, the relatively low drying energy reduction associated with SMP and FFMP in comparison to DWP and WPP, coupled with poor rehydration properties, point to potential limitations of the process to handle higher protein and/or fat containing powders. While the results presented here provide useful indications of behaviour and in the case of WPP align with other pilot-scale studies, future studies are planned to determine the transferability of the lab-based protocol to an industrial scale.

Introduction

Dehydration of perishable liquid dairy streams into stable powder formats is a widely practiced means of increasing ambient shelf life by up to 3 years. Conventional dairy powder manufacturing practice includes production of a viscous concentrate (25 to 60% w/w dry matter (DM) content depending upon composition, especially protein content) using membrane concentration and/or falling film evaporation. Subsequently, the concentrate is transformed into powder using spray drying (SD), wherein atomization of fine droplets in a hot air stream results in rapid dehydration. SD accounts for almost 50% of total energy in dairy powder plants, while the specific energy for water removal is 10 to 20 times higher than falling film evaporators (Schuck et al., 2015). Furthermore, in some cases the high operational costs associated with SD can mean the process returns little or no profit when applied to low value streams such as permeate. Thus, replacing energy intensive SD with alternative, energy efficient operations can result in greatly improve the environmental footprint and profitability of dairy powder manufacturing plants (Patil et al., 2021;Schuck et al., 2016;Tanguy et al., 2017).

Recent investigation of a superconcentration-granulation based process (poudre sans tour [PST] process) estimated energy savings at more than 30% compared to SD during production of permeate powders at a pilot scale (100 kg.h -1 feed rate). The improved performance was mainly attributed to superconcentration to 80% w/w DM (vs 60% w/w DM in conventional process) using high-shear, thin-film horizontal rotary evaporators (Tanguy et al., 2017). In another investigation, a similar agitated thin film drying technology (ATFD) was proposed for energy efficient production of dairy and food powders at a lab scale (~0.5 kg.h -1 feed rate). However, evolution of a sticky rubbery mass significantly impaired agitation while drying products consisting of lower molecular weight sugars [START_REF] Qiu | a. Agitated thin-film drying of foods[END_REF]. In contrast to the ATFD process, the PST process uses an innovative 'granulation' feature, which involves addition (recycling) of dry powder (>97% w/w DM) into superconcentrated paste (up to 80% w/w DM) in order to produce large discrete powder particles which are subsequently dried (Tanguy et al., 2017).

While both PST and ATFD processes differ in final powder production step, superconcentration in high shear conditions is a common feature for both technologies.

Achieving maximum product concentration in contact evaporators (falling film evaporators, thin film high shear evaporators) is a key to sustainable production of dairy and food powders (Patil et al., 2021;Schuck et al., 2016Schuck et al., , 2015;;Walmsley et al., 2018). However, little is known about the nature and physical behaviour of products during superconcentration, particularly, the evolution of rheological properties that restrict the superconcentration step beyond specific DM content. The end-point of superconcentration directly influences the process performance with respect to energy savings and productivity. For instance, reduction in permeate superconcentration from 80% w/w DM to 75% w/w DM, was estimated to increase the recirculation rate by a factor of two-fold for permeate powder (Tanguy, et al., 2017).

Additionally, granulation of wet pastes by addition of dry powders is a relatively new process which has not been studied for dairy systems.

The primary objective of the present investigation was to increase the understanding of dairy superconcentration and granulation processes through application of a variety of rheological (flow curves, flow stress -oscillation amplitude test, cohesionprobe-tack test)

and wet mass characterization techniques. A variety of techniques were applied to gain insights into the complete range of DM contents that will be encountered during both superconcentration and granulation, as some techniques may not be practical to handle higher DM content superconcentrated pastes.

Materials and methods

Materials

Skim-milk microfiltration permeate (MFP) at approximately 6% w/w DM was obtained by membrane filtration (0.1 µm cut-off ceramic membrane, PALL, UK) of fresh skim milk at 50°C in the Dairy Platform (STLO, Rennes, France). In order to obtain sufficient quantity of superconcentrate at the end of the concentration step in rotary evaporator, MFP was preconcentrated to approximately 30% w/w DM using a pilot scale falling film evaporator (GEA Process Engineering, France) at a feed flow rate of 70 kg.h -1 and an evaporation temperature of 60°C. Lastly, MFP concentrate was stored in a cold room (4°C) for subsequent use. No solidification of the MFP concentrate occurred during storage. Only a small quantity (less than 2%) deposited in the bottom of the bottles. The pH of the MFP concentrate was checked before each experiment and was equal to 6.08 ± 0.02.

Preparation of superconcentrates

Concentration and crystallization

The simplified experimental set-up used to approximate the superconcentrationgranulation process is described in Fig. 5.1. A lab scale rotary evaporator (Hei-VAP Value Digital, Heidolph Instruments, Schwabach, Germany) was used to continue the MFP concentration from 30 to 60% w/w DM at 60°C under ~100 mbar vacuum. Batch crystallization was then performed in a 1-liter glass beaker (diameter -11 cm), which was immersed in a temperature-controlled water bath at 30°C and over which a paddle agitator (diameter -4.5 cm) was installed for controlled stirring using an electric motor (Eurostar digital, IKA-Werke, Germany). Around 600 ml of concentrated MFP was poured in the glass beaker and agitated at 600 ± 50 rpm. Lactose crystals (100 mesh; Lactalis Ingredients, France) were seeded at time t=0 min at 0.1% w/w. The crystallization procedure was based on the previous studies of [START_REF] Gernigon | Effects of the addition of various minerals, proteins and salts of organic acids on the principal steps of α-lactose monohydrate crystallisation[END_REF] and [START_REF] Mimouni | Kinetics of lactose crystallization and crystal size as monitored by refractometry and laser light scattering: effect of proteins[END_REF]. A handheld refractometer (Atago, Tokyo, Japan) was used to measure the refractive index (expressed in°Brix) as a function of time and monitor the lactose crystallization. Measurements were made every 30 minutes during the first 6 hours, and then at the end of crystallization (overnight around 16-18 h). Particle size of the crystals was measured using laser light diffraction (Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK); saturated lactose solution (29 g.100 g -1 water) was used as a dispersing medium to avoid solubilization of lactose crystals [START_REF] Mimouni | Kinetics of lactose crystallization and crystal size as monitored by refractometry and laser light scattering: effect of proteins[END_REF]. Crystal morphology was observed under light microscopy (model BX51TF, Olympus, Japan). Lactose crystallization kinetics were derived using the method and equations described by [START_REF] Mimouni | Kinetics of lactose crystallization and crystal size as monitored by refractometry and laser light scattering: effect of proteins[END_REF].

Superconcentration

Crystallized MFP was further concentrated up to about 85% w/w DM in a temperature-controlled mixer (Thermomix, TM5, Vorwerk, Germany) under high shear conditions at 60 ± 2°C. The mixer bowl had an internal diameter of ~15 cm and the 4-blade knife agitator had a diameter of 13.5 cm. High shear was achieved by operating at a high agitation speed (500 rpm). The temperature of 60 ± 2°C was in line with temperatures used during concentration at industrial scale while the speed of 500 rpm was the highest speed that could be employed while avoiding product splashing. During concentration, samples were taken at regular intervals for further characterization. MFP samples with DM higher than 85% w/w, that could no longer be processed in mixer, were obtained by drying superconcentrate (~85% w/w) at 60°C in a hot air oven for 30, 60 and 90 minutes. Drying time was varied to produce a range of samples with increasing DM content. The resulting MFP samples were used for characterization of wet mass flow properties using shear cell measurement as described in following section 5.2.3.4.

Characterization of superconcentrates

Dry matter content

The DM contents of superconcentrates were measured by oven method (105°C, 7 h) according to [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF]. DM content was determined by weight loss using 3-5 g product samples carefully mixed with sand in a capsule.

Rheological properties

Flow curves, flow stress and cohesion were used to characterize the rheological behaviour during superconcentration. All measurements were performed in a stresscontrolled rheometer (DHR2, TA Instruments, France) using a flat parallel plate (50 mm diameter) geometry with a constant gap of 1 mm.

For flow curves, shear rate ranged from 0.01 to 1000 s -1 to obtain 5 measuring points at a constant temperature (25°C). Product discharge and slip were more pronounced at higher temperature and higher shear rates depending on the geometry and surface roughness [START_REF] Sharma | Measurement techniques for steady shear viscosity of Mozzarella-type cheeses at high shear rates and high temperature[END_REF], therefore flow curves were characterized at 25°C. Serrated (hatch-plate) plate (40 mm diameter) was used for higher DM samples to avoid the product slipping. Apparent viscosity at a shear rate of 100 s -1 was presented as a function of DM content. Oscillation amplitude sweep was performed by varying the stress from 0.1 to 1000

Pa logarithmically, at a constant frequency and temperature of 1 Hz and 60°C respectively.

Paraffin oil was used to avoid evaporation from sample while performing measurements.

Stress corresponding to cross over point of the storage modulus (G') and loss modulus (G'') curves was determined, which is described as flow point or flow stress [START_REF] Mezger | The Rheology Handbook: For Users of Rotational and Oscillatory Rheometers[END_REF][START_REF] Zhu | Extrusion-based 3D printing of food pastes: Correlating rheological properties with printing behaviour[END_REF], using the rheometer software (TA Instruments Trios V5.0.0.44616). Cohesion was determined by measuring axial force required to separate the top plate from the bottom plate of the rheometer (similar to Probe Tack test described by Malvern Instruments limited, 2021).

Sample was loaded on the bottom plate and the top plate was lowered to a gap of 1 mm after which excess product was carefully trimmed. Prior to analysis the sample was pre-sheared at 50 s -1 for 1 min followed by equilibration at rest for 1 min. During analysis, the top plate was lifted at a constant velocity of 1 mm.s -1 . The axial force experienced by the top plate is recorded over time until complete separation of both plates. Cohesion is indicated by maximum force (N) required while lifting the top plate. Measurements were performed at 25°C. Higher temperature was avoided due to possibility of evaporation modifying product properties, especially at higher DM. Application of paraffin oil to avoid evaporation was not possible for this technique.

Specific power consumption during superconcentration

The electric current amperage (A), reflecting the electric power consumption of the temperature-controlled mixer, was used for gaining more information about the evolution of the product during superconcentration. Power consumption was measured at 500 rpm using an energy meter (Energy Logger 4000 FR, Voltcraft, Germany). In parallel, the mass of superconcentrate in the mixer was weighed before each sampling for rheology characterization. From both measurements, the specific power consumption, i.e. electrical power consumption per kg of product, was determined at all stages of superconcentration.

Additionally, a disruption (breakdown) in continuous processing of superconcentrated paste was simulated in order to understand the magnitude of potential changes in paste rheological properties arising from said disruption. This information could be crucial in designing the torque requirements of the equipment to handle breakdowns in operations at pilot-scale/industrial scale. Superconcentrated pastes were stored in a closed container under ambient conditions (25°C, 40% RH) for one hour. Flow curves, flow stress and power consumption measurements were again performed to evaluate changes in the superconcentrate behaviour. Subsequently, the cohesiveness and ffc values were derived. Detailed description of the ringshear tester equipment and its operation can be found in recent works [START_REF] Schulze | Flow Properties of Powders and Bulk Solids[END_REF][START_REF] Tobin | Cohesiveness and flowability of particulated solid and semi-solid food systems[END_REF].

Granulation and characterization of granules

Granulation was carried out by fragmenting the superconcentrated paste (82% w/w DM) with addition of sweet whey powder at 97% w/w DM (Lactalis Ingredients, France) in the temperature-controlled mixer. The powder was added gradually in the superconcentrated paste at 60°C under constant agitation of 500 rpm, according to Patil et. al. (2021). The end point of powder addition was determined by following the power consumption and the formation of visible granules in the mixer. The recirculation rate expresses the ratio between the mass of dry powder added and the mass of the superconcentrated paste. The DM content of granules was determined according to the same method described for superconcentrates.

Experimental and analytical repetition

The experiments for concentration, crystallisation, superconcentration and granulation were performed at least in duplicate. The analysis for characterisation of the superconcentrates (section 5.2.3) were performed in duplicate for each run, except for the flow properties (section 5. 2.3.4) where it was not possible to perform duplicate measurement due to limited quantity of product.

Results and discussion

Characterization of MFP superconcentrates

Lactose crystallization

Controlled lactose crystallization of MFP concentrates at around 60% w/w DM was performed with initial seeding of 0.1% lactose. Close to 70% lactose was crystallized at the end of crystallization (~16 h), while more than 60% lactose was crystallized within first 4 hours of crystallization (Fig. 5.2a). The typical tomahawk shape was observed for crystallized lactose with an average size around 50 m and monomodal particle size distribution (Fig.

5.2b

). The results with respect to the extent of lactose crystallization, the shape and size of crystals were in agreement with previous published studies [START_REF] Mimouni | Kinetics of lactose crystallization and crystal size as monitored by refractometry and laser light scattering: effect of proteins[END_REF][START_REF] Simeão | Lactose crystallisation in concentrated whey: The influence of vat type[END_REF].

Rheological properties of superconcentrates

The flow behaviour of the superconcentrates (DM>60% w/w) was analyzed using the rheometer software (TA Instruments Trios V5.0.0.44616). The Herschell-Bulkley model well described MFP superconcentrate flow behaviour (R 2 = 0.99). Table 5.1 presents the yield stress, consistency coefficient and flow behaviour index for MFP superconcentrates (60%<DM<78% w/w). Modelling of flow curves could not be performed for DM higher than 78% w/w due to wall slippage and product discharge. Similar challenges, i.e. wall slippage at higher shear rates, were reported by [START_REF] Sharma | Measurement techniques for steady shear viscosity of Mozzarella-type cheeses at high shear rates and high temperature[END_REF] during characterisation of a different product (cheese) using flow curves. As DM content increased from 61,1 to 70% w/w DM, MFP superconcentrates showed shear thinning behaviour at higher DM i.e. flow behaviour index decreased from 1.0 at 61.1% to < 0.7 at DM > 64.7% w/w. The apparent viscosity was strongly influenced by concentration (Fig. 5.3). Apparent viscosity as a function of DM was correlated by an exponential model (R 2 = 0.98). Additionally, the agitator power consumption, expressed in current (A), demonstrated a robust linear correlation (R 2 = 0.99) with apparent viscosity (Fig. 5.4). A similar trend (R 2 = 0.93) was observed between power consumption and viscosity during pumping of a dairy concentrate [START_REF] Schuck | Pump amperage: A new method for monitoring viscosity of dairy concentrates before spray drying[END_REF].

These results are consistent with previous studies on dairy products which reported increased shear-thinning behaviour at increasing concentrations [START_REF] Morison | Viscosity and Non-Newtonian Behaviour of Concentrated Milk and Cream[END_REF], exponential rise in apparent viscosity with increasing DM contents, and a substantial rise in viscosity corresponding to a small increase in DM at higher concentration levels [START_REF] Snoeren | The viscosity of skim-milk concentrate[END_REF]Vélez-Ruiz and Barbosa-Cánovas, 2000). The typical shear thinning behaviour confirms the advantage of high shear processes (e.g. PST) to produce superconcentrated dairy streams, where viscosity increase is limited by the rigorous mechanical shear. This enables higher water removal in energy efficient contact evaporators, which is a challenge for conventional falling film evaporators (Patil et al., 2021;Tanguy et al., 2017). ' and G'' (Fig. 5.5). This reflects another limitation while processing superconcentrates, namely higher resistance for startups after sudden breakdown during operations, which is substantially amplified at higher DM contents.

A steep increase (4.5X) in stress to initiate flow was observed for a small change in DM content from 78.3% to 81.7% w/w DM. This dramatic increase in flow stress indicating marked evolution in rheological properties points to transformation of suspensions (fluid-like state) into capillary state (strong gel-like state), as observed by [START_REF] Koos | Capillary suspensions: Particle networks formed through the capillary force[END_REF].

a.

b. Values were normalized on Y-axis with respect to the value corresponding to DM on x-axis.

Furthermore, the effect of holding the superconcentrated paste under static conditions was evaluated. The product at around 78% DM and 60°C was left unagitated for 60 min.

Initial product behaviour was like a smooth flowing paste with 0.32 A power consumption. At the end of 60 min, the product appeared like a very thick paste, with an immense resistance to initial flow, increases the amperage requirement from 0.32 to 0.85 A. However, within short time (1-2 min) of continuous shearing, the thick paste transformed into easy flowing paste resembling the consistency before stand-by, with slight increase in power consumption close to 0.40 A, which can also be due to lower temperature or slightly increased DM content. Substantial increase (up to 4X) in the yield stress and flow stress was observed due to aging. This information can be critical in designing the equipment, as an abrupt breakdown can lead to substantially higher torque requirements to re-initiate the production.

Shear cell measurements

The behaviour of superconcentrated MFP pastes were typified by cohesiveness and ffc using a ring shear tester. Cohesiveness represents internal binding strength of the material, whereas ffc indicates if the material has a good or poor flow behaviour. The ffc is a function of consolidation stress and unconfined yield stress. A non-flowing (hardened) state is attributed to ffc <1, whereas higher ffc value indicate greater flowability [START_REF] Schulze | Flow Properties of Powders and Bulk Solids[END_REF].

Flow properties of superconcentrated MFP pastes were highly dependent on DM (Fig. 5.8).

As the DM content increased from 81% to 93% w/w DM, cohesiveness and flowability values evolved virtually in opposite direction, e.g. when cohesiveness increases, ffc decreases and vice versa. Markedly lower cohesiveness was observed at 81% and 89% w/w DM, in contrast to 82% to 86% w/w DM, wherein excessive cohesiveness (3X to 6X) was recorded. Similarly, within the DM range of 82% to 86% w/w DM, ffc values were less than 1, while higher ffc values were noted for 81% and 89% w/w DM. Therefore, a highly cohesive non-flowing hardened state was located within 82% to 86% w/w DM. The clear distinction with respect to the product flowability, indicated by ffc, marked the limiting DM for the superconcentration of MFP.

Inversion of ffc and cohesiveness values has previously been reported to represent the transition from cohesive wet powder regime to a concentrated suspension as DM content was progressively reduced [START_REF] Althaus | Characterization of wet powder flowability by shear cell measurements and compaction curves[END_REF][START_REF] Tobin | Cohesiveness and flowability of particulated solid and semi-solid food systems[END_REF]. In contrast this study progressively increased DM during superconcentration, however, a similar trend was observed at around 81% w/w DM, which can be indicative of a transition from concentrated suspension to cohesive wet powder regime. Moreover, while cohesiveness demonstrated more or less constant values within 89%<DM<96% w/w, an abrupt increase was observed on transition from 81% to 82% w/w. This may indicate a transition from an overwet state (i.e.>80% saturation corresponding to capillary state) as was observed by [START_REF] Althaus | Characterization of wet powder flowability by shear cell measurements and compaction curves[END_REF] albeit for a different composition. Also, the yield locus of MFP as it approached 81% w/w DM was not stress-dependent, which is encountered in a state very close to that of slurry, wherein the applied normal stress does not have an influence on the forces between particles (Schulze, 2021). Overall, multiple indications of transition into capillary state were observed that can be related to onset of highly cohesive phase, based on rheology and shear cell data, which consistently pointed to 81% w/w DM in the case of MFP.

Effect of product behaviour as a function of DM on in-process parameter (motor amperage)

Fig. 5.9 depicts variation in power consumption as a function of DM. Initially, a gradual rise in power consumption was observed as the concentration increased. A dramatic rise can be seen around 81% w/w DM, with a peak at 86% w/w DM and a subsequent sharp fall at a DM greater than 88% w/w.

Remarkably, the overall trend in power consumption was closely related to the product properties determined using rheometer as well as the flow properties measured by ring-shear tester. A close relationship was evidenced between the product characteristics and process parameter (power consumption). Thus, the monitoring of power consumption can be a reliable tool to effectively control the process and determine the highly cohesive regime zone, in addition to the laboratory tools such as rheological and shear cell measurements to characterize the superconcentrated product. In the case of MFP, the highly cohesive regime was observed within the range 81% < DM < 88%, which needs to be overcome for efficient operation. The different techniques used in this study, consistently point out a dramatic evolution as the DM exceeds 81% w/w for MFP; this is likely related to transition from over-wet slurry into a capillary state, as discussed earlier in section 3.1.2 and 3.2. In order to avoid the risk of equipment damage due to excessive resistance in highly cohesive phase, superconcentration to 81% w/w can be considered optimum for MFP. This will be further validated at a pilot scale (100 kg.h -1 ). The highly cohesive phase is overcome in the PST process by recirculation of dry powders in the granulation step.

Granulation and characterization of granules

Granulation of the superconcentrated paste (82% w/w DM) by addition of dry powder was evaluated. The recirculation rate of dry powder for effective granulation was 1.0 ± 0.1 fraction (w/w) of paste. While granulation was successful there was nonetheless elongated and irregular shaped ("dough-like") pieces present (~3% of material). DM content of the dough like pieces was found to be < 86.5% w/w, whereas for discrete granules it was > 88.5% w/w. This illustrates that even within a successfully granulated powder a critical DM content must be reached (>88% w/w DM for MFP), to obtain granules. This is consistent with previous study that suggested >88% w/w DM for granulation of permeate (Tanguy et al., 2017).

Energy saving potential of the superconcentration-granulation based powder manufacturing process is based on the extent of superconcentration, which in turn sets the required dry powder recirculation rate. Extent of superconcentration is limited by onset of cohesive phase, while quantity of dry powder recirculation is defined by DM content at the end of cohesive phase. The information with respect to onset as well as extent of cohesive phase (DM contents at onset and end), is crucial for screening suitability of products for the new approach. For example, a product with earlier onset and longer cohesive phase, will have lower superconcentration potential and require higher dry powder recirculation rate, which can result in poor process performance.

Conclusion

This study characterised dairy superconcentrates using rheological and shear-cell Overall, this study demonstrated that there is a critical DM content that imposes practical limitations on superconcentration as well as achieving discrete granules formation.

Efficient process performance is dependent on achieving maximum superconcentration with minimum recirculation rate for granulation. The techniques presented in this study provide useful insights into the evolution of product behaviour, particularly, identification of a cohesive non-flowing state, with increasing DM content, while reliably demarcating the boundaries for superconcentration and granulation. In the case of MFP, the superconcentration limit was ~82% w/w DM; the target for dry powder addition required to overcome the cohesive phase and achieve effective granulation was greater than 89% w/w DM. Additionally, the protocol of monitoring the power consumption can be a useful inline tool to control the new process. The techniques can be utilised for characterisation of other dairy and food products to understand the feasibility of applying the PST process.

CHAPTER 6:

GRANULATION BEHAVIOUR OF SUPERCONCENTRATED HIGH LACTOSE DAIRY PASTES

In this chapter, the granulation of high lactose dairy superconcentrates was investigated. This study demonstrated decisive role of moisture content on the granulation behaviour of whey permeate (WP) and demineralised whey (DW). A significant difference between the granulation behaviour of WP and DW was observed, likely due to the higher cohesiveness exhibited by the DW. Furthermore, use of WPI as a recirculation powder resulted in more efficient granulation (reduced energy, finer granules and lower recirculation rate) of the superconcentrated WP paste. This was linked to the higher water holding capacity of WPI. Overall, this study sheds light on the granulation step of the novel process.

In particular, high cohesiveness of superconcentrated pastes, which may be related to protein content, was shown to negatively impact granulation. Conversely, addition of protein to the granulating powder was shown to have positive effects, which has potential for formulated products, whereby different powders can be mixed with superconcentrates to prepare products with targeted compositions.

The main aim of this chapter were to:

-evaluate the granulation behaviour of the superconcentrated paste of two dairy side-streams, i.e. whey permeate and demineralised whey -determine the influence of addition of whey protein isolate in the back-mix fraction on the granulation of superconcentrated paste of whey permeate

Introduction

A novel superconcentration-granulation based process for drying of high lactose dairy streams has been proposed which has the potential to reduce energy cost while maintaining key powder functional properties (Patil et al., 2021;Tanguy et al., 2017). It has been shown

that process performance was optimised by ensuring the maximum extent of superconcentration was achieved and that sufficient powder was recirculated to provide effective granulation (Patil et al., 2021;Tanguy et al., 2017). [START_REF] Patil | Characterization of the superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders[END_REF] demonstrated the robustness of superconcentrate characterisation technique, which measured agitator current consumption to predict crucial process parameters. Dairy composition markedly influenced the extent of superconcentration, with higher lactose:protein ratios allowing for a greater extent of superconcentration. For instance, whey permeate and demineralised whey can be superconcentrated to ~80% w/w DM, whereas skim-milk was limited to DM < 62% w/w (Patil et al., 2021). Consequently, this implied that lactose-rich compositions were better suited to the proposed process due to superconcentration challenges associated with protein-rich materials.

These results point to the importance of understanding the limits of superconcentration in the novel process; however, granulation, which may be assumed to have an equally important role, has not yet been studied. Indeed, there is great flexibility within the granulation process, whereby physical properties of the granulated powder may play a role. There also exists the possibility to alter the composition of the granulating powder. It is vital to build insights into the granulation step of relevant dairy superconcentrate in order to assist in the process development and optimisation initiatives associated with the granulation of dairy superconcentrates.

Accordingly, the purpose of this study was two-fold:

i. to evaluate the granulation behaviour of superconcentrated whey permeate and demineralised whey ii.

to determine the influence of addition of whey protein isolate to the back-mix fraction during granulation of superconcentrated whey permeate

Materials and methods:

Materials

Whey permeate (WP), demineralised whey (DW), and whey protein isolate (WPI), were acquired from commercial manufacturers. Table 6.1 presents the specified paste of WP was prepared as described earlier. Then, the granulation was performed using three variablesi) 195 g of WP paste was granulated by 200 g of WPI powder; ii) 195 g of WP paste was granulated by using a previously prepared dry blend WPI10, (i.e. 180 g WP and 20 g WPI) and iii) 195 g of superconcentrated paste was granulated by using a previously prepared dry blend WPI20, (i.e. 160 g WP and 40 g WPI). The other granulating conditions (temperature, time and speed) were consistent throughout the experiments.

Finally, in order to investigate the effect of reduced powder addition on granulation, the superconcentrated WP paste (~195 g) was granulated at two other conditions, i.e. 100 g and 80 g of WPI20 blend which represent 1:0.5 and 1:0. The DM contents of superconcentrates and granules were measured by weight loss method. Approximately, 3-5 g of product samples in an aluminum cup were placed in an oven at 102°C for at least 5 h [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF].

Particle size of granules

The granules were sifted over a set of sieves (pan, 0.25, 0.5, 1, 2 and 4 mm) using a vibratory sieve shaker (AS200, Retsch, Germany). The sieving was performed for 2 min at an amplitude of 1.25 mm. These conditions were established after initial screening tests. The different fractions on the sieve were collected for moisture measurements.

Protein content of powder

The nitrogen content of powder was determined by combustion (LECO FP628 Protein analyser, LECO Corp., St. Joseph, MI, USA). The protein content was obtained from the nitrogen content, using a conversion factor of 6.38.

Resistance to flow measurement of superconcentrates and granules

The dairy products prepared from WP, WPI10, WPI20 and WPI powders were characterised by measuring their resistance to flow at different DM contents. The power consumption (electric current, A) of the mixer was measured at a constant speed (500 rpm).

Demineralised water was progressively added to powder in the mixer to vary the DM content of the resulting product, and the corresponding power consumption was recorded. This technique enabled mapping the variation in power consumption as a function of DM, consequently the determination of crucial parameters for the superconcentration-granulation process (Patil et al., 2021). Additionally, the resistance of bulk granules at the end of granulation was recorded at 500 rpm. This resistance correlates well with the cohesiveness of the product at the end of granulation [START_REF] Patil | Characterization of the superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders[END_REF].

Rehydration and flowability of granulated powders

The rehydration properties of the granulated and dried powders were characterized by measuring wettability and solubility index; which were determined according to [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF]. Flowability is represented as time in seconds required for sample powder to leave a rotary drum through a slit (GEA-Niro, 2006).

Experimental and analytical repetition

The granulation experiments (section 6.2.2) were performed in triplicate. The analysis for the physico-chemical and functional properties (section 6.2.3) were performed in duplicate.

Results and discussion

6.3.1 Granulation of whey permeate (WP) and demineralised whey (DW)

Initially, the granulation of two high lactose dairy side-streams (WP and DW) at similar granulating conditions (back-mix fraction 1:1) was evaluated (Table 6.2, Exp # 2 and 5). In contrast to a previous study (Chapter 4) which showed higher lactose dairy streams had more or less similar limits of cohesive phase and minimum DM for granulation (Patil et al., 2021b), a significant difference between the granulation behaviour of WP and DW pastes was detected. The energy required for granulation of the DW was markedly higher (+55%)

than that for WP. Additionally, the DW granules were almost two-fold larger and only 9% passed through a 2 mm sieve as compared to 65% for the WP granules. The product recovery from the mixer (granules) after the granulation was ~16% higher for WP in comparison to DW (Table 6.3). Clearly, a higher degree of fragmentation of the superconcentrated paste was evident for WP compared to the DW. Additionally, the effect of recirculation rate (back-mix fraction) of powder on the efficacy of granulation was evaluated at three levels of superconcentrate:powder (1:0.8, 1:1 or 1:1.2; Table 6.2, Exp # 1 to 6). This was essential to generate precise data regarding the optimum recirculation rate, and in turn the DM content of resulting product (i.e. bulk granules), beyond which effective granulation can be achieved. Different size fractions of granules were segregated using a vibratory sieve. The granules were classified into three typesi. Large dough like pieces which were retained on a sieve of size 4 mm; ii. Granules (0.25 < size < 4 mm) and iii. Native particles passing through the sieve size 0.25 mm, which were taken to be the fine powder particles and did not participate in the granulation. The efficacy of granulation was related to the final DM content of the granules as shown in Fig. 6.1. The DM content of the granules is a function of the back-mix ratio, with lower quantities of powder reducing in lower mass fraction. Under these conditions (1:0.8) a large quantity of dough-like materials (~70% of total mass) was observed in WP granules. Increasing the back-mix ratio from 1:1 to 1:1.2 resulted in a significant reduction of dough like materials (<17%). In terms of DM content, this is consistent with previous studies which reported DM > 88% w/w was essential for effective granulation (Patil et al., 2021b, Tanguy, et al., 2017).

However, the results here suggest that increasing DM content can have a significant beneficial effect i.e. 0.5% increase in DM resulted in ~57% reduction in oversized granules. In contrast, a back-mix ratio of 1:0.8 was not sufficient to granulate DW superconcentrates. For a back-mix ratio of 1:1, about 60% of DW granules were considered oversized. This is about 2-fold higher than in WP granules manufactured under similar conditions and indicates that a higher recirculation rate of powder should be employed for effective granulation of DW.

Furthermore, the mass median diameter of the granules obtained was well correlated (R 2 >0.9) to the DM content of the bulk granules (Fig. 6.2). An inverse relationship is evident, i.e. the higher the DM, the finer the particle size of the granules. This trend was consistent with the results of a similar granulation process, albeit a different product (wheat flour) [START_REF] Rondet | Agglomeration of wheat powders by a novel reverse wet agglomeration process[END_REF]. This reinforces findings from previous studies (Chapter 4 and 5) where control of DM content, and in turn recirculation rate of the dry powder, was essential [START_REF] Patil | Characterization of the superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders[END_REF]Tanguy et al., 2017).

Cohesiveness of bulk granules

The cohesiveness of the bulk product obtained at the end of granulation (i.e. end of addition of dry powders into superconcentrated paste) was measured by the power consumption of the mixer. A robust correlation was demonstrated between the power consumption of mixer and the products cohesiveness [START_REF] Patil | Characterization of the superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders[END_REF]. Cohesiveness was highly influenced by the DM content of bulk product at the end of granulation, as well as product composition (Fig. 6.3). Regardless of composition, the cohesiveness demonstrated an inverse relationship with the DM content, i.e. cohesiveness decreased with increasing DM. This trend was consistent with previous studies that investigated high shear granulation for different products [START_REF] Patil | Characterization of the superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders[END_REF][START_REF] Rondet | Agglomeration of wheat powders by a novel reverse wet agglomeration process[END_REF]. Furthermore, DW showed markedly higher cohesiveness than WP at all the DM content observed (Fig. 6.3).

Figure 6.3. Cohesiveness, expressed as electric current (A), as a function of dry matter of

bulk product at the end of the granulation of whey permeate (triangles) (exp #1 to 3 in Table 6.2) and demineralised whey (open diamonds) (exp #4 to 6 in Table 6.2). Error bars represent std. deviation, n=3.

Additionally, a linear relationship (R 2 >0.8) was found between mass median particle size of granules and the cohesiveness of the bulk granules (Fig. 6.4). In these lab-scale granulation experiments, achieving temperature control in the mixer was a challenge which limits investigation of the effect of temperature [START_REF] Hoornaert | Agglomeration behaviour of powders in a Lödige mixer granulator[END_REF]. In future studies, rheometers equipped with precise temperature control and geometries should be applied to investigate the effect of temperature on cohesiveness of dairy ingredients at DM content relevant to granulation.

Influence of the addition of whey protein isolate powder on granulation behaviour of superconcentrated WP paste

A major challenge to the implementation of novel superconcentration-granulation is to control and, if possible, reduce the recirculation rates of powders. Therefore, this section evaluates alternative routes to overcome the challenge of high recirculation rates.

The effect of complete or partial replacement of WP powder with WPI on the granulation of superconcentrated WP paste (Table 6.2, Exp # 7-11) is presented in Table 6.4. Addition of WPI instead of WP resulted in up to three times smaller granules size (mass median diameter 0.67 mm), while the energy required for granulation was reduced by half.

Partial replacement of WPI into WP powder (realised by pre-blends -WPI10 and WPI20)

showed improved performance with increased WPI content in the blend. For instance, the WPI10 and WPI20 powders resulted in reduced particle size by 27% and 43% along with decrease in energy by 7% and 14% respectively (Table 6.4). Clearly, the fragmentation of superconcentrated paste was markedly improved with the addition of WPI powder. The increased levels of WPI in the recirculation fraction resulted in elevated fragmentation performance during granulation.

The effectiveness of WPI was noticeable during granulation, which can be attributed to its high water absorption capacity estimated to ~1.5 to 3.5 g.g -1 of protein (Liu et al., 2018). Fig. 6.5 presents the distribution of particle sizes of granules and their water contents for the various blends of WPI and WP. The fine fractions (native particles <0.25 mm) in the case of WPI addition show unusual trend with highest moisture content (~12% w/w) among all granule size fractions obtained. The protein content of these fine fractions was found to be 88.2% dry basis, in contrast to the bulk average of ~50%, which confirms the largest contribution of WPI. This validates the hypothesis that high water holding ingredients deprive the superconcentrated paste of free water [START_REF] Iveson | Growth regime map for liquid-bound granules: further development and experimental validation[END_REF]Liu et al., 2018;Patil et al., 2021). [START_REF] Iveson | Growth regime map for liquid-bound granules: further development and experimental validation[END_REF] observed that bound water does not play a role in wet mass capillary cohesive systems. These observations can be substantiated from the variation in power consumption as a function of DM for WPI, blends of WPI and WP (WPI10 and WPI20)

and WP (Fig. 6.6). The increasing protein content shifts the cohesive phase towards lower DM contents (i.e. higher moisture requirements), as well as reducing the DM required for effective granulation. For instance, the DM for effective granulation was 86.5%, 84.2% and 67.8% (w/w) for WPI10, WPI20 and WPI, respectively. Consequently, the fragmentation of WP paste into discrete particles with finer size can be predicted when using higher protein back-mix fractions. This is also consistent with the inverse relationship between granules mass median size and the bulk DM content (Fig. 6.2), because the further away the DM of bulk product moves from the DM required for effective granulation, the finer the granule size observed. Additionally, a robust correlation was found between the DM required for effective granulation and the protein content for WPI and WP systems (Fig. 6.7). [START_REF] Chitu | Rheology, granule growth and granule strength: Application to the wet granulation of lactose-MCC mixtures[END_REF] found a similar correlation in the optimum liquid to solid ratio required for granulation in a binary mixture of lactose and microcrystalline cellulose in high shear granulation studies. WP granulated by WPI10 1.39 ± 0.01 77.0 ± 1.0 95.0 ± 0.5 14.9 ± 0.4

Furthermore, the feasibility of reducing the dry powder addition rate for granulation was evaluated with the use of WPI20 powder blend. The superconcentrated WP paste at 82.20% w/w DM, was granulated by adding either 100 g or 80 g of WPI20, which corresponds to 1:0.5 and 1:0.4 (superconcentrate:powder) back-mix fraction, respectively (Table 6.2; Exp # 10-11). At 1:0.5 back-mix fraction, the WP superconcentrate was effectively granulated, whereas the 1:0.4 back-mix fraction failed to granulate the mix and led to a thick and cohesive paste with higher resistance. This difference can be anticipated from the correlation between protein and minimum DM for granulation (Fig. 6.7). For instance, at the back-mix fraction of 1:0.5 and 1:0.4, the protein level in the resulting product is 9.2% and 8.3% respectively. From the correlation between protein content and DM for effective granulation, the DM of bulk product required for effective granulation was calculated as 87.1% and 87.3% (w/w) for 1:0.5 and 1:0.4 back-mix fractions respectively. The actual DM measured of the resulting products after powder addition for the back-mix fraction of 1:0.5 and 1:0.4 were 87.3% and 86.5% (w/w) respectively. Therefore, the minimum DM for granulation was not reached at the lower recirculation rate of 1:0.4 (actual 86.5% < required 87.3% w/w DM), in contrast to the recirculation rate of 1:0.5. The two end-points plotted on the typical cohesive phase map represent the process conditions (Fig. 6.8), which are consistent with the physical observations as well as the protein-effective DM for granulation model. From the process performance perspective, reducing the back-mix from 1:1 to 1:0.5 markedly increases the productivity (up to 35%) considering the line capacity is limited by the granulation equipment. Moreover, and from the product perspective, this innovative approach presents an opportunity to formulate relevant value-added products based on low value dairy co-product streams, in a cost-effective way. For instance, permeate powders find application in a wide range of food products (bakery, confectionery, culinary and dairy), primarily as a cost-effective bulking agent, in addition to feed applications. In harmony with target product categories, combining appropriate ingredients (e.g. high water-holding capacity fibres from side streams or proteins etc.) into superconcentrated permeate during granulation can offer benefits to promote circular food systems. Another product proposal to evaluate could be an infant formula (IF) base that satisfies both the requirements, i.e.

protein:lactose and casein:whey protein proportion of the IF formulation. This can be based on superconcentration of demineralised whey into a paste, which would be then granulated by adding a blend of skim milk and lactose powder or milk protein concentrate (MPC) and lactose powder. The potential benefit would be simplified handling of ingredients for IF manufacturers working solely on reconstitution of powders.

Granulated powder properties

The functional properties of powders obtained after granulation of superconcentrated WP paste with WP (recirculation rate 1:1, superconcentrate:powder) and WPI20

(recirculation rate 1:0.5, superconcentrate:powder) were compared. The solubility index for WP and WPI20 granulated powders was 99.9% and 99.8%. The WP granules had an immediate wettability, whereas the wettability of WPI20 was 7.7 s. The drum flowability of the granulated powders was comparable at 12 s for WP and 11 s for WPI20. Overall, the use of WPI to reduce the recirculation rate of powders into superconcentrated WP pastes did not compromise the powder functionality.

Conclusion

This study advances the understanding of the granulation behaviour of two high lactose dairy side-streams (WP and DW). Significant differences in granulation behaviour of WP and DW were observed, as characterised by energy consumed during granulation and particle size of resulting granules. Irrespective of product composition, DM of the granulated systems highly influenced the cohesiveness, mass median particle size and energy requirement, which confirmed the significance of DM in optimising the granulation process.

The granulation of DW paste required markedly higher energy and yielded larger particle size granules as compared to WP paste. This shows that DW possess higher resistance for granulation, that can be related to the higher protein:lactose ratio than WP.

Furthermore, the granulation behaviour of WP paste can be substantially improved by the addition of WPI or blends of WPI and WP. Consequently, recirculation rate can be controlled and even reduced through addition of even small quantities of high protein powder. This also provides significant opportunities to utilise the technology as a means of manufacturing formulated products with targeted composition. The insights provided into the granulation step, particularly use of relevant ingredients to manipulate granulation behaviour, can be leveraged to produce value-added products that are based on high lactose dairy side streams in a cost effective and energy efficient manner.

Granulation process

The granulation process was performed as explained in previous chapter 6 (section 6.2.2). Initially, superconcentrated DW paste at around 82% w/w was prepared in the temperature-controlled mixer at 50°C. The resulting demineralised whey paste was fragmented into discrete granules by addition of a blend of either SMP and Lactose (IF 1) or MPC and Lactose (IF 2) (Table 7.1), in such a way that the ratio of lactose:protein (7.2:1.3) and casein:whey protein (0.4:0.6) was kept constant in resulting granules, which was in accordance with stage 1 of infant formula composition. The powder functional properties were determined as explained in Chapter 6 (section 6.2.3). The experiments were performed in duplicate. lower energy for granulation as compared to the addition of IF 1. The higher resistance to fragmentation of IF 1 was further manifested by a markedly higher cohesiveness than the granules of IF 2. Despite similar composition of both with respect to lactose:protein ratio and casein:whey protein ratio, substantial difference in their granulation behaviour was observed.

Particularly, the use of MPC (in IF 2) resulted in improved granulation behaviour of superconcentrated DW paste. This may be as result of the high protein content of MPC, 7 SMP, skim milk powder; MPC, milk protein concentrate, IF, Infant formula which acted to absorb water and improve granulation efficiency in a similar manner to that observed for WPI in Chapter 6. However, for this to be true, this assumes that MPC-lactose has greater water binding ability compared to a mix of SMP-lactose with the same overall composition. Another possibility is that the greater proportion of lactose crystals present in IF 2 may affect the granulation process. In either case, the results presented here are interesting and should be further investigated. Overall, the granulation was achieved using a back-mix fraction of 1:0.6 (superconcentrate:dry powder) in both the IF 1 and IF 2, in contrast to the excessive backmix fraction of 1:1 required for granulating DW paste with DW powder. This is in line with the results of Chapter 6, which found that WPI markedly reduced the powder required for granulation. Indeed, the results presented in this Chapter may be reflective of the comparatively higher water binding capacity of casein as compared to whey proteins.

Granulated powder properties

The functional properties of the granulated IF 1 and IF 2 were evaluated (Table 7.3).

The solubility index for IF 1 was 98.0%, whereas a lower solubility index of 95.4% was observed for IF 2, which can be explained by the presence of MPC which typically exhibits poor solubility [START_REF] Burrington | The development and application of sticky-point models to spray drying processes for the manufacturing of nutritional powder products and infant formulas[END_REF][START_REF] Bot | Solubility enhancement of milk protein isolate by sodium caseinate addition: Comparison between wet-and dryblending approaches[END_REF][START_REF] Meena | Milk protein concentrates: opportunities and challenges[END_REF][START_REF] Schuck | Rehydration of casein powders: effects of added mineral salts and salt addition methods on water transfer[END_REF][START_REF] Sikand | Solubility of commercial milk protein concentrates and milk protein isolates[END_REF]. The drum flowability of both the granulated powders (Table 7.3) was markedly higher than the standard spray-dried powders, which was consistent with observations in the previous study (chapter 4).

Conclusion

This study briefly presented the potential of superconcentration-granulation process to manufacture value added ingredients by splitting the two operations: a. applying highshear superconcentration process to compatible streams (e.g. demineralised whey) to realise maximum DM content; and b. reformulation during granulation of superconcentrated paste by addition of other ingredients that make it possible to match with the composition target and favour efficient granulation (i.e. lower back-mix fraction), while simultaneously producing value added relevant ingredients. The superconcentrated DW paste was effectively granulated at a lower back-mix fraction of 1:0.6. Furthermore, the nature of the ingredients used to granulate was also a significant factor, with MPC-lactose blends having more favourable granulation performance compared to SMP-lactose blends.

The resulting granulated powders had specific compositions (lactose:protein and casein:protein) which can potentially find application in infant formula manufacture. However, in terms of physical properties, the increased granulation efficiency of MPC-lactose blends may have a negative result on solubility of the resultant product. Overall, a cost-effective energy-efficient approach was presented as a proof of concept to manufacture novel ingredients in a sustainable manner; however further studies are required to determine the mechanisms by which back-mixed powders affect granulation efficiency and finished powder properties.

GENERAL CONCLUSION

Energy efficient drying plays a critical role in determining the environmental sustainability (and profitability) of any drying process. While much work is focused on the optimisation of conventional spray drying processes, novel spray-drying free approaches which promise breakthrough energy savings remain under-investigated. However, some recent work has shown the energy saving potential of a novel superconcentrationgranulation process (patented Poudre sans Tour process, PST) for permeate powder manufacture. Superconcentration using high shear, and, granulation by recirculation of dry powders are the key operations of PST process and their associated processing parameters can be expected to govern energy efficiency and product properties. In order to determine the operational space, it is necessary to understand the behaviour of products during superconcentration under high shear operating conditions and during granulation.

Furthermore, since these are new operations in the field of dairy, the lack of detailed knowledge relating to the superconcentration and granulation behaviour of dairy streams represents a significant challenge for scale-up. Therefore, the strategy developed in this study focused on the superconcentration-granulation steps and the influence of different dairy compositions on these process steps.

In the first part, complimentary analytical techniques were applied to understand what happens during the superconcentration of dairy streams at lab-scale, with the overall goal being to identify the limits of the PST process. Consequently, the granulation behaviour was characterised in order to understand how this operation can be manipulated to alter process and product properties. This was aimed at overcoming the inherent limits of the PST process. Thus, the scientific questions (as outlined in the chapter 3) were addressed:

1. What are the limiting factors for the novel superconcentrationgranulation process?

Shear thinning behaviour of dairy concentrates is widely reported in the literature [START_REF] Patil | Characterization of the superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders[END_REF]Trinh et al., 2007a;Vélez-Ruiz and Barbosa-Cánovas, 2000). In chapter 5, successful modelling of the flow behaviour of the microfiltrate from skim milk confirmed shear thinning behaviour at superconcentrated states. Indeed, it is this intrinsic property of dairy concentrates which allows for the processing of superconcentrates, because the rise in viscosity due to higher dry matter is counteracted by the high shear applied in the turboconcentrators. Therefore, the typical exponential rise in viscosity with increasing dry matter content demonstrated by the dairy concentrates is no longer the limit for the superconcentration process. However, the progressive superconcentration eventually resulted in the formation of a highly cohesive agglomerated wet mass. This hardened nonflowing state, if not controlled, can damage the equipment due to its great resistance to flow.

Moreover, the literature (albeit for non-dairy applications) points to dramatically reduced heat and mass transfer rates in this cohesive phase [START_REF] Bennamoun | Review on fundamental aspect of application of drying process to wastewater sludge[END_REF]. Therefore, the limit for maximum extent of superconcentration can be identified as the dry matter content at the onset of this cohesive phase.

The application of complimentary analytical techniques to characterise the dairy product in the superconcentrated state revealed the location of the highly cohesive phase.

The onset was marked with dramatic increase in flow stress, yield stress, cohesiveness and, most importantly, the value of the flowability function was lower than the hardened state threshold (ffc<1), measured using a ring shear tester. Additionally, the minimum recirculation rate required for effective granulation was related to the end of the cohesive phase.

Interestingly, the evolution in product properties was well correlated with the variation in power consumption of the mixer during superconcentration. This relationship can be applied during processing to better understand and control product behaviour within the system and to assist scale-up and optimisation (Fig. 8.1).

How does the composition of the dairy streams influence superconcentration-granulation process?

The composition of dairy streams showed a marked influence on the location of the cohesive phase. Indeed, the process is much more feasible for high lactose dairy streams (Fig. 8.2). This is because, the higher lactose dairy streams (whey permeate and demineralised whey as considered in this study) could be superconcentrated to higher levels as compared to the skim milk and fat-filled milk, which were restricted to lower DM content likely due to the higher water binding capacity of inherent proteins. The evolution of the cohesive phase during dehydration points towards capillary cohesive mechanisms, wherein the literature suggests that only free water influences the cohesive phase, in contrast the bound water which plays no role. Due to the higher water binding capacity of proteins, the increased protein content resulted in formation of a cohesive mass at a much lower DM content. Interestingly, the corollary of this observation was observed in the final section of the thesis i.e. increased water binding from proteins reduced the required recirculation rate of dry powder. The identification of critical intrinsic parameters, i.e. the cohesive phase for different dairy streams made it possible to determine the energy savings potential. According to literature, the energy required to remove water in conventional drying processes accounts for 90% of the total energy; therefore, a methodology on the sole basis of water removal was used to compare the energy savings potential. The specific energy required to remove water follows the order (from lowest to highest; (Tanguy et al., 2017)): falling-film evaporation (75 kJ.kg -1 evaporated water; 2-stage MVR) < superconcentration (2875 kJ.kg -1 evaporated water) < spray drying (5500 kJ.kg -1 evaporated water) < final drying using turbo concentrator/dryer (5400 kJ.kg -1 evaporated water). Therefore, processing of high-lactose whey permeate and demineralised whey demonstrated markedly higher energy savings potential compared to processing of fat-filled and skim milk powders. The energy savings estimated, combined with lower capital costs due to compact installation, can motivate industry to evaluate process feasibility at pilot scale. At the same time, it must be noted that the energy required for the agitators during granulation and superconcentration can vary significantly as a function of composition (Chapter 6, Table 6.3). Although, thermal operations dominate the total energy consumption in conventional process, the energy required for granulation cannot be neglected, and should be part of the energy estimates at the pilot/industrial scale validation.

What are the characteristics of the powders produced by granulation compared to conventional spray dried powders?

Granulated powders had highly compact structures with higher densities and lower porosities compared to equivalent spray dried powders. The sharp contrast in compactness and opened structures was demonstrated by SEM for granulated and standard spray dried powders respectively. Interestingly, high lactose/low protein powders had excellent rehydration and flow properties, in contrast to medium lactose/medium protein powders (skim milk and fat-filled milk) which both behaved poorly upon rehydration. Light microscopy revealed highly crystalline structures for skim milk and fat-filled milk powders which was in contrast to standard spray dried powders. The nature of high shear granulation, in addition to a longer residence time in the superconcentrated state, wherein lactose is prone to spontaneous crystallisation due to supersaturation, will influence the powder structures and in turn powder functionality and stability (Vuataz, 2002). Although lab scale powders cannot be directly compared to the spray dried powders, the influence of underlying mechanisms reflected in lab scale granulated powder properties can assist in optimising the pilot plant process.

How do whey permeate and demineralised whey differ with respect to their granulation behaviour?

Even though the maximum limit for superconcentration and minimum recirculation rate for granulation were similar, as revealed by the onset and end of cohesive phase for demineralised whey and whey permeate, large differences were observed in their granulation. Granulation of demineralised whey paste consumed markedly more energy than whey permeate and the resulting demineralised whey granules were substantially larger.

Both results are likely due to the higher cohesiveness exhibited by the demineralised whey.

Moreover, increasing the DM content of granules, achieved through higher back-mix ratios, reduced the cohesiveness and in turn improved the granulation performance of both compositions. This indicates that cohesiveness, or perhaps disruption of cohesiveness, is the key property influencing granulation behaviour. Any measure (e.g. modification of temperature, addition of anti-caking aids, etc.) which disrupts cohesiveness can be assumed to optimise the process performance.

5. How can the granulation behaviour be manipulated to overcome the challenge of high recirculation rates?

Recirculation of dry powder is an effective strategy used in the superconcentrationgranulation process to circumvent the highly cohesive phase. However, high recirculation rates (i.e. recirculation of large quantities of powder through the drying stage) may be limiting, both in terms of equipment sizing, process productivity and product properties.

Therefore, it is pertinent to investigate means of reducing recirculation rates. The use of WPI in the recirculation powder resulted in more efficient granulation (reduced energy, finer granules and lower recirculation rate) of the superconcentrated WP paste. This was linked to the higher water holding capacity of WPI. During granulation, high cohesiveness of superconcentrated pastes, which may be related to protein content, has been shown to negatively impact granulation. Conversely, addition of protein to the granulating powder has been shown to have positive effects, which offer new perspectives for formulated products, whereby different powders can be mixed with superconcentrates to prepare products of targeted composition. A proof of concept was demonstrated by producing a formulated infant formula protein base from a superconcentrated paste of demineralised whey mixed with an appropriate amount of a casein rich ingredient and lactose. This has potential to be further expanded to different dairy co-product streams and different target applications for pilot scale validation.

Overall, this PhD generates new understanding of the dairy superconcentrates and their granulation behaviour which has potential to significantly alter the manner in which lactose-rich dairy powders are manufactured. In addition, the lab scale model developed within the project can be applied to a range of dairy and food products to assess their compatibility with the superconcentration-granulation approach, while predicting crucial parameters and energy savings potential. Finally, the project which began with the characterisation of superconcentration-granulation based dehydration technology culminated in presenting a breakthrough technical solution to overcome the inherent processing challenges. The solution, which involves modulating the composition of back-mixed powder, in-effect transforms the granulation step into a formulation opportunity. This expands the scope of the technology beyond relatively low-value high lactose streams to encompass potentially significant applications in the production of value-added products.

FUTURE OUTLOOK

This study was based on a lab scale model, wherein a range of dairy compositions were evaluated. It was shown that the evolution of a cohesive phase predominantly influences the effective functioning of the novel superconcentration-granulation based process, and that the crucial parameters that can be used to control the process can be obtained from a wet mass characterisation protocol. The primary follow-on step is to validate the outcomes from the lab scale study at a manageable scale in the pilot plant. The future work can focus on a number of topics including a) determining the mechanisms of lactose crystallisation within the process, b) further investigations into the reverse phase granulation step and c) optimisation of the final drying step in order to optimise the process and product properties and thus expand the scope of this novel process.

Pilot Plant validation:

The successful implementation of the novel superconcentration-granulation process will depend on the robust validation at the pilot scale. This is because, both superconcentration and granulation are relatively new processes in the dairy industry. The key focus should be on determining the superconcentration and minimum recirculation rates required for effective granulation under industrially relevant conditions. This will determine the economic feasibility of the novel process. The lab scale study proposed to use the agitator power consumption to monitor and control the process. A similar process control strategy should be part of the pilot plant validation, this is particularly important, given the fact that a small decrease in maximum superconcentration can dramatically increase the recirculation rates and consequently the process performance. Energy efficiency is one of the primary motivations to investigate this technology, therefore, a thorough validation of specific energy consumptions is necessary to demonstrate its actual energy savings potential. This is of particular interest, given the fact that higher lactose streams can substantially differ in their granulation behaviour, ultimately influencing the energy efficiencies. Moreover, the extent of production runs in between cleaning cycles and effective cleaning procedures need to be validated because the cleaning process can have considerable impact on overall energy efficiency and productivity. It would be of interest to conduct this pilot scale study using dimensional analysis by analogy with other mixing processes in order to guarantee the validity of the results in further upscaling at the industrial scale [START_REF] Delaplace | Dimensional Analysis of Food Processes[END_REF]. Finally, it is essential to demonstrate the powder functionality and stability produced at a manageable scale in order to compare with standard spray dried powders, thereby building confidence in the technology amongst industry cohorts.

Crystallisation:

Lactose crystallisation is a crucial process in high lactose dairy powders that determines both processability and stability of resultant powders [START_REF] Huppertz | Lactose in dairy ingredients: Effect on processing and storage stability1[END_REF][START_REF] Kalab | Composition and structure of demineralized spray dried milk permeate powder[END_REF][START_REF] Paterson | Lactose processing: From fundamental understanding to industrial application[END_REF]. Typically, crystallisation is performed around ~60% w/w DM. It is a time-consuming process (~16 h) and sensitive to DM content and temperature profiles [START_REF] Mimouni | Kinetics of lactose crystallization and crystal size as monitored by refractometry and laser light scattering: effect of proteins[END_REF][START_REF] Parimaladevi | Influence of supersaturation level on the morphology of α-lactose monohydrate crystals[END_REF][START_REF] Wong | Designing a lactose crystallization process based on dynamic metastable limit[END_REF]. The influence of superconcentration on the crystallisation of lactose should be investigated in order to determine whether it is feasible to replace (or reduce the duration of) the controlled batch crystallisation step which is typically employed in permeate and whey manufacture.

The use of instruments capable of tracking particles in process, such as FBRM -focused beam reflectance measurement [START_REF] Ndoye | In-line characterization of a whey protein aggregation process: Aggregates size and rheological measurements[END_REF][START_REF] Pandalaneni | Focused beam reflectance measurement as a tool for in situ monitoring of the lactose crystallization process[END_REF] preliminary considered during the thesis, may be useful to characterise the superconcentration step with respect to the spontaneous crystallisation of lactose due to progressive supersaturated conditions.

Reverse phase granulation process:

The dehydration technology based on superconcentration-granulation process has been show in this thesis to have potential for manufacture of formulated products (see Chapter 6b). For instance, the limiting feature of higher recirculation rates can be overcome by using different suitable powders. The reverse phase granulation process, although scarcely reported in literature, is employed in a range of processes already applied to nondairy products, for instance agglomeration of wheat flour, manufacture of date powder and pharmaceutical applications [START_REF] Rondet | Agglomeration of wheat powders by a novel reverse wet agglomeration process[END_REF][START_REF] Sablani | A new method of producing date powder granules: Physicochemcal characteristics of powder[END_REF][START_REF] Wade | Feasibility assessment for a novel reverse-phase wet granulation process: The effect of liquid saturation and binder liquid viscosity[END_REF].

This field can be further expanded to not only dairy streams but also products from other food industries. A targeted product development approach combining various side streams can potentially result in value added ingredients in a cost-effective manner. For instance, within the Irish agri-food industrial context, brewers spent grain fibres can be combined with the permeate superconcentrates for animal feed applications. Another approach could be a combination of high-water holding fruit and vegetable side streams (rich in fibres) with superconcentrated permeates for application in bakery products.

Final drying step:

The superconcentration-granulation process is more likely to be applied to dairy permeate streams. The permeate powders find wide applications in both the food and feed industries as a cost-effective ingredient. Some applications (e.g. chocolates, etc) prefer powders with a higher browning potential or powders that show a brown color. The final drying step (temperature-aw-time) has an influence on the browning of the powders, and should be investigated as a means of obtaining desired powder properties.

Expansion of superconcentration-granulation process to produce other food powders:

Finally, in continuation of pilot scale validation, the lab scale protocol presented in this project is a useful starting point for evaluation of different dairy and food ingredients suitability for superconcentration-granulation process. The preliminary screening is possible in a swift manner and requires relatively little in the way of product quantities. For instance, manufacture of powdered ingredients and formulations from fruit and vegetable concentrates. This approach develops further the resourceful drying using agitated thin-film dryers instead of spray drying of viscous liquid foods, pastes or pureed foods [START_REF] Qiu | a. Agitated thin-film drying of foods[END_REF], wherein spinach leaf slurries were successfully dried but tomato and bell pepper concentrates resulted in sticky mass abruptly stopping the equipment. The shift from spray drying to superconcentration-granulation has a potential to significantly reduce the global environmental footprint, and should be explored for other compatible dairy and food powders manufactured by spray drying the concentrates. Values were normalized on Y-axis with respect to the value corresponding to DM on x-axis. La consommation d'énergie pour la fabrication des poudres de perméat (13 860 kJ.kg -1 poudre) est supérieure de 30 % à celle des poudres de lait écrémé (10 656 kJ.kg -1 poudre ; Schuck et al., 2015), du fait de la matière sèche plus faible des perméats en regard de celle du lait écrémé. Dans le même temps, la valeur des poudres de perméat (882 €.t -1 ) est inférieure de 73 % à celle de la poudre de lait écrémé (3328 €.t -1 ). Par conséquent, une technologie alternative à faible coût peut être intéressante afin de réduire l'empreinte environnementale de la transformation de volumes croissants de perméat et gagner simultanément en rentabilité.

TABLE OF FIGURES

Contexte scientifique 9

La déshydratation du lait est une opération clé pour l'industrie laitière, permettant une conservation prolongée de la poudre sous une forme stable à température ambiante. Les procédés thermiques d'élimination de l'eau impliqués représentent près de 90 % de la consommation d'énergie dans les usines de fabrication de lait en poudre. Par conséquent, une diminution significative de la consommation d'énergie peut être espérée en focalisant les processus de déshydratation. Cependant, la prise en compte de l'ensemble de la chaîne de production est cruciale lors de l'optimisation du procédé en raison de l'effet cumulatif des interactions procédé-produit lors des prétraitements et des effets de ceux-ci peuvent avoir sur les procédés de déshydratation, tant conventionnels que disruptifs, et c'est une erreur de considérer la déshydratation seule. Par exemple, les technologies de concentration membranaire telles que l'osmose inverse (RO) et la nanofiltration (NF) séparent l'eau du produit sans impliquer de changement de phase, économisant ainsi la chaleur latente de vaporisation nécessaire pour cet effet de concentration. Cependant, il existe un optimum entre facteur de concentration plus élevés et productivité.

Les prétraitements ont un impact considérable sur les propriétés du produit, de sorte qu'ils jouent un rôle majeur dans la performance de l'étape de SD. L'optimisation des traitements thermiques et la mise en oeuvre de technologies pour réduire la viscosité du produit à l'entrée du sécheur par pulvérisation peuvent permettre d'atteindre des conditions opératoires favorables en autorisant une concentration d'entrée plus élevée du produit et un 9 Revue de littérature publiée : Patil, M.H., Tanguy, G., Floch-Fouéré, C.L., Jeantet, R., Murphy, E.G., 2021a. Energy usage in the manufacture of dairy powders: Advances in conventional processing and disruptive technologies. Dry. Technol. 0, 1-19.

Comment la composition des fluides laitiers influence-t-elle le processus de surconcentration-granulation ?

La composition des fluides laitiers a une influence marquée sur la localisation de la phase cohésive. Ainsi, le procédé de surconcentration s'avère beaucoup plus réalisable pour les fluides laitiers à haute teneur en lactose. En effet, ces fluides (perméat de lactosérum et lactosérum déminéralisé, tels que pris en compte dans cette étude) ont pu être surconcentrés à des niveaux plus élevés par rapport au lait écrémé et au lait entier (Fig. 2), dont la concentration était limitée à une faible teneur en extrait sec probablement du fait d'une plus grande capacité de liaison de l'eau due à leurs protéines. L'évolution de la phase cohésive au cours de la déshydratation pointe vers des mécanismes cohésifs capillaires, où la littérature suggère que seule l'eau libre influence la phase cohésive, alors que l'eau liée ne joue aucun rôle. En raison de la capacité de liaison de l'eau plus élevée des protéines, l'augmentation de leur teneur a conduit à la formation d'une masse cohésive à une teneur en extrait sec beaucoup plus faible. Fait intéressant, le corollaire de cette observation a été observé dans la dernière section de la thèse, c'est-à-dire qu'une liaison accrue de l'eau par les protéines a réduit le taux de recirculation requis de poudre lors de la granulation.

L'identification des paramètres intrinsèques critiques, c'est-à-dire la phase cohésive pour les différents fluides laitiers, a permis de déterminer le potentiel d'économies d'énergie.

Selon la littérature, l'énergie nécessaire pour éliminer l'eau dans les procédés de séchage conventionnels représente 90 % de l'énergie totale ; par conséquent, une méthodologie sur la seule base de l'élimination de l'eau a été utilisée pour comparer le potentiel d'économies d'énergie du procédé de surconcentration granulation. L'énergie spécifique nécessaire pour éliminer l'eau suit l'ordre (de la plus faible à la plus élevée ; (Tanguy et al., 2017) (Vuataz, 2002). Bien que les poudres à l'échelle du laboratoire ne puissent pas être directement comparées aux poudres séchées par pulvérisation, l'influence des mécanismes sous-jacents identifiés au travers des propriétés de la poudre granulée à l'échelle laboratoire peut aider à optimiser le procédé à échelle pilote.

Comment les différences de composition des coproduits laitiers à haute teneur en lactose affectent-elles le comportement de granulation ?

Même si la limite maximale de surconcentration et le taux de recirculation minimal pour la granulation étaient similaires pour le lactosérum déminéralisé et le perméat de lactosérum, comme le révèlent le début et la fin de la phase cohésive, de grandes différences ont été observées dans leur granulation. La granulation de la pâte de lactosérum déminéralisé consommait nettement plus d'énergie que le perméat de lactosérum et les granules de lactosérum déminéralisé résultant étaient sensiblement plus gros. Ces deux résultats sont cohérents, en raison de la cohésivité plus élevée présentée par le lactosérum déminéralisé. L'augmentation de la teneur en extrait sec des granulés, obtenue grâce à des taux de recirculation plus élevés, a permis de réduire la cohésivité et par suite amélioré les performances de granulation des deux compositions. Ceci indique que la cohésivité est la propriété clé influençant le comportement de granulation. On peut ainsi présupposer que toute mesure qui perturbe la cohésivité (par exemple, modification de la température, ajout d'agents anti-mottants, etc.) permettra d'optimiser les performances du procédé. La recirculation de la poudre sèche est une stratégie efficace pour contourner le facteur limitant de la phase hautement cohésive au cours du procédé de surconcentrationgranulation. Cependant, des taux de recirculation élevés (c'est-à-dire la recirculation de grandes quantités de poudre) constitue elle-même une limitation, à la fois en termes de dimensionnement de l'équipement, de productivité du procédé et de propriétés du produit.

Par conséquent, il est impératif d'étudier les moyens permettant de réduire ce taux de recirculation. L'utilisation d'isolat de protéines solubles en tant que poudre de recirculation a entraîné une granulation plus efficace (énergie réduite, granulés plus fins et taux de recirculation inférieur) de la pâte de perméat de lactosérum surconcentrée, du fait d'une plus grande capacité de rétention d'eau. Pendant la granulation, il a été démontré que la cohésivité élevée des pâtes surconcentrées, qui peut être liée à la teneur en protéines, a un impact négatif sur la granulation. A l'inverse, il a été démontré que l'ajout de protéines à la poudre de granulation a des effets positifs, qui offrent de nouvelles perspectives pour les produits formulés, où différentes poudres peuvent être mélangées avec des surconcentrés pour préparer des produits à composition ciblée. Une preuve de concept a été obtenue en produisant une base protéique de formule infantile formulée à partir d'une pâte surconcentrée de lactosérum déminéralisé mélangé avec une quantité appropriée d'un ingrédient riche en caséine et de lactose. Elle démontre le potentiel d'étendre le procédé de surconcentration granulation à différents coproduits laitiers et à différentes applications cibles, qui sera étudié à l'échelle pilote.

Conclusion générale et perspectives

Les approches de sobriété énergétique jouent un rôle essentiel dans la durabilité environnementale et la rentabilité de tout procédé de séchage. Alors que de nombreux travaux se concentrent sur l'optimisation des procédés de séchage conventionnels par pulvérisation, de nouvelles approches sans étape de séchage, qui permettent d'envisager

CHAPTER 7 :

 7 GRANULATION OF SUPERCONCENTRATED DEMINERALISED WHEY PASTE TO MANUFACTURE VALUE ADDED INGREDIENTS: THE EXAMPLE OF INFANT MILK FORMULA .................................................................111 7.1 Introduction ..................................................................................................................111 7.2 Materials and methods: ..............................................................................................111 7.2.1 Materials .......................................................................................................................................... 111 7.2.2 Granulation process ...................................................................................................................... 112 7.3 Results and discussion: .............................................................................................112 7.3.1 Granulation of superconcentrated DW paste by addition of MPC and lactose or SMP and lactose ....................................................................................................................................................... 112 7.3.2 Granulated powder properties ..................................................................................................... 114 7.4 Conclusion ...................................................................................................................114 GENERAL CONCLUSION ..........................................................................................116 FUTURE OUTLOOK ...................................................................................................121 REFERENCES ............................................................................................................124 TABLE OF FIGURES ..................................................................................................142 TABLE OF TABLES ...................................................................................................146 RÉSUMÉ LONG: SÉCHAGE INNOVANT DE CO-PRODUITS LAITIERS …..…….148

  a) the economic and scientific context (chapter 1) b) a literature review on the recent energy saving innovations in dairy powder manufacturing technology (chapter 2) c) the experimental strategy of the PhD project (chapter 3) The second part presented the results and discussion focusing on the superconcentration-granulation based powder manufacturing approach i.e.: a) determination of limiting factors of superconcentration-granulation (chapter 4) b) characterization of a model dairy superconcentrate (chapter 5) c) characterization of granulation behaviour of high lactose dairy side-streams, (chapter 6) d) utilization of results from previous chapters to develop a lab-scale prototype for manufacture of a formulated infant formula ingredient (chapter 7)

Figure 1 . 2 .

 12 Figure 1.2. Manufacture of dairy powders (10 3 tonnes) in Ireland and France between 2017-2019 (CNIEL, 2021).

Figure 2 . 1 .

 21 Figure 2.1. Conventional dairy powder manufacturing process with energy requirement for water removal (in green rectangles), relevant physico-chemical properties from process performance point of view (in red rectangles), technological constraints of equipment's used for water removal (in black rectangles); MVR -Mechanical vapor recompression, TVR -Thermal vapor recompression, RO -Reverse osmosis, NF -Nano-filtration, FFE -Falling-film evaporation, Tgglass transition temperature, awwater activity.

Figure 2 . 2 .

 22 Figure 2.2. Schematic representation of the Poudre sans tour (PST) process for the manufacture of permeate powders from concentrate. 1 -Concentrate entering the superconcentrator; 2 -Superconcentrated paste; 3 -Granules; 4 -Final powder which is partly recycled back into superconcentrated paste to fragment the paste into discrete granules.

  as compared to SD)  substantially lower energy (operational costs) for dehydration  highly compact installation (savings in capital costs, waste and cleaning)  lower drying velocity (controlling the phase transitions in lactose is a challenge)  exposure to higher temperatures (risk of denaturation)  evolution of highly cohesive phase restricts adaptation to narrow formulation range (higher molecular weight)



  relatively simpler operations (few variables)  substantially lower energy for dehydration  Granulation: unique feature to overcome cohesive phase with recirculation of dry powders; potential for adaptation to wide range of formulations by restructuring ingredient addition stages  low temperature can control denaturation  highly compact installation  lower drying velocity (controlling the phase transitions in lactose is a challenge)  higher recirculation rate (~50%), limits production capacity  highly compact product structures with reduced porosity due to high shear granulation may be a ability to process higher solids formulations  reduced energy for dehydration  versatile technology: potential to produce innovative powder micro-structures for wider applications  capital intensive technology  possibility of surface composition modification for complex formulations (increased porosity  higher fat/free fat on surface)

  3.1.Chapter 4 comprised of developing a novel lab-scale model to describe the superconcentration-granulation process. The developed model generated useful insights into the effect of superconcentration on a variety of dairy streams. Additionally, granulated powders were produced at lab scale and powder physical, functional and structural properties were compared with standard spray dried powders. Finally, the energy saving potential of the novel process was estimated for the various dairy streams. In Chapter 5, the lab scale model was used to assess performance of fresh liquid skim-milk microfiltrate. The rheology and flow properties of the product during superconcentration-granulation steps were systematically characterized. The aim was to develop relevant tools and techniques in order to better understand the novel process, which can be used to assist in the scale-up, optimization and assessment of various dairy and food ingredients. In Chapter 6, the granulation step for high lactose dairy streams was evaluated. The aim was to better characterize the granulation step and determine how it could be manipulated through changing of both the composition of the superconcentrate and the powder used for granulation. Chapter 7 briefly provides a proof of concept for the manufacture of value added products from high lactose streams by changing the composition of the recirculation powder used for granulation. The relationship between the experimental chapters 4 -7 is represented in fig.3.2.

Figure 3 . 1 .Figure 3 . 2 .

 3132 Figure 3.1. Objective and experimental strategy for the PhD project

4. 2 . 5

 25 Particle size distribution, rehydration properties and flowabilityA laser scattering granulometer(Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK) was used to determine the particle size distribution. The rehydration properties of experimental granulated and standard spray-dried powders were characterized by dispersibility index and solubility index; which were determined according to[START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF]. Flowability is represented as time in seconds required for sample powder to leave a rotary drum through a slit(GEA-Niro, 2006).

Figure 4 .Figure 4 . 2 .

 442 Figure 4.1. (a) Apparent viscosity and (b) variation in power consumption as a function of dry matter (DM) content for dairy products -WPP, whey permeate powder, DWP, demineralized whey powder, FFMP, fat-filled milk powder and SMP, skim milk powder. R 2 values are for exponential fit of apparent viscosity as a function of DM using average values of apparent viscosity.

Figure 4 . 3 .Figure 4 . 4 .

 4344 Figure 4.3. Polarised light microscopy images of fat-filled milk powder (FFMP): Granulated (left) and standard Spray-dried (right)

4. 4 . 1

 41 Fig. 4.1a clearly shows how viscosity can have a limiting effect on the water removal process. The exponential rise in viscosity with increasing total solids imposes practical limits on the concentration levels achieved and is strongly composition dependent. It is well known increased viscosity negatively influences flow distribution and heat transfer in FFEs and

Figure 4 . 5 .

 45 Figure 4.5. Transition of whey permeate through three different phasesviscous, highly cohesive (sticky) mass and non-sticky discrete granules as a function of dry matter (DM) content.

Figure 4 . 6 .

 46 Figure 4.6. Cohesive phase of whey permeate (WPP), A -Maximum possible superconcentration; B -Minimum dry matter (re-circulation rate) for successful granulation; C -Highly cohesive (so-called sticky) phase which needs to be avoided by recirculation of dry powder for efficient and safe operation.

Figure 4 . 7 .

 47 Figure 4.7. Block diagram with estimation of specific energy consumptions for dehydration steps in (a) conventional spray drying and (b) novel superconcentration-granulation process for manufacture of whey permeate powder (WPP). DM, Dry matter; % (w/w); SEC, Specific energy consumption (kJ.kg -1 dry powder).

Figure 5 . 1 .

 51 Figure 5.1. Lab-scale experimental set-up to simulate the superconcentration-granulation process. a -crystallization set-up; b -temperaturecontrolled mixer for superconcentration; c -granulated product.

5. 2 . 3 . 4

 234 Flow properties of superconcentrated paste using a shear cell measurement Flow properties of superconcentrated pastes were described by cohesiveness and flowability function (ffc) using shear cell tests. A ring-shear tester (RST-XS, Dr. Dietmar Schulze Schüttgutmesstechnik, Wolfenbüttel, Germany) was used to characterize flow properties of superconcentrated pastes at various DM contents at 25°C. Superconcentrated paste was filled in the shear cell (24 cm 3 ) and excess material was scraped off. Shear test measured yield locus of a pre-consolidated bulk solid sample, by applying shear stress until failure. Cohesiveness and ffc were determined by analyzing the yield locus measurements using RST control software (RSV 95). All yield locus measurements were performed at a normal pre-shear stress of 2000 Pa using four shear points (400, 1000, 1600 and 400 Pa).

Figure 5 . 2 .

 52 Figure 5.2. a. Desupersaturation curve of MFP during crystallisation at 60% w/w 30°C. (Lactose g.g -1 wateropen circles; Extent of crystallizationdark circles); b. Particle size of the lactose crystals measured using laser light diffraction.

Figure 5 . 3 .

 53 Figure 5.3. Apparent viscosity of superconcentrates as a function of dry matter (DM). Measurement at 25°C and 100 s -1 . Error bars represent standard deviation of measurements performed in duplicates.

Figure 5 . 4 .

 54 Figure 5.4. Correlation of apparent viscosity of superconcentrates (Pa s) and specific power consumption of mixer expressed as current (A). Viscosity measurement made at 25°C and 100 s -1 . Agitation speed of 500 rpm. Correlation is based on average values of apparent viscosity (n=2).

Figure 5 . 5 .

 55 Figure 5.5. Flow stress (Pa) using oscillation amplitude sweep as a function of dry matter of superconcentrates (% w/w). Measurement made at 60°C and constant frequency of 1 Hz. Error bars represent standard deviation (n = 2).

Figure 5 . 6 .

 56 Figure 5.6. Cohesion (in N) as a function of DM of superconcentrates (% w/w). Measurements at 25°C. Standard deviation less than 3% (n = 2), error bars are within the symbols.

Figure 5 . 7 .

 57 Figure 5.7. Comparison of the three rheological techniques based on normalized results of yield stress (Herschell-Bulkley model), flow stress (oscillation amplitude sweep) and cohesion (probe track test) as a function of DM content of superconcentrates (% w/w).

Figure 5 . 8 .

 58 Figure 5.8. Cohesiveness and flowability function as a function of dry matter (DM) of superconcentrates, dashed line represents threshold for non-flowing state (ffc <1).

Figure 5 . 9 .

 59 Figure 5.9. Specific power consumption of mixer expressed in ampere (A) as a function of dry matter (DM) of superconcentrates. Experiments carried out at 500 rpm and 60°C.

  based techniques to gain relevant insights which can be used to control superconcentration and granulation processes. Flowability function (ffc) and cohesiveness effectively demarcated a highly cohesive non-flowing state (ffc < 1) of the superconcentrated paste. Interestingly, the rheological techniques used (flow curves -Herschel Bulkley model, oscillation amplitude sweep and cohesionprobe-tack test) demonstrated a similar evolution of the product behaviour as a function of DM. A dramatic increase was observed in flow stress and cohesion as the DM approached onset of the highly cohesive non-flowing state, ~82% w/w DM for MFP.

Figure 6 . 1 .

 61 Figure 6.1. Population balance of granules as a function of dry matter content of bulk products at the end of granulation for a.) Whey permeate, WP and b.) Demineralised whey, DW. Error bars represent std. deviation, n=3.

Figure 6 . 2 .

 62 Figure 6.2. Mass median diameter of granules as a function of the final DM content of bulk product in the case of whey permeate (WP) granulation. Error bars represent std. deviation, n=3.

Figure 6 . 4 .

 64 Figure 6.4. Cohesiveness of bulk granules, expressed as electric current (A), and mass median particle size of the granules for all the granulation experiments. Error bars represent std. deviation, n=3.

Figure 6 . 5 .

 65 Figure 6.5. Moisture content distribution of the granules segregated by sieves obtained at the end of granulation of superconcentrated whey permeate (WP) paste by WP, whey protein isolate, WPI powders and dry blends of WP/WPI at two levels (WPI10 -WP:WPI 90:10; WPI20 -WP:WPI 80:20). All granulations performed at a back-mix fraction of 1:1 (superconcentrate:powder).

Figure 6 . 6 .

 66 Figure 6.6. Variation in power consumption as a function of DM content for the whey permeate (WP) and blends of WP and whey protein isolate (WPI) powders, i.e. WPI10 and

Figure 6 . 8 .

 68 Figure 6.8. Location of end-point of granulation for two back-mix fractions on the typical cohesive phase map for whey permeate, open circle for 1:0.4; dark circle for 1:0.5 superconcentrate:powder ratios. The powder added was WPI20a blend of whey permeate and whey protein isolate in 80:20 proportion (w/w).

Figure 8

 8 Figure 8.1. A reliable protocol developed to identify crucial parameters of the superconcentration-granulation process.

Figure 8 . 2 .

 82 Figure 8.2. Influence of composition on superconcentration-granulation parameters.

Figure 1 . 1 .

 11 Figure 1.1. World-wide milk equivalent per capita annual consumption (left axis; kg) and world population (right axis; million inhabitants) between 2010 and 2019 (IDF, 2020).

Figure 1 . 2 .

 12 Figure 1.2. Manufacture of dairy powders (10 3 tonnes) in Ireland and France between 2017-2019 (CNIEL, 2021).

Figure 2 . 1 .

 21 Figure 2.1. Conventional dairy powder manufacturing process with energy requirement for water removal (in green rectangles), relevant physico-chemical properties from process performance point of view (in red rectangles), technological constraints of equipment's used for water removal (in black rectangles); MVR -Mechanical vapor recompression, TVR -Thermal vapor recompression, RO -Reverse osmosis, NF -Nano-filtration, FFE -Fallingfilm evaporation, Tgglass transition temperature, awwater activity.

Figure 2 . 2 .

 22 Figure 2.2. Schematic representation of the Poudre sans tour (PST) process for the manufacture of permeate powders from concentrate. 1 -Concentrate entering the superconcentrator; 2 -Superconcentrated paste; 3 -Granules; 4 -Final powder which is partly recycled back into superconcentrated paste to fragment the paste into discrete granules.

Figure 3 . 1 .

 31 Figure 3.1. Objective and experimental strategy for the PhD project.

Figure 3 . 2 .

 32 Figure 3.2. Schematic representation showing relationships between the experimental chapters (4 to 7).

Figure 4 .

 4 Figure 4.1. (a) Apparent viscosity and (b) variation in power consumption as a function of dry matter (DM) content for dairy products -WPP, whey permeate powder, DWP, demineralized whey powder, FFMP, fat-filled milk powder and SMP, skim milk powder. R 2 values are for exponential fit of apparent viscosity as a function of DM using average values of apparent viscosity.

Figure 4 . 2 .

 42 Figure 4.2. Granulation obtained for different back-mix fractions for dairy products -WPP, whey permeate powder, DWP, demineralized whey powder, FFMP, fat-filled milk powder and SMP, skim milk powder.

Figure 4 . 3 .

 43 Figure 4.3. Polarised light microscopy images of fat-filled milk powder (FFMP): Granulated (left) and standard Spray-dried (right).

Figure 4 . 4 .

 44 Figure 4.4. Scanning electron microscopy images (5000X magnification, scale bar -10 um) of dairy powders: WPP, whey permeate powder; DWP, demineralized whey powder; FFMP, fat-filled milk powder; and SMP, skim milk powder; G, granulated powders; SD, standard spray-dried powders.

Figure 4 . 5 .

 45 Figure 4.5. Transition of whey permeate through three different phasesviscous, highly cohesive (sticky) mass and non-sticky discrete granules as a function of dry matter (DM) content.

Figure 4 . 6 .

 46 Figure 4.6. Cohesive phase of whey permeate (WPP), A -Maximum possible superconcentration; B -Minimum dry matter (re-circulation rate) for successful granulation; C -Highly cohesive (so-called sticky) phase which needs to be avoided by recirculation of dry powder for efficient and safe operation.

Figure 4 . 7 .

 47 Figure 4.7. Block diagram with estimation of specific energy consumptions for dehydration steps in (a) conventional spray drying and (b) novel superconcentration-granulation process for manufacture of whey permeate powder (WPP). DM, Dry matter; % (w/w); SEC, Specific energy consumption (kJ.kg -1 dry powder).

Figure 5 . 1 .

 51 Figure 5.1. Lab-scale experimental set-up to simulate the superconcentration-granulation process. a -crystallization set-up; b -temperature-controlled mixer for superconcentration; c -granulated product.

Figure 5 . 2 .

 52 Figure 5.2. a. Desupersaturation curve of MFP during crystallisation at 60% w/w 30°C. (Lactose g.g-1 wateropen circles; Extent of crystallizationdark circles); b. Particle size of the lactose crystals measured using laser light diffraction.

Figure 5 . 3 .

 53 Figure 5.3. Apparent viscosity of superconcentrates as a function of dry matter (DM).Measurement at 25°C and 100 s -1 . Error bars represent standard deviation of measurements performed in duplicates.

Figure 5 . 4 .

 54 Figure 5.4. Correlation of apparent viscosity of superconcentrates (Pa s) and specific power consumption of mixer expressed as current (A). Viscosity measurement made at 25°C and 100 s -1 . Agitation speed of 500 rpm. Correlation is based on average values of apparent viscosity (n=2).

Figure 5 . 5 .

 55 Figure 5.5. Flow stress (Pa) using oscillation amplitude sweep as a function of dry matter of superconcentrates (% w/w). Measurement made at 60°C and constant frequency of 1 Hz. Error bars represent standard deviation (n = 2).

Figure 5 . 6 .

 56 Figure 5.6. Cohesion (in N) as a function of DM of superconcentrates (% w/w). Measurements at 25°C. Standard deviation less than 3% (n = 2), error bars are within the symbols.

Figure 5 . 7 .

 57 Figure 5.7. Comparison of the three rheological techniques based on normalized results of yield stress (Herschell-Bulkley model), flow stress (oscillation amplitude sweep) and cohesion (probe track test) as a function of DM content of superconcentrates (% w/w).

Figure 5 . 8 .

 58 Figure 5.8. Cohesiveness and flowability function as a function of dry matter (DM) of superconcentrates, dashed line represents threshold for non-flowing state (ffc <1).

Figure 5 . 9 .

 59 Figure 5.9. Specific power consumption of mixer expressed in ampere (A) as a function of dry matter (DM) of superconcentrates. Experiments carried out at 500 rpm and 60°C.

Figure 6 . 1 .

 61 Figure 6.1. Population balance of granules as a function of dry matter content of bulk products at the end of granulation for a.) Whey permeate, WP and b.) Demineralised whey, DW. Error bars represent std. deviation, n=3.

  Cependant, les questions de durabilité prennent aujourd'hui une importance croissante avec les préoccupations liées au changement climatique. Par rapport à d'autres produits laitiers et alimentaires, la fabrication de poudres nécessite l'élimination d'une grande quantité d'eau et mobilise par conséquent une quantité d'énergie importante. De ce fait, la réduction de la consommation d'énergie dans les processus de séchage est une priorité pour les industries qui cherchent à minimiser leur empreinte environnementale. Une efficacité énergétique accrue se traduit également par des coûts d'exploitation réduits et, par conséquent, par des profits plus élevés. Ces dernières années ont été marquées par une demande croissante de protéines laitières et de fromage, générant en retour un grand nombre de flux secondaires riches en lactose (1 kg de fromage ~ 7 à 11 kg de lactosérum), qui ont une valeur intrinsèquement inférieure au flux laitier primaire. Le lactosérum peut être valorisé ultérieurement en concentrés de protéines de lactosérum, tout en générant de grandes quantités de perméat de faible valeur. Ces flux à extrait sec relativement faible peuvent être vendus en alimentation animale ou comme source pour produire du lactose cristallisé, mais de nombreux fabricants choisissent de les stabiliser par séchage par pulvérisation. Cependant, en raison de leur faible valeur et du volume élevé d'eau à éliminer, l'optimisation du séchage est dans ce cas encore plus essentielle. En effet, à capacité de séchage limitée, le traitement de ces flux peut passer au second plan par rapport à la fabrication de poudres laitières riches en protéines et à haute valeur ajoutée. Par conséquent, une technologie alternative compacte et économe en énergie serait d'un intérêt significatif pour traiter les flux secondaires de l'industrie laitière. L'objectif général de ce projet de thèse était d'étudier une nouvelle technologie pour fabriquer des poudres laitières sans passer par l'étape de séchage par pulvérisation, afin de réduire la consommation d'énergie tout en conservant la fonctionnalité attendue des poudres. Les travaux réalisés ont élargi les principes d'un procédé breveté de surconcentration-granulation (poudre sans tour (PST), EP 3174402, Schuck et al., 2016), c'est-à-dire une surconcentration sous fort cisaillement pour maintenir le produit à l'état fluide, suivie de la granulation du surconcentré en particules discrètes à l'aide d'une poudre. L'accent a été mis sur la compréhension de la rhéologie des surconcentrés et sur le développement d'informations sur les paramètres physiques, physico-chimiques et de procédé influençant le processus de surconcentration-granulation. En plus des fluides à haute teneur en lactose évoqués ci-dessus, d'autres fluides laitiers ont été inclus dans l étude pour déterminer le champ d'application potentiel du procédé. La thèse est structurée en deux parties. La première inclut : -le contexte économique du secteur laitier (chapitre 1) -une revue de littérature sur les dernières tendances de recherche sur le séchage des produits laitiers pour améliorer la sobriété et l'efficacité énergétiques (chapitre 2) -la stratégie expérimentale du projet doctoral (chapitre 3) La deuxième partie présente et discute les résultats obtenus sur le procédé de fabrication de poudre par surconcentration-granulation : -détermination des facteurs limitants de surconcentration-granulation (chapitre 4) -caractérisation d'un surconcentré laitier modèle (chapitre 5) -caractérisation du comportement de granulation de coproduits laitiers riches en lactose, (chapitre 6) -utilisation des résultats des chapitres précédents pour établir à l'échelle du laboratoire la preuve de concept pour la fabrication d'un ingrédient de préparation pour nourrissons (chapitre 7) Finalement, les principaux apports et perspectives de la recherche sont synthétisés dans les sections conclusion et perspectives. et les poudres de lactosérum représentaient respectivement 1069, 914 et 520 millions d'euros en France (CNIEL, 2020). De plus, la valeur des poudres laitières exportées d'Irlande a atteint 3 milliards d'euros (0,9 Mt) en 2020, en augmentation de 13 % au cours des cinq dernières années (communication personnelle).

  encrassement réduit, avec une productivité supérieure associée et des temps d'arrêt plus courts. Le séchage à haute teneur en extrait sec est ainsi intéressant du point de vue économique et environnemental. Le goulot d'étranglement qu'il représente peut être surmonté par des technologies alternatives telles que l'extrusion-aération, ou selon les exigences du cahier des charges produit, par l'utilisation de voies innovantes de production de poudre telles que la surconcentration et la granulation. L'efficacité énergétique des équipements existants peut être améliorée par la mise en oeuvre de mesures telles qu'une meilleure récupération de chaleur et des modifications mineures dans les configurations existantes sans que celles-ci n'affectent les propriétés du produit final.Par ailleurs, des améliorations significatives de consommation énergétique peuvent être possibles en utilisant des technologies de rupture. En particulier, les défis associés à la viscosité du produit, qui sont des facteurs limitants dans les évaporateurs, atomiseurs et sécheurs par pulvérisation conventionnels, peuvent potentiellement être contournés. Ces technologies doivent cependant être largement testées pour démontrer leur robustesse aux capacités de production industrielle sans compromis sur les propriétés fonctionnelles et la stabilité du produit. En outre, la compréhension du comportement des liquides laitiers hautement concentrés (> 60 % de MS) doit être élargie pour soutenir ces nouvelles approches.En résumé, de nombreux travaux ont été entrepris pour développer de nouvelles approches d'économie d'énergie, que ce soit concernant des procédés de fabrication de poudres laitières conventionnels ou de rupture. Cependant, un grand nombre des approches décrites sont actuellement à un stade pilote et doivent être validées pour implémentation future en production. La nature complexe des produits laitiers (par exemple, la compositionles interactions entre constituants, leur thermo sensibilité) et l'évolution associée de leurs propriétés physico-chimiques au cours de la transformation rend d'autant plus complexe l'adoption de ces nouvelles technologies. Caractériser les produits laitiers au plan des propriétés qui conditionnent la mise en oeuvre de nouvelles technologies et étudier les opportunités qu'elles présentent peut être une voie à suivre pour produire des poudres laitières avec de nouvelles fonctionnalités de manière durable et rentable. être utilisée pour fabriquer des produits formulés à valeur ajoutée à partir de fluides laitiers à haute teneur en lactose ? La stratégie expérimentale pour répondre à ces questions de recherche a consisté à développer un système à l'échelle du laboratoire pour simuler le procédé de surconcentration-granulation (Fig.1). Un mélangeur à température contrôlée (1,2 L, 1,5 kW Thermomix, Vorwerk Allemagne) a été utilisé pour produire des surconcentrés dans des conditions de cisaillement élevé. La granulation des surconcentrés a été réalisée sous cisaillement dans le même mélangeur, par addition de poudres pour fragmenter la pâte en granulés secs. Un compteur d'énergie (Energy Logger 4000 FR, Voltcraft, Allemagne) a permis de suivre l'intensité de courant électrique consommée par le mélangeur, corrélée aux propriétés du produit et à la résistance mécanique qu'il opposait lors du mélange. Les surconcentrés ont été caractérisés en mesurant les propriétés rhéologiques telles que le comportement d'écoulement (balayage de contrainte), la contrainte d'écoulement (balayage d'amplitude d'oscillation) et la cohésion (test d'adhérence de la sonde) à l'aide d'un rhéomètre (AR G2, TA Instruments, Crawley, Royaume-Uni ; DHR2, TA Instrument, France). La pâte surconcentrée a également été caractérisée au plan de ses propriétés d'écoulement (cohésivité et fonction d'aptitude à l'écoulement, ffc) à l'aide d'un ring shear tester (RST-XS, Dr Dietmar Schulze Schüttgutmesstechnik, Wolfenbüttel, Allemagne). Ces mesures, combinées à celles de courant électrique, ont pu être conduites sur la plage de teneur en matière sèche typiquement observée dans le procédé de surconcentration-granulation. De même, les propriétés physiques, fonctionnelles et structurelles des poudres granulées produites à l'échelle du laboratoire ont été comparées aux poudres standard séchées par pulvérisation. De plus, le potentiel d'économie d'énergie du nouveau procédé a été estimé pour les différents fluides laitiers.Enfin, l'étape de granulation pour les flux laitiers à haute teneur en lactose a été évaluée en mesurant l'énergie nécessaire à la granulation, la cohésion du surconcentré et la distribution granulométrique des granulés. L'objectif était de mieux caractériser l'étape de granulation et de déterminer comment elle pouvait être mise en oeuvre en modifiant à la fois la composition du surconcentré et de la poudre utilisée pour la granulation.

Figure 1 .

 1 Figure 1. Système expérimental à échelle laboratoire développé pour simuler le procédé de surconcentration-granulation. acristallisoir ; bmixer à température contrôlée pour la surconcentration ; cproduit granulé.

  ) : évaporation à flot tombant (75 kJ.kg -1 d'eau évaporée ; MVR à 2 étages) < surconcentration (2875 kJ.kg -1 d'eau évaporée) < séchage final au turbo-concentrateur/sécheur (5400 kJ.kg -1 d'eau évaporée) < séchage par atomisation (5500 kJ.kg -1 d'eau évaporée). Par conséquent, le traitement du perméat de lactosérum et du lactosérum déminéralisé, tous deux à haute teneur en lactose, conduit à un potentiel d'économies d'énergie nettement supérieur par rapport au traitement de lait écrémé et entier. Les économies d'énergie estimées, combinées à des coûts d'investissement inférieurs grâce à une installation compacte, peuvent motiver l'industrie à évaluer la faisabilité du procédé à l'échelle pilote. Bien que les opérations thermiques dominent la consommation d'énergie totale dans les procédés conventionnels, l'énergie requise pour la granulation ne peut être négligée et devrait faire partie des estimations d'énergie lors de la validation à l'échelle pilote/industrielle.

Figure 2 .

 2 Figure 2. Influence de la composition sur les paramètres de surconcentration-granulation.

4. 3

 3 Quelles sont les caractéristiques des poudres issues du procédé de surconcentration-granulation par rapport aux poudres obtenues classiquement par séchage spray ? Les poudres granulées présentaient des structures très compactes avec des densités plus élevées et des porosités plus faibles que des poudres équivalentes séchées par pulvérisation. Le contraste marqué entre la compacité et les structures ouvertes a été confirmé par microscopie électronique pour les poudres séchées par pulvérisation granulées et standard, respectivement. Fait intéressant, les poudres à haute teneur en lactose/faible teneur en protéines avaient d'excellentes propriétés de réhydratation et d'écoulement, contrairement aux poudres à teneur moyenne en lactose/teneur moyenne en protéines (lait écrémé et lait entier), qui se ont toutes deux présenté une mauvaise aptitude à la réhydratation. La microscopie optique a révélé des structures hautement cristallines pour le lait écrémé et les poudres de lait écrémé, contrairement aux poudres standard séchées par pulvérisation. Le fort cisaillement appliqué lors de la granulation, couplé à un temps de séjour plus long à l'état surconcentré dans lequel le lactose est à sursaturation et par suite sujet à une cristallisation spontanée, influence la structure des particules et par suite la fonctionnalité et la stabilité de la poudre

4. 5

 5 Comment atteindre des taux de recirculation élevés, en s'appuyant sur la connaissance du comportement du produit à la granulation ? Cette approche pourrait-elle être utilisée pour fabriquer des produits formulés à valeur ajoutée à partir de fluides laitiers à haute teneur en lactose ?

  des économies d'énergie conséquentes, restent sous-étudiées. Cependant, des travaux récents ont montré le potentiel d'économie d'énergie d'un nouveau procédé de surconcentration-granulation (procédé breveté Poudre sans Tour, PST) pour la fabrication de poudre de perméat. La surconcentration utilisant un cisaillement élevé et la granulation par recirculation de poudres sèches sont les opérations clés du procédé PST, et leurs paramètres gouvernent l'efficacité énergétique du procédé et les propriétés du produit. Afin de déterminer le domaine opératoire du procédé, il est nécessaire de connaître le comportement des produits sous forte contrainte de cisaillement lors de la surconcentration et de la granulation. Tout particulièrement, le manque de connaissances détaillées concernant le comportement des fluides laitiers représente un défi important pour le changement d'échelle. Par conséquent, la stratégie développée dans cette étude s'est concentrée sur les étapes de surconcentration et de granulation et l'influence de la composition des différents fluides laitiers sur ces étapes du procédé. The future work can focus on a number of topics including a) determining the mechanisms of lactose crystallisation within the process, b) further investigations into the reverse phase granulation step and c) optimisation of the final drying step in order to optimise the process and product properties and thus expand the scope of this novel process. Finally, the primary follow-on step is to validate the outcomes from the lab scale study at a manageable scale in the pilot plant. Dans la première partie, des techniques analytiques complémentaires ont été développées et mises en oeuvre pour comprendre les facteurs limitants de l'étape de surconcentration appliqué aux fluides laitiers. Leur comportement à la granulation a ensuite été caractérisé afin de comprendre comment cette opération peut être mobilisée pour

per capita consumption and population World: self-sufficiency rate (

	2019 mln tonnes						Other Africa Oceania 5% 4%
	2017 2018							America Europe South 8%
	2016							9%
	2015							North and
								Central
	2010							America
	0		200	400	600	800	1 000	18%	24% EU 28
		Cow's milk	Buffalo milk	Other milk
								World total 2019: 714 bill
	500 mln tonnes						25 mln tonnes	21%
	470							20
				CAGR '10-'19: +1.6%
	440							15
								30%
	410							10
	380		C					5	32
	350							0
	2010	2013	2016	2019	Butter/-oil	Cheese	SM
	kg					mln inhabitants	Region
	115		k				8 000	Asia
	110						7 700	Europe	1
	105						7 400		EU	1
	100		110.3 112.0 113.4 114.7	7 100	North America	Non-EU	1 1
	95	105.2					6 800	South America	1
	90						6 500	Africa
		2010	2016	2017	2018	2019		Central America
		Per capita consumption		World population	Oceania	3
	Figure 1.1. World-wide milk equivalent per capita annual consumption (left axis; kg) and
	world population

(A) Countries included in total: 55.

(B) Countries included: 56 (butter/-oil); 56 (chees

CONSUMPTION

World:

  agglomeration, if not controlled, it can lead to excessive build-up blocking the tower. Process factors include, primarily ambient air humidity (highly variable), along with other factors (inlet

	and outlet air temperature and relative humidity, air flowrate, product total solids and
	flowrate), while product factors include hygrostability (glass transition temperature [Tg], aw),
	hygrosensitivity (Tg, % moisture [M]) and hygrocapacity (aw, %M). End point of dehydration
	(residual moisture content) together with exhaust air RH and temperature, relative to product
	Tg are crucial factors in control of stickiness. Additionally, variation in ambient air humidity
	directly influences drying capacity and impacts process consistency with a tendency to
	operate in safe zone (i.e., residual moisture considerably lower than target). Operating closer

  [START_REF] Chen | A pilot scale study on the concentration of milk and whey by forward osmosis[END_REF] examined concentration of skim milk and whey using FO at pilot scale.

	They achieved VRR of 2.5 increasing the concentration to 21% and 15% w/w DM for skim

milk and whey, respectively. Recently,

[START_REF] Chen | The application of forward osmosis to dairy processing[END_REF]

demonstrated application of FO to obtain elevated total solids (up to 40% w/w DM) in retentates compared to conventional NF and RO systems (15-20% w/w DM).

[START_REF] Aydiner | Techno-economic viability of innovative membrane systems in water and mass recovery from dairy wastewater[END_REF]

evaluated membrane technologies (FO/MD and MD/RO) with respect to water recovery, whey valorization and economic feasibility. They reported substantial improvement in water recovery during raw whey concentration up to 25.8% w/w DM. They concluded that implementation of membrane systems was attractive, with paybacks of less than 1 year. Although the initial results look promising, FO is restricted by availability of compatible draw solutions and their recovery.

Table 2 .1

 2 Emerging technologies and innovations in pre-treatments to control rising viscosity 1

	Technology	Principle/Mechanism	Operating conditions	Key findings

Table 2 .

 2 1 continued

	Technology	Principle/Mechanism	Operating conditions	Key findings

Table 2 .

 2

	Technology Principle/key feature	Advantages	Disadvantages
	Spray drying	 large increase in	 rapid dehydration (<30 secs); better
	(SD)	surface area by	control on lactose phase transitions
		atomising bulk into fine	 minimum heat exposure
		droplets, leading to
		rapid mass and heat
		transfer

2 Comparison of Spray-drying (SD), Agitated thin-film drying (ATFD), Towerless drying (PST) and Extrusion porosification technology (EPT TM )  mature technology in dairy industry  maintains key powder functionality (rehydration and nutritional properties)  higher operating capacity  energy intensive (lower thermal efficiency)

 exhaust air restrictions (emissions control)

 highly complex and skilled operations required  huge drying towers, higher capital and maintenance/waste, etc.

Table 2 .

 2

	2 continued
	Technology Principle/key feature Advantages	Disadvantages
	Towerless
	drying
	technology
	(PST)

Table 4

 4

.1 presents specifications of the dairy powders used in experiments.

Table 4 .

 4

	(FFMP)
		WPP	DWP	SMP	FFMP
	Lactose (% w/w)	85.5	83.5	54.8	38.0
	Protein (% w/w)	2.5	12.0	34.0	24.0
	Fat (% w/w)	1.2	1.2	1.2	29.0
	Moisture (% w/w)	2.8	3.5	4.0	3.0
	Ash (% w/w)	8.0	< 1	6.0	6.0

1. Composition of dairy powders used in the study; whey permeate powder (WPP), demineralized whey powder (DWP), skim milk powder (SMP) and fat-filled milk powder

Table 4 . 2 .

 42 Experimental conditions for superconcentrated paste and granulation by addition of dry powder at three levels

	Superconcentration

Table 4 . 3 .

 43 Estimation of the specific energy consumptions (SEC) for dehydration in conventional spray drying (SD) and novel superconcentration-granulation process.2

			Energy		Skim milk		Fat-filled milk	Demineralised whey		Whey permeate
			consumption	Inlet	Outlet	SEC	Inlet	Outlet	SEC	Inlet	Outlet	SEC	Inlet	Outlet	SEC
			ratio (ECR)	DM	DM		DM	DM		DM	DM		DM	DM
			(kJ.kg -1 of	%	%	(kJ.kg -1	%	%	(kJ.kg -1	%	%	(kJ.kg -1	%	% (w/w)	(kJ.kg -1
			evaporated	(w/w)	(w/w)	powder)	(w/w)	(w/w)	powder)	(w/w)	(w/w)	powder)	(w/w)		powder)
			water)
	Conventional	Two-stage MVR evaporator a	75	9	50	683	12.5	55	464	17	60	316	5.5	60	1,239
		SD a	5,500	50	97	5,330	55	97	4,330	60	97	3,497	60	97	3,497
	Novel	Two-stage MVR evaporator a	75	9	50	683	12.5	55	464	17	60	316	5.5	60	1239
		Superconcentration b	2,875	50	62	1113	55	65	804	60	80	1,198	60	80	1,198
		Post-granulation	5,400	80	97	3,143	80	97	2,741	88.5	97	1,183	88.5	97	1,183
		drying b
	% energy savings c (global process)				18			16			29			24
	% energy savings c (minus evaporator)				20			18			32			32

Table 4 . 4 .

 44 Physical properties, rehydration properties and flowability of dairy powders: lab scale granulated powders (G) and conventional

	spray-dried powders (SD) 3
			WPP		DWP		SMP		FFMP
			G	SD	G	SD	G	SD	G	SD
	Dry matter	(g.kg -1 )	990	977	993	982	990	975	993	975
	Bulk density (kg.m -3 )	717 ± 3	568 ± 2	730 ± 5	532 ± 4	800 ± 6	550 ± 7	606 ± 4	405 ± 5
	Tapped density	(kg.m -3 )	787 ± 6	686 ± 4	877 ± 8	657 ± 5	862 ± 7	660 ± 3	694 ± 0	508 ± 3
	Porosity	(%)	47	55	39	57	38	41	35	60
	d(0.1)	(µm)	139 ± 4	20 ± 0	33 ± 0	16 ± 0	289 ± 35	37 ± 0	231 ± 9	62 ± 2
	d(0.5)	(µm)	615 ± 14	148 ± 1	582 ± 9	73 ± 1	711 ± 37	105 ± 3	681 ± 7	169 ± 6
	d(0.9)	(µm)	1441 ± 9	324 ± 1	1380 ± 9	188 ± 13	1384 ± 28 224 ± 23	1428 ± 4	428 ± 40
	Span	(-)	2.1	2.0	2.3	2.4	1.5	1.8	1.8	2.2
	Solubility	(%)	96.8 ± 0.4 98.8 ± 0.0 99.8 ± 0.0 99.5 ± 0.0 76 ± 0.0	99.8 ± 0.0 76 ± 0.0	99.8 ± 0.0
	Dispersibility (%)	85.4 ± 1.2 93.6 ± 0.5 92.8 ± 0.7 93.5 ± 0.4 20.0 ± 3.0 94.3 ± 0.9 18.0 ± 3.6 86.4 ± 0.8
	Flowability	(g.min -1 )	47.8 ± 1.0 46.2 ± 1.8 39.9 ± 1.2 12.2 ± 0.3 48.9 ± 0.9 14.2 ± 0.4 26.1 ± 0.7 9.1 ± 0.2

3 WPP, whey permeate powder; DWP, demineralized whey powder; SMP, skim milk powder and FFMP, fat-filled milk powder; d(0.1), d(0.5) and d(0.9) represent the particle sizes below which 10%, 50% and 90% of the powder volume exists, respectively.

Table 4 .

 4

	5. Dry matter contents (w/w) for viscous phase (maximum superconcentration),
	cohesive phase and discrete non-sticky granules for different dairy compositions -whey
	permeate powder (WPP), demineralized whey powder (DWP), skim milk powder (SMP) and
	fat-filled milk (FFMP)
		WPP	DWP	SMP	FFMP
	Viscous phase	< 80%	< 80%	< 62%	< 65%
	Cohesive phase 80 -88% 80 -87%	62 -78%	66 -77%
	Discrete phase	>88%	>87%	>78%	>77%

Table 5 .

 5

	Dry matter	Yield stress	Consistency coefficient	Flow behaviour index
	% (w/w)	σy (Pa)	K (Pa s n )	n (-)
	61.1	13.1 ± 1.0	0.1 ± 0.01	1.0 ± 0.01
	64.7	5.9 ± 0.4	1.5 ± 0.3	0.7 ± 0.03
	70.0	10.1 ± 0.2	11.8 ± 0.6	0.6 ± 0.01
	72.9	17.2 ± 10.0	12.1 ± 5.5	0.6 ± 0.08
	77.8	128 ± 17.0	22.6 ± 11.0	0.6 ± 0.07
	Oscillation amplitude sweeps were performed to determine the stress required to
	initiate flow in the superconcentrates. Across all the DM contents studied (i.e. DM > 60%
	w/w), G' was higher than G'' at lower stresses. Concentration highly influenced the stress

1. Flow behaviour of skim milk microfiltrate (MFP) superconcentrates at different DM (% w/w) represented by Herschel-Bulkley model (R 2 =0.99) 4 . Measurements at 25°C. required to initiate flow, i.e. cross over point of G

  4 (superconcentrate:powder) backmix fractions. All experimental conditions are presented in Table 6.2.

	6.2.3 Physico-chemical and functional properties measurement
	6.2.3.1 Dry matter content of superconcentrates and granules

Table 6 . 2 .

 62 Description of experimental conditions to investigate granulation behaviour of superconcentrated dairy pastes 5

			Superconcentration			Granulation
	Exp. No.	Dairy ingredients	Dry matter of paste	Mass of paste	Experimental Factor	Powder added	Mass of powder added	Back-mix ratio (superconcentrate: powder)
			% (w/w)	g		-	g	-
	1. 2. 3.	WP WP WP	82.20 ± 0.06	195	Back-mix ratio (3-levels)	WP WP WP	160 200 240	1:0.8 1:1 1:1.2
	4. 5. 6.	DW DW DW	82.56 ± 0.09	195	Back-mix ratio (3-levels)	DW DW DW	160 200 240	1:0.8 1:1 1:1.2
	7. 8. 9.	WP WP WP	82.20 ± 0.06	195	Powder used for granulation	WPI WPI20 WPI10	200 200 200	1:1 1:1 1:1
	10. 11.	WP WP	82.20 ± 0.06	195	Back-mix ratio	WPI20 WPI20	100 80	1:0.5 1:0.4

Table 6 . 3 .

 63 Granulation performance of superconcentrated WP and DW pastes granulated with WP and DW powder respectively, at a back-mix fraction of 1:1 (superconcentrate:powder).

		Mass median particle size	Mass fraction passing 2 mm sieve	Product recovery from the mixer	Energy required for granulation
		mm	%	%	kJ
	Whey permeate, WP	1.82 ± 0.06	65.0 ± 2.0	96.0 ± 0.0	15.7 ± 0.3
	Demineralised whey, DW	3.40 ± 0.17	9.3 ± 4.0	80.0 ± 9.4	24.3 ± 1.7

Table .

 .

		7.1 Description of experimental conditions to investigate granulation behaviour of
	superconcentrated demineralised whey (DW) paste. 7
	Exp.	Dry powder	DW Superconcentration	Granulation
	No.	blend added	Dry matter	Mass of	Composition of dry	Mass of powder
			of paste	paste	powder blend	added
			% (w/w)	g	-	g
	1. 2.	IF 1 IF 2	82.20 ± 0.06	195	SMP-lactose (57:61) MPC-lactose (25:92)	118 117
	7.3 Results and discussion:
	7.3.1 Granulation of superconcentrated DW paste by addition of MPC and lactose or
	SMP and lactose
		The granulation behaviour of the superconcentrated DW paste (~82% w/w DM) by
	the addition of dry powder blends of SMP and lactose or MPC and lactose is presented in

Table 7 .

 7 2. Addition of dry blends of IF 2 resulted in up to 48% smaller granules size and 35%

Table 7 .

 7

	2 Granulation performance of superconcentrated demineralised whey (DW) paste
	granulated with dry blend of SMP and lactose (IF 1) or MPC and lactose (IF 2) at a back-mix
	fraction of ~1:0.6 (superconcentrate:powder).
	Dry powder blend added for granulation	Mass size median particle	Mass mm sieve fraction passing 2	Product mixer recovery from	Energy granulation required for	Cohesiveness
		mm	%	%	kJ	A
	IF 1	3.29	7.0	82.4	18.0	1.97
	IF 2	1.7	75.5	93.0	11.6	1.01

Table 7 . 3 .

 73 Functional properties of granulated powders 8

	Dry powder
	blend added for	Wettability Solubility index Drum flowability	Particle density
	granulation
		s	%	g.min -1	g.cm -3
	IF 1	immediate	98.0	194.4	1.48
	IF 2	immediate	95.4	183.0	1.42
	8 IF 1: superconcentrated demineralised whey paste granulated by a dry blend of skim milk
	powder and lactose,
	IF 2: superconcentrated demineralised whey paste granulated by a dry blend of milk protein
	concentrate powder and lactose,

TABLE OF TABLES

 OF

Table 2 .1

 2 Emerging technologies and innovations in pre-treatments to control rising viscosity.

Table 2 .

 2

	2 Comparison of Spray-drying (SD), Agitated thin-film drying (ATFD), Towerless
	drying (PST) and Extrusion porosification technology (EPT TM ).

Table 4 .

 4

	1. Composition of dairy powders used in the study; whey permeate powder (WPP),
	demineralized whey powder (DWP), skim milk powder (SMP) and fat-filled milk powder
	(FFMP).

Table 4 . 2 .

 42 Experimental conditions for superconcentrated paste and granulation by addition of dry powder at three levels.

Table 4 . 3

 43

. Estimation of the specific energy consumptions (SEC) for dehydration in conventional spray drying (SD) and novel superconcentration-granulation process.

Table 4 . 4

 44

. Physical properties, rehydration properties and flowability of dairy powders: lab scale granulated powders (G) and conventional spray-dried powders (SD).

Table 4 .

 4

5. Dry matter contents (w/w) for viscous phase (maximum superconcentration), cohesive phase and discrete non-sticky granules for different dairy compositions -whey

permeate powder (WPP), demineralized whey powder (DWP), skim milk powder (SMP) and fat-filled milk (FFMP).

Table 5 .1. Flow

 5

behaviour of skim milk microfiltrate (MFP) superconcentrates at different DM (% w/w) represented by Herschel-Bulkley model (R 2 =0.99). Measurements at 25°C.

Table 6 . 1 .

 61 Composition of dairy powders.

	Table 6.2. Description of experimental conditions to investigate granulation behaviour of
	superconcentrated dairy pastes.
	Table 6.3. Granulation performance of superconcentrated WP and DW pastes granulated
	with WP and DW powder respectively, at a back-mix fraction of 1:1
	(superconcentrate:powder).

Table 6 . 4 .

 64 Granulation performance manipulated by addition of WP, WPI or dry blends of WP/WPI into the superconcentrated WP paste at 82.20% w/w DM. All granulations were performed at a back-mix fraction of 1:1 (superconcentrate:powder).

Table 7

 7

	.1 Description of experimental conditions to investigate granulation behaviour of
	superconcentrated demineralised whey (DW) paste.
	Table 7.2 Granulation performance of superconcentrated demineralised whey (DW) paste
	granulated with dry blend of SMP and lactose (IF base -1) or MPC and lactose (IF base -
	2) at a back-mix fraction of ~1:0.6 (superconcentrate:powder).

Table 7 . 3 .

 73 Functional properties of granulated powders.

	RÉSUMÉ LONG:
	SÉCHAGE INNOVANT DE CO-PRODUITS LAITIERS
	1. Introduction générale
	Le lait est une source nutritionnelle de première qualité ; mais il est également
	rapidement périssable sous sa forme liquide d'origine. Par conséquent, la transformation du
	lait liquide en poudre stable avec une durée de conservation plus longue (jusqu'à 3 ans) est
	une opération essentielle. Le séchage par atomisation est le procédé de référence pour
	fabriquer industriellement des poudres laitières, car il permet d'obtenir d'atteindre des
	productivités élevées (jusqu'à 30 t.h -1 ) tout en garantissant les propriétés nutritionnelles et
	fonctionnelles souhaitées.

ECR values from Tanguy et. al. (2017); MVR, mechanical vapour recompression; a DM are typical values observed in industrial practice; b DM values taken from current study; c Novel vs. Conventional

Standard deviation (n = 2)

WP: whey permeate, DW: demineralised whey, WPI: whey protein isolate, WPI10 and WPI20: powders resulting from blending of WP and WPI in 90:10 and 80:20 proportion, respectively.

WPI20 and WPI10: dry blends of WP and WPI at 80:20 and 90:10 proportion. (std deviation, n = 3)

WPI20 in 90:10 and 80:20 proportion of WP:WPI, respectively. Error bars represent std. deviation, n=2.

LIST OF SYMBOLS

DAIRY POWDERS

The aim of this chapter was to contribute to the understanding of the dairy superconcentrates by evaluating the flow properties of pre-concentrated skim milk microfiltrate as model dairy system. The evolution of physical properties during superconcentration and granulation was investigated using rheological and shear-cell based techniques. The results showed that progressive increase in dry matter content during superconcentration led to a cohesive non-flowing state. The transition from concentrate to non-flowing regime manifested as a sharp increase in cohesiveness. A robust correlation was observed between the laboratory based rheological and shear cell measurements and the amperage of the mixer (that is to say the power consumption) during superconcentration.

The application of analytical techniques in order to establish critical process parameters were discussed. The chapter was published in the Journal of Food Engineering (Paper 3 ).

3 Published as: Patil, M.H., Tanguy, G., Floch-Fouéré, C.L., Murphy, E.G., Jeantet, R. Characterisation of superconcentration and granulation steps of a disruptive spray-drying free process for the manufacture of dairy powders. Journal of Food Engineering, 317, 110865. The main aim of this chapter were to:

-present relevant tools and techniques for the characterization of dairy superconcentrates -provide useful information regarding the flow behaviour during superconcentration to determine critical process parameters composition of these ingredients. WP and WPI were manually mixed in 90:10 and 80:20 proportion (w/w) to prepare blended powders, referred as WPI10 and WPI20 respectively. These blends were used to investigate the partial replacement of WP with WPI for granulation of superconcentrated paste, described in the following section 2.2. Firstly, a superconcentrated dairy paste at around 82% w/w DM was produced from powder using a high shear temperature-controlled mixer (TM6, Thermomix, Vorwerk, Germany). The powder (163 g) was mixed with demineralised water (37 g) under constant agitation (500 rpm) at a temperature of 50°C for 30 mins. The resulting pastes (~195 g) were immediately granulated by addition of equal quantity of respective dairy powders. The powder addition was performed manually over 50 s under constant agitation of 500 rpm, followed by high speed granulation for 10 s at 2000 rpm. The compacted layer along the wall of the mixer bowl was scraped manually and granulated for further 30 s at 500 rpm. The resulting granules were discharged from the mixer. An energy meter (Energy Logger 4000 FR, Voltcraft, Germany) logged power consumption (expressed as electric current, A) during the granulation process. Additionally, the superconcentrated paste was granulated with different superconcentrate:powder ratios (1:0.8, 1:1 and 1:1.2), to determine the effect of recirculation rate (back-mix fraction) on granulation. The granules obtained were stored in airtight containers for further physico-chemical analysis.

Furthermore, in order to determine the effect of the addition of WPI powder on the granulation behaviour, WPI was used to granulate the superconcentrated WP paste by full or partial replacement of the WP powder used for granulation. For partial replacement, preprepared dry blends WPI10 and WPI20 (section 2.1) were used. First, a superconcentrated the manufacture of infant formula in order to closely mimic the formula's desired composition, particularly lactose:protein and casein:whey protein ratios [START_REF] Blanchard | 18 -Infant formula powders[END_REF]Masum et al., 2020). Indeed, 25%, 40% and 43% of SMP, DW and lactose production, respectively find their final use in infant formula products application [START_REF] Franceagrimer | Etude sur l'évolution et les perspectives d'utilisation des matières grasses et protéiques d'origine laitière par les industries agro-alimentaires dans l'Union Européenne[END_REF]Future Market Insights, 2017). The novel superconcentration-granulation approach promised energy efficient manufacture of formulated powders from high lactose dairy streams and therefore may be applicable to the manufacture of IF protein bases. In this study, a superconcentrated demineralised whey paste was granulated by addition of a blend of SMP and lactose or milk protein concentrate (MPC) and lactose, with an objective to produce a formulated base powder with lactose:protein ratio and casein:whey protein ratio of 7.2:1.3 and 0.4:0.6, in conjunction with the stage 1 of infant formula composition [START_REF] Blanchard | 18 -Infant formula powders[END_REF]Masum et al., 2020). This base powder has the potential to reduce the number of individual macro ingredients for the infant formula manufacturers that can be investigated.

Materials and methods:

Materials

All dairy powders -DW, SMP, milk protein concentrate (MPC) and lactose were acquired from the commercial manufacturers. The two dry blends of SMP and lactose (SMP:lactose, 57:61) and MPC and lactose (MPC:lactose, 25:92) were prepared by manual mixing in a container.

Titre : Séchage innovant de co-produits laitiers innovant

Mots cléfs : Séchage -Surconcentration -Granulation -Phase cohésive -Formulation

Résumé :

Procédé de référence pour la production de poudres laitières, le séchage par atomisation est extrêmement énergivore. Son remplacement par un nouveau procédé en deux étapes, surconcentration et granulation, permettrait de réduire l'empreinte énergétique. Cette thèse visait à étudier l'évolution des propriétés physiques de différents co-produits laitiers au cours de la surconcentration et leur comportement lors de la granulation, afin de déterminer les paramètres critiques du procédé. La conduite de la surconcentration est limitée quand le surconcentré atteint un état peu fluide et hautement cohésif. L'extrait sec marquant le début de la phase cohésive dépend de la composition des produits : il est notamment supérieur pour les liquides à fortes teneurs en lactose. A l'inverse, le taux de recirculation de la poudre nécessaire à une granulation efficace est minimal en fin de phase cohésive. A phase cohésive comparable, les liquides à fortes teneurs en lactose présentent des différences notables de comportement à la granulation par rapport à d'autres fluides. Enfin, la granulation est améliorée avec des poudres d'isolats de protéines solubles. Dans l'ensemble, la forte cohésivité des surconcentrés, corrélée à leur teneur en protéines, impacte négativent le procédé de surconcentration-granulation. A l'inverse, la granulation avec des poudres protéiques améliore la conduite du procédé, ouvrant la voie à de nouvelles possibilités de formulation de produits complexes. En complément des outils et méthodes développés au cours de ce travail, une meilleure compréhension du comportement des surconcentrés, notamment à la granulation, est nécessaire pour développer et optimiser ce nouveau procédé aux fins de production durable de poudres laitières.

Title : Drying of dairy co-products through innovative processing

Keywords : Drying -Superconcentration -Granulation -Cohesive phase -Formulation Abstract : Spray drying (SD), although being the standard for producing dairy powders, is an extremely energy intensive process. Replacing SD by disruptive superconcentration-granulation process can reduce energy footprint as much as superconcentration is maximized. This PhD project aimed at understanding the science of dairy superconcentrates in order to identify critical parameters controlling the process. Namely, the evolution of physical properties and the granulation behaviour were characterized for different dairy streams. A dry matter (DM) dependent highly cohesive non-flowing state was shown to limit the extent of superconcentration, whereas the minimum dry powder recirculation for effective granulation was related to the end of cohesive phase. The onset of the cohesive phase was related to the composition of dairy products, and occurred at higher DM for high lactose streams.

Moreover, a striking dissimilarity in the granulation behavior was detected for high lactose streams with similar cohesive phase, which was likely due to their higher or lesser cohesiveness.

Finally, more efficient granulation was obtained by using whey protein isolate in relation to higher water holding capacity. Overall, high cohesiveness of superconcentrates, which may be related to protein content, negatively impacted superconcentration-granulation process. Conversely, granulation with protein powder showed positive effects, which opens new routes for formulating complex products. The better understanding of the superconcentrates and granulation behavior, in addition to tools and techniques provided by this work, enable the optimization, scale-up and expansion of the novel process for sustainable and profitable manufacturing of powders.