Imène Felfoul
email: imenef@gmail.com

Jennifer Burgain

Carole Perroud

Claire Gaiani

Joël Scher

Hamadi Attia

Jérémy Petit

Impact of spray-drying conditions on flow properties of skim dromedary 2 and cow's milk powders using the FT4 powder rheometer 3 4

Keywords: spray-drying, flowability, dromedary milk powder, basic flowability energy, specific energy spray drying, flowability, dromedary milk powder, basic flowability energy, 33 specific energy, shear test

Two sets of spray-drying conditions were applied on skim dromedary and cow's milks 15 to determine the effect of air outlet temperature (75 and 85 °C) and milk type on proximate 16 composition, particle size distribution, and flow properties of spray-dried powders. The particle 17 size distribution was more influenced by the chemical composition than the spray-drying 18 conditions. The air outlet temperature impacted on dromedary milk powder flowability, as the 19 basic flowability energy was significantly increased from 620 to 1018 mJ when the outlet air 20 temperature was increased from 75 to 85 °C, respectively. Specific energy, associated with the 21 flow performance of a powder in a low stress environment, were significantly higher for 22 dromedary milk powder at the higher air outlet temperature. However, spray-drying conditions 23 had no significant effect on specific energy values of cow's milk powder. Dromedary milk 24 powders had better flow behavior than cow's milk powders. Shear tests confirmed that powders 25 spray-dried at 75 °C had better flowability than powders spray-dried at 85 °C.

NOVELTY IMPACT STATEMENT

27 Powdering is particularly interesting for dromedary milk industrialization, as a way to 28 overcome problems related to production seasonality and commercialization in low production 29 regions. Using a lower air outlet temperature (75 °C) led to better flowability and lower 30 compressibility. Shear tests showed that cow's milk powders were more cohesive than 31 dromedary milk powders. 32

INTRODUCTION

35 Milk transformation into a powdered product by means of a drying process is widely used in 36 the dairy industry [START_REF] Hammes | Study 551 of the influence of soy lecithin addition on the wettability of Buffalo milk powder obtained by 552 spray-drying[END_REF]. Milk powder can be preserved 37 for longer times and its production reduces the cost of certain operations, such as packing, 38 storage, and transportation [START_REF] Kitamura | Experimental vacuum spray drying of 578 probiotic foods, included with lactic acid bacteria[END_REF]. Powdering processes allow the 39 almost complete preservation of nutrients [START_REF] Gaiani | 541 How surface composition of high milk proteins powders is influenced by spray-drying 542 temperature[END_REF]. In the case of dromedary milk, 40 powdering is particularly interesting for product industrialization, as a way to overcome 41 problems related to production seasonality and commercialization in low production regions.

42 Indeed, it is possible to store powdered milk in areas and periods of high production, in order 43 to fulfill the demand where and when dromedary milk is not produced enough.

44

The drying step determines the quality of resulting powder; the choice of drying method 45 and conditions is dictated by the nature of the material to be dried, the desired powder 46 functionalities, and economic factors [START_REF] Jinapong | Production of instant soymilk 566 powders by ultrafiltration, spray drying and fluidized bed agglomeration[END_REF]Esquivel & 47 Jiménez, 2012). Spray-drying is the most commonly used technique for industrial production 48 of milk powder, because it generates a product of good quality with better organoleptic and 49 nutritional features than drum-drying, while being faster and cheaper than freeze-drying 50 [START_REF] Ceballos | Effect of freezing rate on quality 513 parameters of freeze-dried soursop fruit pulp[END_REF][START_REF] Reddy | Influence of processing conditions on functional and reconstitution properties of milk 611 powder made from Osmanabadi goat milk by spray drying[END_REF].

51

To the best of our knowledge, only a few studies in the literature have been dedicated 52 to spray-dried dromedary milk powder and they were restricted to the determination of the 53 influence of the drying process on the nutritional, physicochemical, and organoleptic properties 54 of powders, as well as their storage stability. For instance, Sulieman, Elamin, Elkhalifa, and 55 Laleye (2014) reported that the physicochemical and functional properties of spray-dried 56 dromedary milk powder (water activity, powder color, solubility, flowability) were greatly 57 affected by the velocity difference (both in terms of intensity and direction) between feed and 58 air and they described thermal properties of freeze-dried whole and skim dromedary milks on

F o r R e v i e w O n l y F o r R e v i e w O n l y
5 84 achieved with the FT4 powder rheometer (Freeman Technology). Flow properties of various 85 food products have been determined using the FT4 powder rheometer: for instance, powdered 86 dairy products [START_REF] Lapčík | Surface energy analysis (SEA) and rheology of powder milk dairy products[END_REF], fermented milk-cereal mixtures [START_REF] Salameh | Physico-614 chemical and rheological properties of Lebanese kishk powder, a dried fermented milk-cereal 615 mixture[END_REF], and 87 plantain-based formulated powders (Gnagne, Petit, Gaiani, Scher, & Amani, 2017).

88

Two sets of spray-drying process conditions were applied on skim dromedary and cow's 89 milks in order to evaluate the impact of drying air temperature and milk type on the flow 90 properties of resulting milk powders. The proximal composition and the particle size 91 distribution were also evaluated in order to better understand the links between physicochemical 92 properties and flowability of studied powders. 98 whose lactation time ranged between 2 and 8 months. Cow's milk, collected from healthy 99 Holstein cows, was provided by a farm located in the north-east of France (experimental domain 100 of La Bouzule, ENSAIA, Laneuvelotte, France). Both milks were collected in sterile milking 101 cans, in which sodium azide was promptly added up to 0.02 % (w/w) to ensure microbiological 102 stability; after that, they were conveyed into a cool box filled with ice to the laboratory and then 103 stored at 4 °C until skimming. Both milks were skimmed at 3 000 × g during 20 min at 4 °C 104 (Gyrozen 1580MGR, Multi-purpose Centrifuge, Daejeon, Korea). Then, both skim milks were 105 stored at -20 °C until use for spray-drying. 111 equipped with a bi-fluid nozzle (Fluid Cap 60 100 + Air Cap 120, Spraying System, Wheaton, 112 Illinois, USA). Aspiration rate was set at 70 % for all performed assays. In the drying chamber, 113 the drying air cocurrently flowed with milk droplets sprayed from the bi-fluid nozzle. The 114 spray-dryer was fed at 27 °C with a FAST Load peristaltic pump (VWR,Leuven,Belgium,48 115 mm internal pipe diameter). Dromedary and cow's milks were first skimmed, then stored at -116 20 °C. For spray-drying experiments, both skim milks were thawed out at 4 °C overnight. Last, 117 both milks were prewarmed at 27 °C in a water bath to improve spray-drying production 118 efficiency (mainly, pumping of the concentrate through the bi-fluid nozzle).

119

The spray-drying conditions such as air inlet and outlet temperatures as well as drying 120 air flow rate suitable for the production of skim milk powder were selected not only based on 121 literature [START_REF] Schuck | Analytical Methods for Food and Dairy 617 Powders[END_REF] and but also after preliminary experimentations in 122 order to obtain skim milk powders suitable for storage, i.e. having a moisture content not 123 exceeding 5 % (w/w) [START_REF] Schuck | Analytical Methods for Food and Dairy 617 Powders[END_REF][START_REF] Felfoul | Impact of spray-drying conditions on physicochemical properties and rehydration 520 ability of skim dromedary and cow's milk powders[END_REF]. Two assays 124 differing in spray-drying conditions were carried out with a view to evaluate the influence of 125 air outlet temperature (Table 1). It is worth noting that C1 and C2 spray-drying conditions 126 differed in outlet air temperature (circa 74.5 and 84.5 °C, respectively) and feed flow-rate (about 127 75 and 67 mL.min -1 , respectively).

128 With this experimental setup, the process parameter having the most influence on 129 powder physicochemical and functional properties should be the air outlet temperature rather 130 than the feed flow-rate. The feed flow rate was a little higher in C1 conditions (Table 1), 131 meaning that liquid velocity at the nozzle outlet was a little higher and resulting in slightly 132 lower air-liquid velocity difference at the nozzle outlet; this should result in slightly larger 133 droplets [START_REF] Petit | A dimensional analysis approach for modelling the size of droplets 605 formed by bi-fluid atomisation[END_REF][START_REF] Mandato | 598 Liquids atomization with two different nozzles: modelling of the effects of some processing 599 and formulation conditions by dimensional analysis[END_REF] and thus slightly larger spray-dried particles

F o r R e v i e w O n l y 7
134 in C1 conditions. However, the effect of feed flow-rate was likely negligible compared to the 135 one of drying air temperature, because the air-liquid velocity difference at the nozzle outlet was 136 not markedly affected by changing the feed flow-rate, as the air velocity is much greater than 137 the liquid velocity at the outlet of a bi-fluid nozzle [START_REF] Petit | A dimensional analysis approach for modelling the size of droplets 605 formed by bi-fluid atomisation[END_REF][START_REF] Mandato | 598 Liquids atomization with two different nozzles: modelling of the effects of some processing 599 and formulation conditions by dimensional analysis[END_REF].

138

The powders were then collected, immediately packed in sealed polyethylene 139 terephthalate bags, and stored at 10 °C until analysis. All analyses were carried out within a 140 week after powder production. Spray-drying yield was calculated as the ratio between the dry 141 mass of powder and the dry mass of feed skim milk.

Proximate composition of skim dromedary and cow's milks and powders

143 Moisture, protein, and ash contents of skim milks and powders were determined 144 according to the AOAC standard methods (A.O.A.C., 2016). Moisture content was determined 145 by mass loss after drying 10 mL milk or 3 g powder in an air oven at 105 °C for 7 h. Ash content 146 was obtained by mass loss after incinerating 10 mL milk sample or 3 g powder in a furnace at 147 550 °C during 6 h. Proteic nitrogen content was determined by the Kjeldahl method (Vapodest, 148 Gerhardt GmbH & Co. KG, Königswinter, Germany) and protein content was deduced from 149 nitrogen content using a conversion factor of 6.38 [START_REF] Felfoul | Impact of spray-drying conditions on physicochemical properties and rehydration 520 ability of skim dromedary and cow's milk powders[END_REF].

150 Fat content of milks (using 11 mL milk sample) was determined according to the Gerber 151 acid-butyrometric method (ISO 11870:2009) using 10 mL of sulfuric acid in order to dissolve 152 milk proteins. The fat separation was carried out by centrifugation in a butyrometer at 1100 153 tr/min for 5 min at 65 °C in the presence of 1 mL of isoamyl alcohol. Fat content of powders 154 was determined using the Folch method (1957) by performing solvent extraction of 2 g powder 155 using a mix of 2:1 (v/v) chloroform/methanol.

156

Carbohydrates content was deduced by difference with other components assuming that 157 vitamins were weakly represented [START_REF] Felfoul | Impact of spray-drying conditions on physicochemical properties and rehydration 520 ability of skim dromedary and cow's milk powders[END_REF].

Flow properties of skim dromedary and cow's milk powders 170

Powder flowability was evaluated using the FT4 powder rheometer (Freeman 171 Technology, Worcestershire, UK) as previously reported in the literature [START_REF] Freeman | Characterisation of powder bulk, dynamic flow and shear 537 properties in relation to die filling[END_REF] 172 [START_REF] Freeman | Characterizing Powder Flow[END_REF]. Stability, compressibility, and rotational shear tests were performed according 173 to the FT4 standard methods. For each test, a powder sample was poured into a FT4 cylindrical 174 vessel of 50 mm diameter. To ensure analytical reproducibility, FT4 analyses include a 175 preliminary step of powder conditioning, performed by a rotational and vertical moving (both 176 downwards and upwards) through the powder sample of a blade (5° helix angle and 48.0 mm 177 diameter) at constant tip speed of 60 mm.s -1 . This operation removes any packing history such 178 as pre-consolidation or excess entrapped air and generates a uniform powder packing in order 179 to ensure repeatable and comparable data [START_REF] Freeman | Measuring shear properties 534 and normal stresses generated within a rotational shear cell for consolidated and non-535 consolidated powders[END_REF]. The mass of 180 skim milk powders was automatically recorded by the FT4 apparatus. The different tests carried 181 out with the FT4 powder rheometer are described hereafter. [START_REF] Freeman | Measuring the flow properties of consolidated, conditioned and 531 aerated powders-a comparative study using a powder rheometer and a rotational shear cell[END_REF]. The bulk 195 density (BD) was also obtained from the stability test and was calculated as the ratio of sample 196 mass to vessel volume (85 mL).

197

The specific energy (SE) is the energy needed to displace the conditioned powder bed 198 during upwards testing divided by the mass of analyzed powder sample (Equation 2). This gives 199 an indication of powder flow properties in a loosely packed and unconfined state (Mitra et al., 200 2017). SE is mostly related to intrinsic particle properties, such as cohesion, particle size and 201 shape distributions, and surface roughness. 215 [START_REF] Freeman | Measuring the flow properties of consolidated, conditioned and 531 aerated powders-a comparative study using a powder rheometer and a rotational shear cell[END_REF][START_REF] Bian | Effect of chaff on bulk flow 511 properties of wheat[END_REF]. Compressed bulk density (CBD) is 216 calculated from bulk density (BD) and compressibility (Equation 3). 235 applied normal stress is thus the so-called yield locus (Gnagne, Petit, Gaiani, Scher, Amani, 236 2017). Shear cell parameters, major principal stress (MPS), unconfined yield stress (UYS), 237 cohesion, and flow function (FF), were then determined with the FT4 software by the Mohr 238 circle approach using following equations 4-6 (ASTM, 20044-6 (ASTM, , 2005)).

239 𝑀𝑃𝑆 = (𝜎 𝑥 + 𝜎 𝑦 ) 2 + ( 𝜎 𝑥 -𝜎 𝑦 2 ) 2 + 𝜏 2 𝑥𝑦 ( 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛

247

The cohesion was evaluated as the intercept of the linear regression of the yield locus curve.

248 The flow factor (FF) is the most commonly used parameter for the description of powder 249 flowability [START_REF] Bian | Effect of chaff on bulk flow 511 properties of wheat[END_REF]. (Equation 6):

250 𝐹𝐹 = 𝑀𝑃𝑆 𝑈𝑌𝑆 (𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 6) 251
The higher the flow factor, the better the powder flows. [START_REF] Schuck | Analytical Methods for Food and Dairy 617 Powders[END_REF]. 308

Particle size distribution of dromedary and cow's milk powders 309

The granulometric characteristics of powders are shown in Table 3. All powders 310 exhibited monomodal particle size distributions centered around 14 -20 µm (Figure 1), 311 consistently with previous results obtained with the same combination of pilot spray-dryer and 312 bi-fluid nozzle (Guérin et al., 2017;[START_REF] Khanji | Structure, gelation, and antioxidant properties of curcumin-doped casein micelle 573 powder produced by spray-drying[END_REF]. It should be mentioned that a significant 313 effect of both milk type and spray-drying conditions was observed on the particle size 314 distribution of spray-dried milk powders. The median particle sizes (D 50 ) as well as the particle 315 size distributions of spray-dried milk powders significantly differed according to spray-drying 316 conditions and milk type. As for the milk type, significantly higher median particle sizes (D 50 )

317 were obtained for partially skim cow's milk powders whatever the spray-drying conditions.

318 This result could be explained by the difference in fat content between milk samples (Table 2):

319 indeed, partially skim cow's milk being richer in lipids, obtained powder was expected to be 320 stickier and then have a higher tendency to form agglomerates, leading to increased median 321 particle size. This hypothesis was confirmed by the significantly higher fines proportion of 322 partially skim cow's milk powders whatever the spray-drying conditions. As regards the 323 influence of spray-drying conditions, when increasing the air outlet temperature, median 324 particle size of partially skim dromedary and cow's milk powder was significantly higher but 325 no significant effect was observed for partially skim dromedary milk powders (Table 3). The 326 results obtained for skim cow's milk powders could be attributed to the fact that a higher drying 327 temperature and a lower feed-rate accelerates the drying rate of droplets, promoting the fast 328 formation of a crust at their surface and therefore limit shrinkage, which leads to larger particles 329 (Kim, Chen, & Pearce, 2002). The fact that the particle size distribution of skim dromedary 330 milk was unaffected by spray-drying conditions may be an indication that crust formation did

F o r R e v i e w O n l y F o r R e v i e w O n l y F o r R e v i e w O n l y F o r R e v i e w O n l y F o r R e v i e w O n l y F o r R e v i e w O n l y 20
456 also studied the flowability of milk powders differing in particle size and composition (skim 457 milk and whey powders), concluding that the particle size significantly affects flowability:

458 smaller particles possess more contact points with the neighboring particles, thus making 459 rearrangements of the powder bed more difficult and impairing its flowability. The poor 460 flowability of milk powders was also probably due to their irregular surface particles, leading 461 to increased interparticular interactions and particle interlocking [START_REF] Hart | Effect of Particle Size on Detergent Powders Flowability and 554 Tabletability[END_REF]. Similar

462 observations were reported for different pharmaceutical powders by [START_REF] Lindberg | Flowability measurements of pharmaceutical powder mixtures with poor 594 flow using five different techniques[END_REF]. The spray-drying process has a significant impact on milk powder flow properties. In 465 this study, partially skim dromedary and cow's milks were spray-dried in different conditions 466 (of air outlet temperature and feed flow-rate) without prior concentration to avoid introducing 467 another source of process variability. Partially skim dromedary and cow's milk powders were 468 very cohesive. The spray-drying conditions, the milk type, the particle size distribution as well 469 as the residual fat content showed a great influence on flow properties of partially skim 470 dromedary and cow's milk powders. Using a higher air outlet temperature (85 °C) led to poorer 471 flowability and higher compressibility whatever the milk type. Shear tests showed that partially 472 skim cow's milk powders were more cohesive than partially skim dromedary milk powders.

473 The shear flow properties of milk powders are expected to be influenced by fat content, particle

  dromedarius) milk was obtained from a local breeding located in 96 the south of Tunisia (Gabès governorate, Tunisia). Dromedary milk was constituted of pooled 97 milk samples assembled from twenty different healthy dromedaries (Camelus dromedarius),

106 2 . 2 .

 22 Spray-drying of skim dromedary and cow's milks107 Skim dromedary and cow's milks were sieved at 50 µm mesh size (test sieve, Mesh S-108 Steel, 200 mm diameter, 50 mm height, Retsch GmbH, Germany) in order to remove the coalesced fat globules. Spray-drying of skim milks was performed in a 110 MicraSpray MS 150 simple effect pilot plant spray-dryer(Anhydro, Soeborg, Denmark)

  142

  158

  of powders were determined with a laser granulometer 161 (Mastersizer 2000 Malvern Instruments, UK), equipped with He-Ne laser (632.8 nm 162 wavelength), using the Aero S module for dry dispersion in the following conditions: 2 bar; 100 163 % air pressure; 100 % feed rate; 3 mm hopper gap. The mean diameter in volume was chosen 164 as particle size estimator. Classical granulometric parameters D 10, D 50 , and D 90 were recorded 165 (D X corresponds to the size for which X % of the particle population has smaller size). Span 166 was calculated to evaluate the width of the particle size distribution (equation 1)(Felfoul et al., 167 2020): 168 𝑆𝑝𝑎𝑛 = (𝐷 90 -𝐷 10 ) 𝐷 50 Equation (1) 169

  is designed to assess the flow energy changes upon repeated testing, it 186 consists in seven successive conditioning and test cycles. Stability test cycles were conducted 187 by the rotating movement of the blade into and through a powder bed contained in the 188 measurement cell (85 mL glass cylinder of 50 mm diameter) at 100 mm.s -1 tip speed and 5° 189 helix angle. During both downwards and upwards rotational movement of the blade, the torque 190 (related to shear stress τ) and axial force (related to normal stress σ) required to move the blade 191 though the powder were recorded and used to calculate the total energy input required to make 192 the powder bed flow. The basic flowability energy (BFE), corresponding to the stabilized flow 193 energy, was evaluated as the flow energy required to displace a conditioned powder sample 194 during downwards movement of the blade in the seventh test cycle

202

  𝑆𝐸 = (𝑢𝑝𝑤𝑎𝑟𝑑𝑠 𝑓𝑙𝑜𝑤 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑐𝑦𝑐𝑙𝑒 6 + 𝑢𝑝𝑤𝑎𝑟𝑑𝑠 𝑓𝑙𝑜𝑤 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑐𝑦𝑐𝑙𝑒 7) (2 × 𝑠𝑝𝑙𝑖𝑡 𝑚𝑎𝑠𝑠) evaluates the change in powder density according to applied 206 normal stress. A powder sample was first placed in the measurement cell (i.e. two overlaid 207 50 mm × 85 mL glass cylinders) and submitted to three conditioning cycles performed as 208 previously described. After that, the vessel was split to remove any excess powder and the 209 dynamic blade was replaced by the 48 mm vented piston. Then, powder was slowly compressed vented piston vertically moving at 0.05 mm.s -1 and applying normal stresses σ from 0.5 211 to 15 kPa(0.5, 1, 2, 4, 6, 8, 10, 12, and 15 kPa) while measuring its volume change. After 212 reaching equilibrium at the target stress, the distance traveled by the vented piston was 213 measured and the compressibility was calculated as a percentage change in volume after 214 compression (C p ). Compressibility is indicative of the level of cohesion between particles

  are widely used to evaluate the flowability of powders in high-220 shear applications. The rotational shear cell of the FT4 powder rheometer consists of a vessel 221 containing the powder samples and a FT4 shear head applying both normal and rotational shear 222 stresses to the powder bed. Powder sample was first conditioned using the helical blade as 223 previously described[START_REF] Quintanilla | Correlation between 607 bulk stresses and interparticle contact forces in fine powders[END_REF]. After the conditioning step, 224 powder sample was slowly subjected for 60 s to a normal stress of 9 kPa carried out with a 225 vented piston vertically moving at a maximum speed of 0.08 mm.s -1 . Then, the vessel 226 containing the powder sample was split, the vented piston was changed for the shear cell head, 227 and the powder sample was recompressed at 9 kPa to remove any disturbances caused by the 228 split and ensure that the powder bed surface was properly consolidated. The preconsolidated 229 powder sample was then submitted to a preshearing stage (60 s at 9 kPa normal stress) carried 230 out with the rotational shear cell accessory (48 mm diameter), to achieve a critically 231 consolidated state. The normal stress was then reduced lowered and the incipient shear stress 232 necessary to cause powder bed failure and initiate flow was measured. The preshear/shear 233 sequence was repeated five times at decreasing normal stresses σ from 7 to 3 kPa by 1 kPa steps .min -1 rotational speed. The curve representing the evolution of incipient shear stress with

  stress (Pa), σ x : stress at horizontal axis (Pa), σ y : stress at vertical axis (Pa) 𝜏 𝑥𝑦 242 𝑈𝑌𝑆 = 𝑃 𝐴 (𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 5) 243 Where P: axial load at failure (N), 245 With: A 0 is the initial horizontal area of the powder bed (m²) and ε designates the axial strain 246 of powder sample (s -1 ).

633 Fig. 1 .Fig. 2 .Fig. 3 .

 123 Fig. 1.

  Powders were classified by Jenike

	306 the ideal temperature range for spray-drying cow's milk are 175 -240 °C for air inlet
	307 temperature and 70 -90 °C for air outlet temperature
		F F F
		o r o r o r
		R R R
		e v i e w e v i e w e v i e w
		O O O
		n l n l n l
		y y y
	252 (1980) according to FF values as follows:
	253	-FF < 1: not flowing,
	254	-1 < FF < 2: very cohesive,
		14

255

-2 < FF < 4: cohesive, 256 -4 < FF < 10: easy-flowing,

  474 size distribution, particle morphology, particle interlocking, packing of smaller particles in void 475 spaces, and surface irregularity. Furthermore, in order to help industrializing dromedary milk 476 powder production, further investigations should focus on biochemical and technological 477 (gelation, coagulation, etc.) properties of dromedary milk powders.

	632 Figure captions
	F F F F F F F F
	o r o r o r o r o r o r o r o r
	R R R R R R R R
	e v i e w e v i e w e v i e w e v i e w e v i e w e v i e w e v i e w e v i e w
	O O O O O O O O
	n l n l n l n l n l n l n l n l
	y y y y y y y y
	478 CONFLICT OF INTEREST
	479 The authors declare no conflict of interest.
	22 25 28

Table 1 .

 1 Spray-drying conditions used for the production of skim dromedary and cow's milk powders.

	Skim milk	Powder sample	Spray-drying	Air inlet	Air outlet	Feed flow
	sample	acronym	conditions	temperature	temperature	rate
				(°C)	(°C)	(mL.min -1 )
	Dromedary Dromedary_C1	C1	230.0 +/-1.3 74.5 +/-0.7	74.7 +/-2.1
	Cow	Cow_C1	C1	230.4 +/-1.6 74.4 +/-0.9	76.5 +/-2.2
	Dromedary Dromedary_C2	C2	230.3 +/-1.8 84.3 +/-1.1	65.4 +/-3.7
	Cow	Cow_C2	C2	230.6 +/-1.3 84.8 +/-0.8	69.5 +/-2.9

Table 2 .

 2 Proximate composition of dromedary and cow's skim milks and powders produced in the two spray-drying assays.

		SDM	SCM	Dromedary_C1 Dromedary_C2	Cow_C1	Cow_C2
	Total solids (%(w/w)	9.52 ± 0.51 x	12.05 ± 0.29 x	92.96 ± 0.33 c	97.01 ± 0.16 a	94.44 ± 0.12 b	96.63 ± 0.08 a
	on wet basis)
	Proteins (%(w/w) on	32.88 ± 1.97 x	29.46 ± 2.70 y	22.50 ± 2.48 a	23.71 ± 0.90 a	25.73 ± 2.24 a	27.00 ± 2.38 a
	dry basis)
	Fat (%(w/w) on dry	7.35 ± 0.09 y	10.04 ± 0.13 x	12.20 ± 0.47 b	12.33 ± 1.89 b	16.35 ± 1.00 a	15.89 ± 0.68 a
	basis)
	Ash (%(w/w) on dry	8.93 ± 0.04 x	6.64 ± 0.09 y	10.74 ± 0.38 a	8.90 ± 0.18 b	6.63 ± 0.23 c	6.41 ± 0.34 c
	basis)
	Carbohydrates	50.84 ± 0.02 y	53.86 ± 0.05 x	53.74 ± 2.07 ab	55.66 ± 0.21 a	51.31 ± 2.62 b	50.71 ± 0.15 b
	(%(w/w) on dry basis)
	Water activity (-)	-	-	0.391 ± 0.002 a	0.203 ± 0.003 d	0.385 ± 0.002 b 0.274 ± 0.002 c

For milks and powders separately, means with different superscripted letters in the same row were significantly different according to Tukey's HSD test (p ˂ 0.05). SDM: skim dromedary milk; SCM: skim cow's milk.

Table 3 .

 3 Granulometric characteristics of skim dromedary and cow's milk powders.Means with different superscripted letters in the same row were significantly different according to Tukey's HSD test (p ˂ 0.05).

		Dromedary_C1	Dromedary_C2	Cow_C1	Cow_C2
	D 10 (μm)	6.41 ± 0.06 d	6.55 ± 0.10 c	7.54 ± 0.06 b	8.28 ± 0.03 a
	D 50 (μm)	14.83 ± 0.08 c	14.84 ± 0.21 c	17.09 ± 0.18 b	19.19 ± 0.10 a
	D 90 (μm)	35.09 ± 0.36 c	32.68 ± 0.34 d	37.43 ± 0.50 b	43.04 ± 0.33 a
	Span (-)	1.93 ± 0.02 a	1.76 ± 0.01 c	1.75 ± 0.01 c	1.81 ± 0.01 b

Table 4 .

 4 Flow properties issued from stability, compressibility, and shear cell tests of skim dromedary and cow's milk powders deduced from the FT4 stability test.Means with different superscripted letters in the same row were significantly different according to Tukey's HSD test (p ˂ 0.05).

		Dromedary_C1 Dromedary_C2	Cow_C1	Cow_C2
	BFE (mJ)	620 ± 27 b	1 018 ± 65 a	917 ± 81 a	907 ± 68 a
	SE (mJ.g -1 )	8.49 ± 0.46 c	12.73 ± 0.12 b	15.52 ± 0.14 a	15.59 ± 0.30 a
	BD (g.mL -1 )	0.31 ± 0.00 a	0.31 ± 0.00 b	0.24 ± 0.00 d	0.25 ± 0.00 c
	CBD at 15 kPa	0.47 ± 0.00 b	0.53 ± 0.00 a	0.48 ± 0.01 b	0.49 ± 0.01 b
	(g.mL -1 )
	Compressibility at	34.2 ± 1.1 c	42.6 ± 0.7 b	48.8 ± 1.2 a	49.1 ± 1.2 a
	15 kPa (%)
	Cohesion (kPa)	4.07 ± 0.13 b	4.01 ± 0.35 b	5.16 ± 0.05 a	5.06 ± 0.36 a
	FF (-)	1.21 ± 0.04 a	1.24 ± 0.11 a	1.02 ± 0.02 b	1.09 ± 0.06 ab

Table 5 .

 5 Pearson's correlation coefficients between flow and physicochemical properties.

		Compressibility	Cohesion	FF (-)	BFE (mJ)	SE (mJ.g -1 )	Fat content	D 50 (μm)
		at 15 kPa (%)	(kPa)				(%(w/w) on
							dry basis)
	Compressibility at 15
		-	0.85	-0.77	0.74	0.99	0.80	0.80
	kPa (%)
	Cohesion (kPa)	0.85	-	-0.97	0.28	0.83	0.99	0.88
	FF (-)	-0.77	-0.97	-	-0.19	-0.75	-0.97	-0.77
	BFE (mJ)	0.74	0.28	-0.19	-	0.76	0.34	0.28
	SE (mJ.g -1 )	0.99	0.83	-0.75	0.76	-	0.86	0.78
	Fat content (%(w/w)
		0.88	0.99	-0.97	0.34	0.86	-	0.87
	on dry basis)
	D 50 (μm)	0.80	0.88	-0.76	0.28	0.78	0.87	-

Journal of Food Processing and Preservation


PhD Jeremy Petit
email: jeremy.petit@univ-lorraine.fr

PhD Tristan Fournaise

Jennifer Burgain

Carole Perroud-Thomassin

Jeremy Petit

Carole Perroud- Thomassin

Jérémy Petit

/casein ratio on the reconstitution and flow properties of spray-dried dairy protein powders

Keywords: Dairy powders, Formulation, protein composition, Spray-drying, reconstitutability, Flowability Dairy powders, formulation, protein composition, spray-drying, reconstitutability, flowability Highlights Conceptualization, Investigation, Methodology, Validation, Writing, Review & Editing Project administration, Conceptualization, Supervision, Review Project administration, Conceptualization, Supervision, Methodology, Validation

Impact of the whey protein

Reviewer 1: This study focuses on the impact of the whey protein/casein ratio on the physicochemical and functional characteristics of dairy protein powders, which has been widely investigated.

The novelty of this paper can be improved if the spray-dried samples are furthur investigated from a chemical perspective.

It is true that the link between physicochemical properties and functionalities of powdered whey proteins and caseins has been widely investigated, but the novelty of this study comes from the focus on the impact of formulation (including chemical composition but also the preparation protocol of the concentrate to be spray-dried). The study presented in this manuscript was a part of a larger study who delt with the impact of formulation (pretreatment of concentrates prior to spray-drying, lipid content, protein content, and whey proteins/caseins ratio) on physicochemical and functional properties of spray-dried dairy powders. The part of this larger study dealing with concentrate pretreatment (air incorporation by stirring with a blade usually used for foaming purposes) and lipid content has already been published in Powder Technology (https://doi.org/10.1016/j.powtec.2020.05.085).

1. DSC, XRD, XPS and Raman investigations are suggested to reveal the mechanisms of the known phenomena at a chemical/molecular level. DSC and XPS have already been carried out on the samples of the present study but showed no differences between samples (from DSC, it was observed that all samples were amorphous and their glass transition temperature was hardly measurable; from XPS, it was shown that their surface was enriched in lipids and proteins, as lipids tend to migrate to the droplet surface by buoyancy during the drying process, whereas proteins, being surface-active molecules, are generally found more concentrated at water/air interfaces (i.e. at the surface of droplets). Thus, it was decided not to present these analyzes in the manuscript.

Response to Reviewers DRX would not be more informative, as samples were amorphous. As for Raman spectroscopy, it could be useful to elucidate the structure of dairy proteins in produced samples, but the aim of the larger study was focused on physicochemical and functional properties of spray-dried dairy powders, not on the structure of individual particles constituting the powders.

2. Thermal stability effects are suggested to be studied for different whey protein/casein ratio. This would be interesting, but the objective of the study was to find appropriate formulation conditions to improve reconstitutability and flowing properties of spray-dried dairy powders.

Thermal stability, preservation ability, gelation properties, water-holding capacity, etc. are another functional properties of interest for spray-dried dairy powders, but they did not fall into the scope of the larger study.

3. The crumpled surfaces can be calculated with math models with diffusion coefficiency.

The present study was intended to elucidate the most influent parameters (amongst formulation and powder physicochemical properties) on powder reconstitutability and flowing properties.

For example, it was evidenced that particle shape can counteract the influence of particle size on powder flowing properties (i.e. samples richer in caseins had larger particles of more wrinkled shape and flowed more poorly). However, the mechanisms leading to particle shape were not the subject of the present study. The subject of research includes in this journal. Research work is interesting. In the paper titled "Impact of the whey protein/casein ratio on the reconstitution and flow properties of spray-dried dairy protein powders".

The authors in their article discuss a very important aspect study focuses on the impact of the whey protein/casein ratio on the physicochemical and functional characteristics of dairy protein powders spray-dried. Research work is interesting.

Thank you for your interest in our work. We hope that modifications performed following your comments will help in improving manuscript quality.

The paper needs minor revision because:

1.

The quality of the language is insufficient. Have a native speaker or similar assist you.

The language level was improved by using a more sustained language when possible (cf.

modifications in blue in revised manuscript lines 16-18, 20-23, 24, 27, 40, 43, 64, 131, 190, 212, 231, 248, 253, 255, 280 and 282-310).

2.

The Introduction is quite random with broad and varying statements and not very well focused.

Indeed, the introduction was a little too vague, as we initially wanted to make the link with the

3.

In my opinion, the final conclusions should also be drawn up as they are not as specific as the scope of work.

The final conclusion was too exhaustive, we have modified it to make it more concise and better focused (cf. modifications in blue in the manuscript lines 280 and 282-310).

4.

Please standardise the number of decimal places in the tables. Please use a precision of at least two decimals places.

We initially chose to present results with 3 significant numbers, thus the number of decimals depended on the values. Thus, we followed reviewer #2's advice for table 3 (cf. modifications in blue). For tables 1 and 2, it was not possible to add more decimals, as data of table 1 came from supplier sheets which were not more accurate than presented values and data of table 2 were calculated from date of table 1. Moreover, one decimal was added to water activity results, as the accuracy of the analytical equipment was sufficient to allow this (cf. modifications in blue).

5.

Please explain why the authors chose such a statistical analysis?

ANOVA and Tukey's HSD test are very classical statistical analysis methods: the first one allows identifying the presence of significant differences amongst sample results, whereas the second one permits to sort samples according their significant differences. Pearson's correlation coefficients are also rather classically used to link parameters between them: it permits to calculate the degree of linear correlation between two variables; in the present study, the degree of correlation between analyzed parameters was determined to help identifying the main physicochemical parameters influencing each analyzed functional properties.

6.

Could an additional PCA analysis have been performed?

Yes, indeed, PCA could have been performed, and it would have given similar information as Pearson's correlation coefficients calculated for all pairs of investigated parameters (as PCA intends to correlate all investigated parameters in a single statistical analysis, whereas Pearson's correlation coefficients are able to indicate the degree of correlation between two parameters only). We chose to discuss results with Pearson's correlation coefficients that we estimate easier to understand for readers than PCA.

7.

Improve the readability of the Figure 3,4.

The readability of figures 3 and 4 has been enhanced (cf. figures 3 and 4 in figures file).

8.

References is incorrectly formatted number [17 -21 and 31].

The reference 17 has been modified, but it's a French PhD so the title should be kept in French.

Other concerned references are norms that were already correctly formatted: these references have already been published in Powder Technology with this format (https://doi.org/10.1016/j.powtec.2020.05.085).

INTRODUCTION

Technofunctional properties of dairy powders such as reconstitutability (wettability, swelling, dispersibility and solubility) and flowability are crucial for the agri-food industry and consumers.

Wettability is the ability of powders to immerse without stirring [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF]. Powder wettability is sensitive to several parameters like particle size: smaller particles generally induce poorer wettabilityPowder surface composition also affects the wetting time: an increase in the proportion of surface lipids is deleterious for powder wettability due to higher hydrophobicity [START_REF] Sharma | Functionality of Milk Powders and Milk-Based Powders for End Use Applications-A Review[END_REF][START_REF] Kim | Surface characterization of four industrial spray-dried dairy powders in relation to chemical composition, structure and wetting property[END_REF][START_REF] Fournaise | Main powder physicochemical characteristics influencing their reconstitution behavior[END_REF].

Dispersibility corresponds to the dissociation of particle agglomerates into single particles and, in a lesser extent, to the attrition of particle agglomerates and single particles. For dispersibility indices lower than 95 %, the powder is considered as non-dispersible: for example, in the case of whole milk (containing 26 % lipids) for which the high proportion of surface lipids impairs interactions with water, the dispersibility index is 89.5 %. Moreover, The presence of caseins has been identified known to strongly reduce dairy powder dispersibility (indeed, the dispersibility index of casein powders is around 26 %) [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF].

Solubility represents the ability of powders to dissolve into water, which corresponds to the disappearance complete loss of granular structure once the product is reconstituted [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF]. Like wettability, solubility decreases with lipids proportion or particle size. In addition, it was evidenced that a high casein content is related to a poor solubility, whereas whey proteins promote dairy powder solubilization [START_REF] Felix Da Silva | Casein-Based Powders: Characteristics and Rehydration Properties[END_REF][START_REF] Singh | Controlling milk protein interactions to enhance the reconstitution properties of whole milk powders -A minireview[END_REF][START_REF] Mimouni | Investigation of the microstructure of milk protein concentrate powders during rehydration: Alterations during storage[END_REF]. Powder flowability is highly linked to its physicochemical properties: large, smooth, and spherical particles having a low water content are known to have good flow properties [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF][START_REF] Mitra | Influence of moisture content on the flow properties of basundi mix[END_REF][START_REF] Kim | Effect of surface composition on the flowability of industrial spray-dried dairy powders[END_REF]. As matters the influence of the whey protein/casein ratio, a high proportion of caseins is known to lead to wrinkled and dense particles, with poorer flow properties [START_REF] Sadek | To What Extent Do Whey and Casein Micelle Proteins Influence the Morphology and Properties of the Resulting Powder?[END_REF]. The flowability can be characterized in low and high stress conditions by a rotational powder rheometer such as the FT4 in order to obtain several flow parameters, especially basic flowability energy (BFE) and SE (specific energy) in low stress conditions or flow factor (ff) and cohesion in high stress conditions [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF][START_REF] Jenike | Gravity Flow of Bulk Solids[END_REF][START_REF] Freeman | Measuring the flow properties of consolidated, conditioned and aerated powders -A comparative study using a powder rheometer and a rotational shear cell[END_REF][START_REF] Freeman | Measuring shear properties and normal stresses generated within a rotational shear cell for consolidated and non-consolidated powders[END_REF].

The present study focuses on the impact of composition on reconstitutability and flowability of dairy protein powders. Investigated powders differed in protein composition (variable casein/whey protein ratio) and were produced by spray-drying at pilot scale. AfterwardsThen, the reconstitutability of spray-dried powders was evaluated through the wettability, dispersibility, and solubility indices classically employed in the dairy industry.

MoreoverBesides, flowability of dairy protein powders was assessed through measurement of BFE, SE, compressibility, cohesion, and ff with the FT4 powder rheometer. Finally, physicochemical properties of spray-dried powders (particle size distribution, appearance of particle surface, water content, and water activity) were determined and related to powder reconstitution and flow behavior.

MATERIAL AND METHODS

Material

Whey proteins isolate (PRODIET 90 S) and micellar casein powder (PRODIET 87 B) were provided by Ingredia (Arras, France). Their proximate composition in wet basis was calculated from technical sheets (Table 1).

Table 1. Proximate composition expressed on wet basis of whey protein isolate and phosphocaseinate powders employed for the preparation of dairy concentrates to be spray-dried.

Preparation of milk concentrates

Five concentrates differing in protein composition were produced from 100 % whey protein isolate (protein mixture 1, PM1), 75 % whey protein isolate and 25 % casein powder (protein mixture 2, PM2), 50 % whey protein isolate and 50 % casein powder (protein mixture 3, PM3), 25 % whey protein isolate and 75 % casein powder (protein mixture 4, PM4), and 100 % casein powder (protein mixture 5, PM5). Concentrates were prepared by reconstituting adequate amounts of powders in distilled water at 15 % (w/w) dry extract under mechanical stirring (ZR 2102 control, Heidolph, Germany) between 200 and 500 rpm with a 4-bladed impeller (2 cm length, 1 cm width). Once the powder mixture was completelytotally immersed, the solution was stirred during 2 h at room temperature (20 °C), then at 4 °C under slight agitation at 300 rpm overnight to ensure complete powder reconstitution.

The proximal composition of spray-dried concentrates was calculated from technical sheets of reconstituted powders (Table 2). The dispersion conditions were adapted to each powder: air pressure between 2 and 3 bar, feed rate between 60 and 90 %, and hopper length between 2 and 3 mm. Three replicates were analyzed for each sample. The mean diameter in volume was chosen as size estimator. Classical granulometric parameters (D10, D50, and D90) were calculated, where Dx designates the diameter for which x % of the particles have a smaller size. The width of the particle size distribution was evaluated with the span (Eq. ( 1)) [START_REF] Salameh | Physico-chemical and rheological properties of Lebanese kishk powder, a dried fermented milk-cereal mixture[END_REF]. Thermo VG Scientific, East Grinstead, England). Observations were performed at × 500 magnification under partial vacuum using a scanning electron microscope (Hitachi S-4800,

Japan) operating at 1.0 kV acceleration voltage [START_REF] Gaiani | Étude des mécanismes de réhydratation des poudres laitières[END_REF].

Powder interactions with water

Water activity

Water activity was measured with a HygroPalm23-AW portable water activity-meter (Rotronic, France) at room temperature (20 °C), using a 3 cm powder layer (15 ± 1 g) in a cup similarly to ISO 21807:2004 [START_REF] Afnor | Microbiology of food and animal feeding stuffs -Determination of water activity[END_REF].

Water content

The water content was measured by the loss of water mass of 2.0 g ± 0.1 mg powder after drying during at least 3 h at 103 °C until a constant dry powder mass was obtained following ISO 5537:2004 [START_REF] Afnor | Dried milk -Determination of moisture content (Reference method)[END_REF].

Reconstitutability

Reconstitutability of spray-dried powders was evaluated through wettability, dispersibility and solubility indices.

2.5.3.1.Wettability

The wettability index is defined as the time in seconds taken by 13.0 ± 0.1 g powder to become wet when gently deposited at the surface of 125.0 g ± 0.1 g distilled water in a 250 mL beaker, following IDF 87:1979 [START_REF] Idf | Détermination de la dispersibilité et de la mouillabilité[END_REF].

2.5.3.2.Dispersibility

Dispersibility was measured by a method adapted from IDF 87: 1979 [START_REF] Idf | Détermination de la dispersibilité et de la mouillabilité[END_REF]. 13.0 ± 0.1 g powder was gently deposited on the surface of 125.0 g water at 20 °C placed in a 400 mL beaker. The mixture was manually stirred for 20 s, then it was allowed to rest for 30 s. After that, the mixture was passed through a 120 μm-mesh sieve. The total dry matter of the liquid, corresponding to the dispersibility index, was determined by drying at 103 ° C for 5 h following (Eq. ( 2)).

𝐷𝑖𝑠𝑝𝑒𝑟𝑠𝑖𝑏𝑖𝑙𝑖𝑡𝑦 (%) =. 𝑊×𝑇 100-(𝐸+𝑇) × 100 𝑆 (2) with:
-W: distilled water mass (g); -T: dry matter content (% w/w) of the liquid;

-E: dry matter content (% w/w) of sampled powder;

-S: sampled powder mass (g).

2.5.3.3.Solubility

Solubility was characterized as the percentage of powder able to dissolve in distilled water following ISO 8156:2005 [START_REF] Afnor | Dried milk and dried milk products -Determination of insolubility index[END_REF]. It was determined by the addition of 2.5 ± 0.1 g powder in 17.5 mL distilled water in a 50 mL centrifugation tube followed by 30 s stirring and centrifugation at 2 294 × g during 10 min (Megafuge 8R, Thermoscientific, France).

AfterwardsThen, the supernatant was removed, and the pellet was added to 20 mL distilled water, followed by stirring and centrifugation at 2 294 × g during 10 min. The pellet was desiccated at 103 °C overnight to determine the mass of insoluble residues, which allows determining the solubility index (Eq. ( 4)).

𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 (%) =.

(𝑆-𝑀) × 100 𝑆 [START_REF] Fournaise | Main powder physicochemical characteristics influencing their reconstitution behavior[END_REF] with:

-M: undissolved sediment mass after desiccation (g);

-S: sampled powder mass (g).

Flow properties of dairy protein powders

Flowability properties were evaluated using a FT4 powder rheometer with 25 mm accessories (Freeman Technology, Worcestershire, UK).

Stability test

The stability test was performed through a series of seven conditioning cycles by rotating the blade (-5° helix angle, 23.5 mm diameter, 100 mm.s -1 tip speed) and measurements of the flow energy of the powder bed [START_REF] Mitra | Influence of moisture content on the flow properties of basundi mix[END_REF][START_REF] Freeman | Measuring the flow properties of consolidated, conditioned and aerated powders -A comparative study using a powder rheometer and a rotational shear cell[END_REF].

This test allows determining two flow indices: BFE and SE. The BFE is linked to the flow behavior of the powder bed, and a low BFE generally corresponds to an easy-flowing behavior, whereas a high BFE is often obtained for poorly-flowing powders. The SE is much linked to powder cohesion [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF][START_REF] Mitra | Influence of moisture content on the flow properties of basundi mix[END_REF] with high SE values associated with cohesive powders.

Freeman Technology provides a classification of powder flow behavior on the basis of the SE calculated by the FT4 powder rheometer by:

-SE < 5 mJ/g: low cohesion -5 mJ/g < SE < 10 mJ/g: moderate cohesion -SE > 10 mJ/g: high cohesion.

Compressibility test

The compressibility test is used to measure the evolution of powder density as a function of normal stress from 0.5 to 15 kPa applied by a 23.5 mm vented piston to the powder sample placed in a 25 mm × 10 mL measurement cell [START_REF] Mitra | Influence of moisture content on the flow properties of basundi mix[END_REF][START_REF] Silva | Flowability and wetting behaviour of milk protein ingredients as influenced by powder composition, particle size and microstructure[END_REF][START_REF] Vasilenko | Shear and flow behavior of pharmaceutical blends -Method comparison study[END_REF].

The compressibility test allows obtaining the bulk density (BD), i.e. powder density before compression, similar to aerated density, and the compressed bulk density (CBD) for powder density after compression. Particle size, shape, surface composition, and roughness are known to influence these compressibility parameters [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF].

Shear cell test

The shear cell test was carried out to evaluate powder flowability and cohesion under high shear stress after a precompaction at 9 kPa. The powder bed was subjected to normal stresses of 7 to 3 kPa by 1 kPa steps in a 25 mm × 10 mL measurement cell [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF][START_REF] Mitra | Influence of moisture content on the flow properties of basundi mix[END_REF][START_REF] Freeman | Measuring the flow properties of consolidated, conditioned and aerated powders -A comparative study using a powder rheometer and a rotational shear cell[END_REF][START_REF] Freeman | Measuring shear properties and normal stresses generated within a rotational shear cell for consolidated and non-consolidated powders[END_REF][START_REF] Vasilenko | Shear and flow behavior of pharmaceutical blends -Method comparison study[END_REF].

The shear test is used to determine the powder cohesion and the flow factor (ff), calculated by the yield locus method from the shear stress vs. normal stress curve [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF]. Powder flowability has been classified by Jenike [START_REF] Jenike | Gravity Flow of Bulk Solids[END_REF] from flow factor values:

-ff < 1: not flowing -1 < ff < 2: very cohesive -2 < ff < 4: cohesive -4 < ff < 10: easy flowing -f > 10: free flowing.

Statistical analysis

All analyses were triplicated to ensure a good analytical repeatability and presented values correspond to means ± standard deviations. Data were statistically analyzed by one-way ANOVA performed with Excel version 2016 and means were separated by Tukey's HSD test at p < 0.05 significance level. The degree of correlation between powder physicochemical and functional properties was evaluated by calculating Pearson's correlation coefficients.

RESULTS AND DISCUSSION

Particle size distribution

Dairy protein powders spray-dried at pilot scale were composed of fine particles ranging from 1 to 200 µm (Figure 1 & Table 3), consistently with the particle size range generally obtained with the employed pilot spray-dryer [START_REF] Guerin | Lactobacillus rhamnosus GG encapsulation by spray-drying: Milk proteins clotting control to produce innovative matrices[END_REF][START_REF] Khanji | Structure, gelation, and antioxidant properties of curcumin-doped casein micelle powder produced by spray-drying[END_REF][START_REF] Nikolova | Is it possible to modulate the structure of skim milk particle through drying process and parameters?[END_REF][START_REF] Mandato | Liquids' atomization with two different nozzles: Modeling of the effects of some processing and formulation conditions by dimensional analysis[END_REF]. Their median particle size D50 was comprised between 11.5 and 18.9 µm, particle size distributions were monomodal, and moderate dispersion around the median value was denoted by the span values ranging from 2.1 to 2.5.

Consistently with literature, the median particle size of protein powders increased with casein proportion of spray-dried concentrates (Table 2) [START_REF] Crowley | Influence of protein concentration on the physical characteristics and flow properties of milk protein concentrate powders[END_REF], as evidenced by the significantstrong positive correlation obtained between these two parameters according to a Pearson's correlation coefficient of 0.932. This may be explained by the fact that during drying, droplets of concentrates rich in caseins quickly form a crust at their surface [START_REF] Sadek | To What Extent Do Whey and Casein Micelle Proteins Influence the Morphology and Properties of the Resulting Powder?[END_REF], which results in larger particles than spray-dried protein powders containing a greater whey protein proportion. No significant influence of protein composition on the span of the particle size distribution was denoted. Table 3. Granulometric parameters of PM 1, PM 2, PM 3, PM 4, and PM 5 powders.

3.2.Particle appearance

SEM images of protein powders (Figure 3) showed that protein composition influenced particle shape: particle sphericity decreased at higher casein proportion, confirming previous literature results [START_REF] Sadek | To What Extent Do Whey and Casein Micelle Proteins Influence the Morphology and Properties of the Resulting Powder?[END_REF][START_REF] Shrestha | Effect of protein concentration on the surface composition, water sorption and glass transition temperature of spray-dried skim milk powders[END_REF][START_REF] Kelly | Influence of protein concentration on surface composition and physico-chemical properties of spray-dried milk protein concentrate powders[END_REF]. This may confirm that the surface of droplets of concentrates rich in caseins had rapidly formed a crust when put in contact with the hot drying air and warped during the drying step, leading to non-spherical particles. It may be concluded that whey protein concentrate droplets did not form a crust upon drying, thus the spherical shape of droplets was kept during the drying step, leading to globally spherical whey protein particles.

Powder interactions with water

Water activity of spray-dried dairy protein powders was low, ranging from 0.16 to 0.26 (Table 4), ensuring an excellentgood protection against spoilage during storage [START_REF] Roos | Importance of glass transition and water activity to spray drying and stability of dairy powders[END_REF], as the ideal water activity of food products, especially dairy, is comprised between 0.2 and 0.3 [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF]. The good preservability of produced powders was confirmed by their water content, not exceeding the threshold of 5 % (w/w) on wet basis generally advised for dairy powders [START_REF]FAO (Food and agriculture organization of the united nations)[END_REF]. The slight variation of water content of spray-dried dairy protein powders in spite of identical manufacturing conditions may come from the difference in hygroscopicity of whey proteins and caseins along with the impact of formulation on the drying kinetics.

The fineness of spray-dried dairy protein powders caused their poor wettability, as expected for powders spray-dried at pilot-scale [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF][START_REF] Guerin | Lactobacillus rhamnosus GG encapsulation by spray-drying: Milk proteins clotting control to produce innovative matrices[END_REF][START_REF] Neves | Encapsulation of curcumin in milk powders by spray-drying: Physicochemistry, rehydration properties, and stability during storage[END_REF]. Their dispersibility was very low (around 5 -10 %), highlighting a high level of powder cohesion (which may be an indication of poor flowability). The influence of each dairy protein type could not be deduced from these results, because all powders were hardly dispersible [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF][START_REF] Sadek | To What Extent Do Whey and Casein Micelle Proteins Influence the Morphology and Properties of the Resulting Powder?[END_REF], leading to insignificant differences between powder samples [START_REF] Freudig | Dispersion of powders in liquids in a stirred vessel[END_REF][START_REF] Hla | Wetting behaviour of instantized cocoa beverage powders[END_REF]. The impact of protein type was much more interesting on powder solubility. Indeed, a linear decrease (Eq. 5, R² = 98.62 %) of powder solubility was observed when increasing the casein content of spray-dried dairy protein powders (Table 4-B & Figure 5).

𝑠𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 (%) = 99.554 -0.698 × 𝑐𝑎𝑠𝑒𝑖𝑛 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (%) (Eq. 5)

This can be related to literature indicating that the presence of casein constitutes a limiting factor for powder reconstitutability, especially by impairing solubility [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF][START_REF] Felix Da Silva | Casein-Based Powders: Characteristics and Rehydration Properties[END_REF][START_REF] Burgain | Surface chemistry and microscopy of food powders[END_REF][START_REF] Burgain | Links between particle surface hardening and rehydration impairment during micellar casein powder storage[END_REF].

MoreoverBesides, as whey proteins and lactose from dairy protein powders are known to solubilize well more easily and quickly than caseins [START_REF] Mimouni | Investigation of the microstructure of milk protein concentrate powders during rehydration: Alterations during storage[END_REF], the decrease in their contents upon increase in casein content (cf. Table 2) may contribute to the observed solubility decrease. As regards the impact of mineral content, which increased with casein content in studied powders (cf. Table 2), Mimouni et al. (2010) [START_REF] Mimouni | Investigation of the microstructure of milk protein concentrate powders during rehydration: Alterations during storage[END_REF] showed that calcium, phosphorus, and magnesium, which constitute the majority of dairy minerals as they are principally associated with casein micelles, have rather slow reconstitution kinetics; this also contributes to the solubility decrease of investigated powders upon casein content increase. Last, lipids content in studied powders was low and very close (cf. Table 2), thus the negative impact of these highly hydrophobic Fig. 3. Evolution of powder solubility with casein content for spray-dried dairy protein powders.

Powder flow properties

Flow parameters deduced from the stability, compressibility, and shear cell tests are gathered in Table 5.

BFE of spray-dried dairy protein powders increased in the following order: PM1 < PM5 ≈ PM2 < PM3 < PM4. No significantclear influence of protein composition on basic flowability energy was denoted, except that 100 % whey protein powder (PM1) had slightly better flowability. As its smaller median particle size and larger span should be deleterious for flow properties, it can be hypothesized that its better flowability may arise from the more spherical shape of its particles. Specific flow energy is generally more informative regarding flowability classification. In the present studyHere, specific flow energy results indicate that PM1 and PM5 are expected to better flow and be moderately cohesive, followed by PM2, PM3, and PM4 that are expected to flow more poorly and have higher cohesion. AfterwardsThen, powders with better flowability were either composed of very small particles of relatively high heterogeneity but rather spherical shape (PM1) or small particles of moderate heterogeneity and wrinkled shape (PM5). Indeed, powder flowability in generally improved for large particles, of low heterogeneity, and/or of spherical shape [START_REF] Fournaise | Impact of formulation on reconstitution and flowability of spray-dried milk powders[END_REF][START_REF] Mitra | Influence of moisture content on the flow properties of basundi mix[END_REF][START_REF] Kim | Effect of surface composition on the flowability of industrial spray-dried dairy powders[END_REF][START_REF] Sadek | To What Extent Do Whey and Casein Micelle Proteins Influence the Morphology and Properties of the Resulting Powder?[END_REF].

Compressibility at 15 kPa was identical for all spray-dried dairy protein powders (Figure 4), presumably owing to their relatively close particle size distributions and their composition dominated by dairy proteins. It can be guessed that their cohesion was not too different, as no marked differences of compressibility have been obtained. In fact, cohesion, deduced from the shear cell test, was comprised between 1.55 and 1.75 kPa for all investigated powders, indicating a rather high level of cohesion, and thus a low flowability. ff values were found in the range from 2.78 to 3.12, showing that all investigated powders had close flow properties, exhibiting a cohesive behavior and a low flowability. No significant impact of protein composition was evidenced, but trends indicated that cohesion was slightly raised and ff slightly reduced for powders containing more caseins (Figure 5); this was confirmed by the highly negative Pearson's correlation coefficient between casein proportion and ff (R = -0,762) and the moderately positive Pearson's correlation coefficient between casein proportion and cohesion (R = 0,570). Thus, flowability under normal stress was a little improved for powders richer in whey proteins.

CONCLUSION

This study is part of a wider research project intending to evaluate the relative influences of formulation (whey proteins/caseins ratio preparation and composition of concentrates) and spray-drying conditions on physicochemical and functional properties of spray-dried dairy powders. The final aim of the project is to establish process relationships relating formulation and spray-drying conditions to powder reconstitutability, flowability, and preservability.

In the present study, a focus was carried out on the influence of protein composition on functional properties of spray-dried dairy protein powders. Thus, dairy protein powders differing in whey protein/casein ratio were spray-dried in identical the same conditions in order to isolate the influence of protein composition.

In the present study, a focus was carried out on the influence of whey protein/casein ratio on functional properties in identical spray-drying conditions.

All powders presented similar particle size distributions, which were monomodal with a moderate dispersion of particle sizes around the median value. A marked influence of formulation was denoted by the small increase in median particle size at higher casein proportion.

Particle size distributions of spray-dried powders varying in whey proteins/caseins ratio were similar (they were monomodal and had comparable span), but a marked influence of formulation was denoted by the small increase in median particle size at higher casein proportion.

This was explained by the different drying behaviors of droplets of whey protein and casein concentrates: formulations richer in caseins are known to lead to the formation of a crust at the droplet surface, resulting in larger and wrinkled particles, whereas droplets containing more whey proteins are known to lead to more spherical particles of smaller size. These hypotheses of different drying mechanisms, consistent with the literature, were validated by the differences in particle appearance between powders containing more whey proteins and those composed of more caseins.

Formulations richer in caseins are known to lead to the formation of a crust at the droplet surface, resulting in larger and wrinkled particles, whereas droplets containing more whey proteins are known to lead to more spherical particles of smaller size.

Water content and water activity of all spray-dried powders were low, ensuring their good preservability.

Owing to the low median particle size obtained for powders spray-dried at pilot scale, wettability and dispersibility of produced powders were very low according to industrial standards, and no significant difference due to powder formulation was evidenced. Solubility linearly varied with the whey protein/casein ratio (it was presumably impaired by the presence of a casein crust at the particle surface) and this process relationship may help industrials formulating their spray-dried dairy protein powders.

Flowability of spray-dried dairy protein powders was poor and interparticular cohesion was high, presumably due to the small median particle size of powder spray-dried on the employed pilot-scale equipment. For all powders, the influence of their particle size distribution seemed to overwhelm the impact of powder formulation. However, a slight improvement of flow properties was observed for dairy protein powders containing more whey proteins and less caseins. This may arise from the more spherical shape of this kind of powders. Powder flowability, mainly controlled by median particle size, should be improved for powders spraydried at industrial scale, which are generally well larger.

Reviewer 2 :

 2 Manuscript Number: POWTEC-D-21-00864

Fig. 1 .

 1 Fig. 1. Particle size distributions (in volume) of PM 1, PM 2, PM 3, PM 4, and PM 5.

Fig. 2 .

 2 Fig. 2. SEM micrographs of spray-dried protein powders varying in protein composition (in the increasing order of casein proportion: PM 1 (A); PM 2 (B); PM3 (C); PM 4 (D); PM 5 (E)) acquired at × 500 and × 2000 magnifications.

components [ 1 , 2 , 3 ]

 123 [START_REF] Gaiani | Surface composition of dairy powders observed by X-ray photoelectron spectroscopy and effects on their rehydration properties[END_REF] [START_REF] Schuck | Analytical Methods for Food and Dairy Powders[END_REF][START_REF] Sharma | Functionality of Milk Powders and Milk-Based Powders for End Use Applications-A Review[END_REF][START_REF] Kim | Surface characterization of four industrial spray-dried dairy powders in relation to chemical composition, structure and wetting property[END_REF][START_REF] Gaiani | Surface composition of dairy powders observed by X-ray photoelectron spectroscopy and effects on their rehydration properties[END_REF] on powder solubility could not be observed in the present study.

Fig. 4 :

 4 Fig. 4: Compressibility results for PM 1, PM 2, PM 3, PM 4, and PM 5.

Fig. 5 :

 5 Fig. 5: Shear test results for PM 1, PM 2, PM 3, PM 4, and PM 5.

Fig. 1 .

 1 Fig. 1. Particle size distributions (in volume) of PM 1, PM 2, PM 3, PM 4, and PM 5.

Fig. 2 .

 2 Fig. 2. SEM micrographs of spray-dried protein powders varying in protein composition (in the increasing order of casein proportion: PM 1 (A); PM 2 (B); PM3 (C); PM 4 (D); PM 5 (E)) acquired at × 500 and × 2000magnifications.

Figure( s )

 s Figure(s) Click here to access/download;Figure(s);Revised figures.docx

Figure 3 .

 3 Figure 3. Evolution of powder solubility with casein content for spray-dried dairy protein powders.

Fig. 4 :

 4 Fig. 4: Compressibility results for PM 1, PM 2, PM 3, PM 4, and PM 5.

Fig. 5 :

 5 Fig. 5 : Shear test results for PM 1, PM 2, PM 3, PM 4, and PM 5.

Table 2 .

 2 Proximate composition on dry basis of spray-dried concentrates.

	2.3. Production of dairy powders by spray-drying
	Spray-drying was carried out on a Micra Spray MS 150 single effect spray-dryer (Anhydro,
	Soeborg, Denmark), equipped with a bi-fluid nozzle (Fluid Cap 60 100 + Air Cap 120, Spraying
	System, Wheaton, Illinois, USA). Spray-drying conditions were set at 200 °C drying air inlet
	temperature and 85 °C outlet temperature in view to obtain powders with good preservation
	abilities (i.e., water activity ranging from 0.2 to 0.3) [1,15]. The concentrate was fed by a
	FASTLoad peristaltic pump (VWR, Leuven, Belgium; 4.8 mm internal pipe diameter) and the

feed concentrate flow rate was regulated at circa 68.0 mL.min -1 to maintain the drying air outlet temperature at 85 °C

[START_REF] Guerin | Lactobacillus rhamnosus GG encapsulation by spray-drying: Milk proteins clotting control to produce innovative matrices[END_REF]

. Powders were bagged just after sampling to limit degradations susceptible to occur during storage (such as caking induced by moisture uptake) and stored at 10 °C until analyses.

2.4. Physicochemical characterization

2.4.1. Particle size distribution

Particle size measurements were carried out with a laser granulometer (Mastersizer S, Malvern Instruments Ltd, Malvern, UK) supplied with a 5 mW He-Ne laser operating at a wavelength of 632.8 nm with a 300 RF lens. Air dispersion was achieved with the Aero S module (Malvern).

Table 4 .

 4 Water content, water activity, wettability, dispersibility, and solubility of spray-dried dairy protein powders.

Table 5 :

 5 Flowability characteristics (from stability, compressibility, and shear cell tests) of PM 1, PM 2, PM 3,

	PM 4, and PM 5.

Table 1 .

 1 Proximate composition expressed on wet basis of whey protein isolate and phosphocaseinate powdersemployed for the preparation of dairy concentrates to be spray-dried.

Table 2 .

 2 Proximate composition on dry basis of spray-dried concentrates.

			Whey
		Proteins		Caseins	Lipids	Lactose	Minerals
	Concentrates		proteins
		(%)		(%)	(%)	(%)	(%)
			(%)
	PM 1	90.5	90.5	0.0	1.1	5.8	2.6
	PM 2	89.7	70.6	19.1	1.2	4.9	4.2
	PM 3	88.9	50.7	38.3	1.3	4.0	5.8
	PM 4	88.1	30.7	57.4	1.5	3.0	7.4
	PM 5	87.4	10.8	76.6	1.6	2.1	9.0

Table 3 .

 3 Granulometric parameters of PM 1, PM 2, PM 3, PM 4, and PM 5 powders. ± 0.03 18.92 a ± 0.13 51.33 a ± 0.15 2.42 a ± 0.00 Table(s) Click here to access/download;Table(s);Revised Tables.docx Means with different superscripted letters in the same column were significantly different according to Tukey's HSD test (p ˂ 0.05; n = 3).

	Moisture	Proteins	Whey
	Powders PM 1 PM 2 PM 3 PM 4 PM 5 (%) 5.5 5.61 Powder D10 (µm) 3.79 e ± 0.01 11.82 d ± 0.11 31.02 d ± 0.53 2.31 b ± 0.05 D50 (µm) D90 (µm) Span (-) 3.92 d ± 0.04 11.53 d ± 0.04 28.51 e ± 0.34 2.15 c ± 0.02 4.34 c ± 0.03 12.80 c ± 0.23 33.81 c ± 0.43 2.29 b ± 0.02 5.20 b ± 0.00 16.93 b ± 0.14 47.04 b ± 0.22 2.47 a ± 0.01 (%) proteins (%) Caseins (%) Lipids (%) Lactose (%) Minerals (%) Whey proteins isolate 85.5 85.5 0 1.0 5.5 2.5 Casein powder 5.0 83.0 10.8 72.2 1.5 2.0 8.5

a

Table 4 .

 4 Water content, water activity, wettability, dispersibility, and solubility of spray-dried dairy protein powders. ± 3.52 11.74 a ± 0.98 0.41 a ± 0.01 0.22 a ± 0.01 47.15 a ± 2.27 1.75 a ± 0.24 2.79 a ± 0.25 PM5 50.28 ab ± 0.90 8.35 c ± 0.99 0.39 a ± 0.03 0.22 a ± 0.00 44.31 a ± 4.27 1.72 a ± 0.24 2.80 a ± 0.25

	Dairy protein	Water content (%
			Water activity	Wettability	Dispersibility	Solubility
	powder	(w/w) on wet
			(-)	(min)	(%)	(%)
	sample	basis)
	PM 1	4.82 a ± 0.14	0.21 c ± 0.00	> 90	5.38 b ± 0.08	99.26 a ± 0.00
	PM 2	3.82 c ± 0.12	0.20 d ± 0.00	> 90	4.83 c ± 0.09	88.35 b ± 0.21
	PM 3	3.78 c ± 0.04	0.23 b ± 0.00	> 90	5.06 c ± 0.14	65.58 c ± 2.58
	PM 4	3.78 c ± 0.15	0.16 e ± 0.00	> 90	8.91 a ± 0.13	53.11 d ± 2.80
	PM 5	4.38 b ± 0.17	0.25 a ± 0.00	> 90	3.26 d ± 0.08	40.12 e ± 2.90
	Basic
		Specific flow	Compressed
	flowability			Bulk density	Compressibility	Cohesion	Flow factor ff
		energy SE	bulk density
	energy BFE			BD (g/mL)	at 15 kPa (%)	(kPa)	(-)
		(mJ/g)	CBD (g/mL)
	(mJ)
	PM1 38.72 b ± 3.87 8.85 bc ± 0.62 0.34 b ± 0.03 0.19 b ± 0.01	43.28 a ± 2.33	1.65 a ± 0.16	3.01 a ± 0.16
	PM2 50.87 a ± 5.19 10.55 ab ± 0.39 0.36 ab ± 0.01 0.20 b ± 0.00	46.16 a ± 1.62	1.55 a ± 0.09	3.12 a ± 0.10
	PM3 56.45 a ± 6.01 11.12 a ± 0.77 0.37 ab ± 0.00 0.20 ab ± 0.01 45.73 a ± 2.96	1.79 a ± 0.24	2.78 a ± 0.25
	PM4 59.43

Means with different superscripted letters in the same column were significantly different according to Tukey's HSD test (p ˂ 0.05; n = 3). Tableau 5 : Flowability characteristics (from stability, compressibility, and shear cell tests) of PM 1, PM 2, PM 3, PM 4, and PM 5. a

ACKNOWLEDGEMENTS

The authors acknowledge support of the LIBio by the "Impact Biomolecules" project of the "Lorraine Université d'Excellence" (Investissements d'avenir -ANR). Moreover, the authors acknowledge the help given by the LIBio scientific team for the training and provision of analytical devices.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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