Alla Nesterenko

Isabelle Alric

Françoise Silvestre

Francoise Silvestre

Vanessa Durrieu
email: vanessa.durrieu@ensiacet.fr

Vegetable proteins in microencapsulation: a review of recent interventions and their effectiveness

Keywords: vegetable proteins, microencapsulation, soy proteins, pea proteins, spray-drying, coacervation coacervation/phase separation, gelation, solvent evaporation, supercritical fluid expansion, interfacial polymerization

Introduction

Microencapsulation consists of the isolation of active substances (in the liquid, solid or gas state), to obtain products with spherical form and micrometric size, in which the active material or core, is shielded by a membrane from the surrounding environment. This technique can be applied for different purposes: protecting sensitive substances from the surroundings, development of controlled release properties, masking of unpleasant taste and odor of the substances, dilution of core material when it must be used in very small amounts or transformation of liquid compounds into mobile solids. Microencapsulation allows the creation of a physical barrier between the core and wall materials and the protection of sensitive ingredients (flavors, antioxidants, polyunsaturated oils, vitamins, drugs…) from the external medium, particularly, moisture, pH and oxidation. The release of microparticle content at controlled rates can be triggered by shearing, solubilization, heating, pH or enzyme action. This technology has different applications in the food, biomedical, pharmaceutical and cosmetic industries as well as in agriculture and catalysis [START_REF] Dubey | Microencapsulation technology and application[END_REF]. The structure of microparticles is generally classified into microcapsules with a single core surrounded by a Wall material particularly affects the microparticles' stability, the process efficiency and the degree of protection of the active core. Materials commonly used as carriers in the makeup of encapsulated ingredients, are synthetic polymers and co-polymers, and bio based materials such as carbohydrates, fats, waxes, and animal and plant derived proteins. chemical agents [START_REF] Patel | Molecular dynamics study of the encapsulation capability of a PCL-PEO based block copolymer for hydrophobic drugs with different spatial distributions of hydrogen bond donors and acceptors[END_REF]. Polysaccharides studied as a matrix for microencapsulation are starches [START_REF] Jeon | The suitability of barley and corn starches in their native and chemically modified forms for volatile meat flavor encapsulation[END_REF][START_REF] Murúa-Pagola | Preparation of starch derivatives using reactive extrusion and evaluation of modified starches as shell materials for encapsulation of flavoring agents by spray drying[END_REF], maltodextrin [START_REF] Krishnan | Microencapsulation of cardamom oleoresin: Evaluation of blends of gum arabic, maltodextrin and a modified starch as wall materials[END_REF][START_REF] Saénz | Microencapsulation by spray drying of bioactive compounds from cactus pear (Opuntia ficus-indica)[END_REF][START_REF] Semyonov | Microencapsulation of Lactobacillus paracasei by spray freeze drying[END_REF], gum arabic [START_REF] Kim | Microencapsulation properties of gum arabic and several food proteins: spray-dried orange oil emulsion particles[END_REF][START_REF] Shaikh | Microencapsulation of black pepper oleoresin[END_REF], pectin [START_REF] Drusch | Sugar beet pectin: A novel emulsifying wall component for microencapsulation of lipophilic food ingredients by spray-drying[END_REF][START_REF] Gharsallaoui | Utilisation of pectin coating to enhance spray-dry stability of pea protein-stabilised oil-in-water emulsions[END_REF], chitosan [START_REF] Higuera-Ciapara | Microencapsulation of astaxanthin in a chitosan matrix[END_REF][START_REF] Pedro | Chitosan: An option for development of essential oil delivery systems for oral cavity care? Carbohyd[END_REF], and alginates [START_REF] Yoo | Micro encapsulation of -tocopherol using sodium alginate and its controlled release properties[END_REF][START_REF] Huang | Microfluidic device utilizing pneumatic microvibrators to generate alginate microbeads for microencapsulation of cells[END_REF][START_REF] Wikstrom | Alginate-based microencapsulation of retinal pigment epithelial cell line for cell therapy[END_REF]. The major advantages of these biopolymers are their good solubility in water and low viscosity at high concentrations, compared to proteins. Often carbohydrates are mixed with proteins [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF]Ducel et al., 2004b;[START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF][START_REF] Pereira | Legumes seeds protein isolates in the production of ascorbic acid microparticles[END_REF][START_REF] Pierucci | New microencapsulation system for ascorbic acid using pea protein concentrate as coat protector[END_REF][START_REF] Pierucci | Comparison of a-tocopherol microparticles produced with different wall materials: pea protein a new interesting alternative[END_REF]Yu et al., 2007) to improve the emulsifying and filmogenic properties during microencapsulation. Furthermore, protein-carbohydrate conjugates covalently cross-linked by the Maillard reaction had shown interesting functional properties [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF][START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF]. Various lipophilic substances such as glycerides, oils, phospholipids, carotenoids and waxes are also used as carrier materials in microencapsulation [START_REF] Eldem | Optimization of spray-dried and -congealed lipid micropellets and characterization of their surface morphology by scanning electron microscopy[END_REF][START_REF] Lee | Microencapsulation of lipid nanoparticles containing lipophilic drug[END_REF][START_REF] Mcclements | Emulsion-based delivery systems for lipophilic bioactive components[END_REF][START_REF] Muller | Nanostructured lipid matrices for improved microencapsulation of drugs[END_REF][START_REF] Patel | Molecular dynamics study of the encapsulation capability of a PCL-PEO based block copolymer for hydrophobic drugs with different spatial distributions of hydrogen bond donors and acceptors[END_REF]. They permit barrier creation for the protection of sensitive ingredients against moisture, plus their transport in aqueous media.

Proteins extracted from animal derived products (whey proteins, gelatin, casein) and from vegetables (soy proteins, pea proteins, cereal proteins) are widely used for encapsulation of active substances. These natural polymers present several advantages: biocompatibility, biodegradability, good amphiphilic and functional properties such as water solubility, and Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 6 emulsifying and foaming capacity. The use of vegetable proteins as wall-forming materials in microencapsulation, reflects the present "green" trend in the pharmaceutical, cosmetics and food industries. In food applications, plant proteins are known to be less allergenic compared to animal derived proteins [START_REF] Jenkins | Evolutionary distance from human homologs reflects allergenicity of animal food proteins[END_REF][START_REF] Li | Effects of high hydrostatic pressure treatment on allergenicity and structural properties of soybean protein isolate for infant formula[END_REF]. For these reasons, over the past few years, the development of new applications for plant products rich in proteins has became an increasingly interesting area for research. For the last decade, the protein ingredient industry has been turning towards plants as a preferred alternative to animal-based sources, e.g. in vegetarian diets, due to increased consumer concerns over the safety of animal-derived products [START_REF] Jiménez-Yan | Energy balance of Litopenaeus vannamei postlarvae fed on animal or vegetable protein based compounded feeds[END_REF][START_REF] Sawashita | Effect of dietary vegetable and animal proteins on atherothrombosis in mice[END_REF][START_REF] Choi | Effects of replacing pork back fat with vegetable oils and rice bran fiber on the quality of reduced-fat frankfurters[END_REF].

Currently, the widespread presence of microparticles based on animal proteins, contrasts with the very limited use of plant proteins in industry. This tendency should be reversed in coming years.

Vegetable proteins consist of several fractions: the major fraction is glutenin, soluble in alkaline water solutions; the globulin fraction, soluble in salt solutions, followed by the albumin and prolamin, fractions soluble in water and ethanol respectively [START_REF] Osborne | The vegetable proteins[END_REF].

Among vegetable proteins used as a wall material in microencapsulation, we find mainly soy protein isolate, pea protein isolate and cereal proteins. Soybean proteins have functional properties suitable for microencapsulation, such as solubility, water and fat absorption, emulsion stabilization, gelation, foaming, plus good film-forming and organoleptic properties [START_REF] Franzen | Functional properties of succinylated and acetylated soy protein[END_REF]. Soy glycinin and conglycinin are somewhat similar (comparable molecular weights, amino acid composition, subunit structures) to pea legumin and vicilin [START_REF] Koyoro | Functional properties of pea globulin fractions[END_REF]. The globulins of pea protein have all the functional properties necessary for successful incorporation into microencapsulation systems as a wall material. The proteins of cereals (oat, wheat, barley and corn) are more advantageous from the nutritional standpoint, and they have attracted research and commercial attention for this

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reason. Due to their interesting functional properties and potential food applications, these proteins were also studied as wall material for microencapsulation [START_REF] Ducel | Rheological interfacial properties of plant protein-arabic gum coacervates at the oil-water interface[END_REF]Ducel et al., 2004b;Wang et al., 2011b). Sunflower proteins have particularly interesting thermal behavior, gelling properties and surface activity. Compared with other sources of vegetable proteins, sunflower seeds have been reported to have a low content in anti-nutritional factors.

These proteins have often been compared to commercial soy proteins, extensively researched on functionality, with regard to their functional properties [START_REF] Gonzalez-Perez | Sunflower proteins: overview of their physicochemical, structural and functional properties[END_REF].

This review presents the recent works dealing with the use of vegetable proteins in microencapsulation. The influence of the proteins functional properties as well as the microencapsulation technique on process efficiency and properties of obtained microparticles is particularly discussed.

The microencapsulation techniques applied to vegetable proteins

The two techniques mainly used for microencapsulation of active material by vegetable proteins are spray-drying and coacervation. Both processes share the aspect of "green chemistry" with vegetable proteins as renewable and biodegradable resources, plus, the two techniques do not need the use of organic solvents. Other processes such as gelation or solvent evaporation techniques can be also considered [START_REF] Dubey | Microencapsulation technology and application[END_REF][START_REF] Gouin | Microencapsulation: industrial appraisal of existing technologies and trends[END_REF].

Spray-drying is a continuous process to convert an initial liquid into a solid powder of microparticles (Figure 2a). It is a very common dehydration process used to form a continuous matrix surrounding the active substances. The initial liquid (solution, emulsion or suspension) containing wall and core materials is sprayed into a stream of heated air. The solvent, almost always water, is evaporated to give instantaneous powder production. This technology offers several advantages: it is simple, relatively inexpensive, rapid and thus Microencapsulation by coacervation is carried out by precipitation of wall forming materials around the active core under the effect of one of the following factors: change of pH or temperature, addition of a non-solvent or electrolyte compound (Figure 2b). This controlled desolvation results in the formation of a polymeric network around the core. This shell of coacervates can be solidified using a chemical or enzymatic cross-linker [START_REF] Gouin | Microencapsulation: industrial appraisal of existing technologies and trends[END_REF].

Coacervation occurs either via a simple or complex method. Simple coacervation involves only one colloidal solute and thus formation of a single polymer envelope. Complex coacervation is produced by mixing two oppositely charged polyelectrolytes for shell formation around an active core [START_REF] Wilson | Microencapsulation of Vitamins[END_REF]. Finally, one of the factors that 9 limits the use of coacervates in encapsulation is their sensitivity to pH and ionic strength [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF].

The important point to consider for the use of proteins in encapsulation systems is their instability in acid media. The isoelectric point of proteins means they are insoluble in most acidic systems and sensitive to precipitation with pH values lower than 7, especially when acidic core materials are used (e.g. ascorbic acid). For the majority of vegetable proteins in aqueous solution, the isoelectric point is located in a pH range between 3 and 5. For this reason these biopolymers are usually used in alkaline conditions in order to obtain good solubility of proteins and to efficiently encapsulate active substances.

Particle properties, such as morphology, size and releasing characteristics, can be very different depending on the process chosen. Two forms are mainly obtained by the spraydrying and coacervation method: microcapsules and microspheres (Figure 1). Particle size obtained by the spray-drying process is typically between 1 µm and 50 µm (Richard and Benoit, 2000) while size of particles obtained by the coacervation method can vary from nanometers to several hundred microns [START_REF] Merodio | Ganciclovir-loaded albumin nanoparticles: characterization and in vitro release properties[END_REF][START_REF] Bayomi | Preparation of casein-chitosan microspheres containing diltiazem hydrochloride by an aqueous coacervation technique[END_REF][START_REF] Gan | Evaluation of microbial transglutaminase and ribose cross-linked soy protein isolate-based microcapsules containing fish oil[END_REF].

These two processes give high values (up to 100%) of microencapsulation efficiency (MEE). The latter is defined as the ratio between the percentage of active core encapsulated in the powder and the percentage of active core in the initial liquid.

Vegetable proteins in microencapsulation

Soy proteins

Soy bean seeds contain an important fraction (35-40%) properties [START_REF] Gu | Effects of different oils on the properties of soy protein isolate emulsions and gels[END_REF]. These protein characteristics and their solubility are strongly dependent on pH, heat treatment, and the presence and concentration of salts or other ingredients (oil, carbohydrate, surfactant).

Soy protein isolate (SPI) use in microencapsulation has already been studied by various authors (Table 1). SPI is generally used as an individual coating material, but can also be mixed with polysaccharides [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF][START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF]Yu et al., 2007). The combination of proteins with carbohydrates as a carrier material favors better protection, oxidative stability and drying properties [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF]. Due to SPI hydrosolubility, microparticles are mainly produced using the spray-drying technique [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF][START_REF] Charve | Encapsulation performance of proteins and traditional materials for spray dried flavors[END_REF][START_REF] Favaro-Trindade | The use of spray drying technology to reduce bitter taste of casein hydrolysate[END_REF][START_REF] Kim | Microencapsulation properties of gum arabic and several food proteins: spray-dried orange oil emulsion particles[END_REF][START_REF] Ortiz | Production and properties of casein hydrolysate microencapsulated by spray drying with soybean protein isolate[END_REF][START_REF] Rascon | Carotenoid retention and storage stability of spray-dried paprika oleoresin using gum arabic and soy protein isolate as wall materials[END_REF][START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF]Yu et al., 2007), but coacervation and gelation have also been investigated [START_REF] Chen | Elaboration and characterization of soy/zein protein microspheres for controlled nutraceutical delivery[END_REF][START_REF] Gan | Evaluation of microbial transglutaminase and ribose cross-linked soy protein isolate-based microcapsules containing fish oil[END_REF]Lazko et al., 2004a;Lazko et al., 2004b;[START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF][START_REF] Nori | Microencapsulation of propolis extract by complex coacervation[END_REF].

Comment citer ce document : 12

Microencapsulation by spray-drying

In the case of hydrophobic core microencapsulation, an oil-in-water emulsion is prepared before the encapsulation step [START_REF] Augustin | Maillard reaction products as encapsulants for fish oil powders[END_REF][START_REF] Kim | Microencapsulation properties of gum arabic and several food proteins: spray-dried orange oil emulsion particles[END_REF][START_REF] Rascon | Carotenoid retention and storage stability of spray-dried paprika oleoresin using gum arabic and soy protein isolate as wall materials[END_REF][START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF]Yu et al., 2007;[START_REF] Nesterenko | Influence of soy protein's structural modifications on their microencapsulation properties: α-tocopherol microparticles preparation[END_REF]. These emulsions are often carried out by high pressure homogenization because of its interesting results in terms of emulsion properties and stability [START_REF] Gharsallaoui | Applications of spray-drying in microencapsulation of food ingredients: An overview[END_REF]. Moreover, increasing homogenization pressure gives a slight decrease in oil droplet size [START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF] and emulsion viscosity (Yu et al., 2007). [START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF] also noted a slight improvement in microencapsulation efficiency with an increase of homogenization pressure. Some results are given in Table 2. (

) 2012
The intense mechanical forces undergone by globular proteins and oil droplets during homogenization, promotes oil droplet dispersion and protein structure modification [START_REF] Rampon | Evidence that homogenization of BSA-stabilized hexadecane-in-water emulsion induces structure modification of the nonadsorbed protein[END_REF], such as unfolding of proteinic chains. This unfolding causes the exposure of polar and non polar protein regions, and movement of charged amino acids to new local environments and makes them more surface-active. The main changes are in the secondary

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Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035

13 and tertiary structure that can modify the surface appearance of the amino acids. As proteins are the most surface-active components in the emulsion they accumulate at the oil-water interface, enwrapping the newly formed oil droplets. This can be explained by the tendency of proteins to adsorb more extensively and less reversibly at hydrophobic surfaces than at hydrophilic surfaces [START_REF] Dickinson | Adsorbed protein layers at fluid interfaces : interactions, structure and surface rheology[END_REF]. The resulting stabilizing layer provides immediate protection of the fine droplets against re-coalescence and thus gives physical stability to the emulsion [START_REF] Dickinson | Milk protein interfacial layers and the relationship to emulsion stability and rheology[END_REF].

The solid level in the emulsion is also a key parameter influencing active core retention [START_REF] Charve | Encapsulation performance of proteins and traditional materials for spray dried flavors[END_REF], and it has been shown that the MEE is improved with increasing solid content. This could be explained by the reduction of core molecule mobility in wall material and of the time needed to form the protective shell, both induced by a high solid content. On the other hand, past a critical concentration (generally 20% w/w solid content) an abrupt increase in viscosity is observed, which involves a significant fall in process efficiency (Yu et al., 2007).

From the process point of view, it has been shown that drying inlet temperature also affects the MEE. Indeed, a high drying temperature supports the formation of a rigid wall material shell on the microparticle surface, limiting core molecule migration and release [START_REF] Rascon | Carotenoid retention and storage stability of spray-dried paprika oleoresin using gum arabic and soy protein isolate as wall materials[END_REF].

In the spray-drying microencapsulation process, efficiency can also be influenced by volatile properties of active core material, e.g. a decrease of MEE for encapsulation of volatile composites such as aromas compared to classic oils. Indeed, during the drying process, the liquid preparations are subjected to high temperatures (150-180°C) and this provokes core material evaporation, which explains the low efficiency of microencapsulation of citral (35%) [START_REF] Charve | Encapsulation performance of proteins and traditional materials for spray dried flavors[END_REF] compared to stearin (91%) [START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF] by SPI.

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Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035

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In our recent study [START_REF] Nesterenko | Influence of soy protein's structural modifications on their microencapsulation properties: α-tocopherol microparticles preparation[END_REF], it has been shown that grafting of hydrophobic fatty acid chain to soy proteins by acylation can enhance the retention of hydrophobic active material (α-tocopherol) during microencapsulation by spray-drying.

Process efficiency was improved from 79.7% to 94.8% when soy proteins were acylated with dodecanoyl chloride. Moreover, this increased retention efficiency after protein acylation was observed for different core/wall ratios, demonstrating that soy proteins in native and modified state represent a relevant encapsulant agent for hydrophobic substances.

Microencapsulation by coacervation

Soy proteins have also been studied in microencapsulation as wall materials using the coacervation method, and several parameters influencing the coacervation MEE have been found. Among them: the active core and wall material concentrations, the temperature and the pH of the media. When active hydrophobic core concentration exceeds 50% w/w, a decrease in process efficiency is generally observed (Lazko et al., 2004a;[START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF][START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF][START_REF] Jun-Xia | Microencapsulation of sweet orange oil by complex coacervation with soybean protein isolate/gum Arabic[END_REF]. This phenomenon is particularly well illustrated in the work of [START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF] where a change of wall/core ratio from 1/1 to 1/3 involved a decrease of MEE from 92% to 79%. [START_REF] Jun-Xia | Microencapsulation of sweet orange oil by complex coacervation with soybean protein isolate/gum Arabic[END_REF] attributed this tendency to incomplete emulsification after addition of excessive oil in system. Unemulsified oil affected the electrostatic interactions between soy proteins and gum arabic, and thus emulsion destabilization.

The protein concentration (as wall material) during the emulsification step is strongly related to the stability and size of coacervates. This could be explained by the specific surface of oil droplets, which is inversely proportional to their mean diameter in emulsion (Lazko et al., 2004a). Due to protein surfactant properties, increasing protein concentration would result in an increase in oil droplet specific surface, improving the adsorption of proteins on the oil- Some authors (Lazko et al., 2004a) have noticed that microencapsulation by the coacervation method is more effective under acidic pH and high temperature conditions (pH 2 and 55°C respectively). Acid mediums favor 11S globular protein denaturation, characterized by deformation of their quaternary structure to secondary and tertiary structures. The overall accessibility of hydrophobic protein sites inside the spherical formations can be improved by this structure change [START_REF] Magdassi | Surface activity of proteins chemical and physico-chemical modifications[END_REF][START_REF] Wagner | Effects of dissociation, deamidation, and reducing treatment on structural and surface active properties of soy glycinin[END_REF]. At pH's below the isoelectric point, the protein COO -functions become uncharged COOH groups, and protein hydrophilicity decreases. Thus, an acidic medium would favor the affinity between proteins and the hydrophobic active core in the emulsions, which should result in improved MEE.

When microencapsulation is undertaken by coacervation, a cross-linking step is often added at the end of the process, mainly to reinforce the microcapsule shells. This supplementary step would not affect the efficiency of protein precipitation around oil droplets, but would play an important role in emulsion stability, and consequently on microcapsule size and dispersion, particularly with prolonged stirring. Without reticulation, the coalescence of microcapsules was observed, resulting in a significant increase in microcapsule average diameter (from 90 µm to more than 200 µm), whereas this coalescence is absent when microcapsules were cross-linked (no change in microcapsule diameter was observed) (Lazko et al., 2004a). Glutaraldehyde is the most commonly used cross-linking agent, allowing stable microcapsule dispersion to be obtained over time (Lazko et al., 2004b), plus better mechanical properties. But glutaraldehyde is a relatively toxic product, which limits its use in applications such as the food industry [START_REF] Leung | Ecotoxicology of glutaraldehyde: review of environmental fate and effects studies[END_REF].

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035

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In an effort to mask the bitter taste of some hydrophobic hydrolysates (e.g. casein hydrolysate) and be able to incorporate them into food products, their microencapsulation by soy proteins was investigated by spray-drying [START_REF] Favaro-Trindade | The use of spray drying technology to reduce bitter taste of casein hydrolysate[END_REF][START_REF] Ortiz | Production and properties of casein hydrolysate microencapsulated by spray drying with soybean protein isolate[END_REF] and coacervation [START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF] techniques. During encapsulation, the hydrophobic interactions between casein hydrolysate and soy proteins lessen the undesirable taste of casein. In both cases, the authors demonstrate the decrease in microencapsulation efficiency and the increase in particle size with increasing active core concentration.

Chen and Subirade (2009) reported the preparation of soy protein based microspheres by cold gelation method (initiated by glacial acetic acid in the presence of calcium carbonate)

to elaborate delivery systems for nutraceutical products (riboflavin). Obtained microparticles had spherical morphology with the diameter about 15 µm. Active material was efficiently encapsulated by soy proteins (process efficiency of 79-88%) at ambient temperature without using cross-linking reagents, which could be interesting for various food and pharmaceutical applications.

Overall, numerous studies have shown the abilities of SPI as an encapsulating agent, using both spray-drying and coacervation techniques. In both methods, specific parameters affect microencapsulation efficiency and microparticle size, particularly the active core concentration (Lazko et al., 2004a;[START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF][START_REF] Rusli | Stabilization of oils by microencapsulation with heated protein-glucose syrup mixtures[END_REF]Yu et al., 2007), but authors showed that high values of MEE could be attained in both cases, by using suitable experimental conditions. The main differences between these two methods are the structure of the microparticles obtained, and consequently the release of the active core, and the microparticle sizes, usually higher with coacervation (less than 100 µm for spray dried microparticles instead of generally more than 100 µm for coacervated microcapsules).

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Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 17

Pea proteins

Pea proteins are extracted from pea seeds where they represent a 20% to 30% fraction including mainly globulins (65-80%) and two minority fractions, albumins and glutelins.

Globulins comprise three different proteins -legumin, vicilin and convicilin [START_REF] Koyoro | Functional properties of pea globulin fractions[END_REF]. Pea legumin represents the 11S globulin fraction with a molar mass between 350 and 400 kDa, while vicilin and convicilin represent the 7S globulin fraction with a molar mass of about 150 kDa.

Pea proteins extracted from grains possess interesting gel-forming [START_REF] Akintayo | Effects of NaCl, ionic strength and pH on the foaming and gelation of pigeon pea (Cajanus cajan) protein concentrates[END_REF] and emulsifying [START_REF] Raymundo | Fat mimetic capacity of Chlorella vulgaris biomass in oil-in-water food emulsions stabilized by pea protein[END_REF] properties. However, in the literature for microencapsulation uses, these proteins are generally associated with polysaccharides (Ducel et al., 2004b;[START_REF] Gharsallaoui | Utilisation of pectin coating to enhance spray-dry stability of pea protein-stabilised oil-in-water emulsions[END_REF][START_REF] Pereira | Legumes seeds protein isolates in the production of ascorbic acid microparticles[END_REF][START_REF] Pierucci | New microencapsulation system for ascorbic acid using pea protein concentrate as coat protector[END_REF][START_REF] Pierucci | Comparison of a-tocopherol microparticles produced with different wall materials: pea protein a new interesting alternative[END_REF]. Indeed, polysaccharide/protein interactions give new functions to pea proteins without chemical or enzymatic modification, particularly solubility, foaming and surfactant properties [START_REF] Liu | Effect of pH on the functional behaviour of pea protein isolate-gum arabic complexes[END_REF]. These interactions can also create stable emulsions and thus give better particle size distribution and improve the efficiency of the microencapsulation process.

Microencapsulation by spray-drying

Several studies deal with pea proteins as wall material for microencapsulation, using the spray-drying technique. The main properties of the microparticles obtained are summarized in Table 4. Utilization of protein/polysaccharide mixtures allows the possibility of combining the specific properties of each of these polymers. Polysaccharide products possess low emulsification properties compared to proteins, and are usually used as wall materials in the presence of a surface-active constituent. The incorporation of carbohydrates with proteins as the encapsulated matrix, gives increased emulsion stability and better protection of active ingredients against oxidation [START_REF] Young | Microencapsulating properties of whey proteins 2. Combination of whey proteins with carbohydrates[END_REF]. Gharsallaoui et al.

(2010) noted that in protein/carbohydrate blends, proteins serve as an emulsifying and filmforming agent, while polysaccharides act as a matrix forming material. The retention of the 19 active core observed in pea protein microparticles is similar to (or better than) in the particles with a protein/carbohydrate mixture. This, presumably, is induced by electrostatic interactions between proteins and encapsulated material [START_REF] Pierucci | Comparison of a-tocopherol microparticles produced with different wall materials: pea protein a new interesting alternative[END_REF]. As stated in the literature, the addition of maltodextrin to the pea protein wall material increases the particle size, in particular for a hydrosoluble active material [START_REF] Pierucci | New microencapsulation system for ascorbic acid using pea protein concentrate as coat protector[END_REF]. The authors justified this increase by the fact that maltodextrin can induce the rapid formation of a glassy surface which would allow air expansion inside microparticles, giving an increase in particle diameter.

The results reported, demonstrate that pea proteins alone or in association with polysaccharides are totally appropriate for the microencapsulation of hydrophilic (ascorbic acid [START_REF] Pereira | Legumes seeds protein isolates in the production of ascorbic acid microparticles[END_REF][START_REF] Pierucci | New microencapsulation system for ascorbic acid using pea protein concentrate as coat protector[END_REF]) hydrophobic (α-tocopherol [START_REF] Pierucci | Comparison of a-tocopherol microparticles produced with different wall materials: pea protein a new interesting alternative[END_REF], and triglyceride (Ducel et al., 2004b;[START_REF] Gharsallaoui | Utilisation of pectin coating to enhance spray-dry stability of pea protein-stabilised oil-in-water emulsions[END_REF]) active core materials.

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 20

Microencapsulation by coacervation

Ducel et al. ( 2004) examined the use of pea globulin (isoelectric point in a pH range 4.4-4.6) for triglyceride microencapsulation by complex coacervation (Table 3), plus the influence of pH and polymer concentration on the microcapsule size. Increasing pea globulin/ gum arabic (50:50) blend concentration in the initial makeup, resulted in increased microcapsule size. For example, at pH 3.5, microcapsule diameters varied from 28 µm to 97 µm with a concentration change of 1 g/L to 10 g/L respectively. Conversely, Lazko et al.

(2004a) observed a decrease of coacervate size with an increase of soy protein concentration.

In fact, the mean diameter of microparticles obtained, decreased from 153 µm to 88 µm as the protein concentration increased from 0.5 g/L to 5 g/L respectively. This discordance between published results was probably due to coacervation process differences. Complex coacervation was used in the case of pea proteins, and particle agglomeration and coalescence increased their size. The presence of polysaccharides in the initial preparation can also influence coacervate agglomeration [START_REF] Klassen | Effect of pH on the formation of electrostatic complexes within admixtures of partially purified pea proteins (legumin and vicilin) and gum Arabic polysaccharides[END_REF]. On the other hand, simple coacervation was used for preparing soy protein microparticles. Higher concentrations of surface active protein in the emulsion increased the coalescence resistant coacervates. In addition, the two coacervation processes were not made under exactly the same conditions, for example temperatures of 30°C and 55°C, pH values of 3.5 and 2.0, mechanical stirring at 500 rpm and magnetic stirring at 600 rpm, for pea proteins and soy proteins respectively.

It is apparently also possible to prepare microparticles from one fraction of pea proteins (legumin [START_REF] Irache | Optimization and in vitro stability of legumin nanoparticles obtained by a coacervation method[END_REF] or vicilin [START_REF] Ezpeleta | Properties of glutaraldehyde cross-linkes vicilin nano-and microparticles[END_REF]Ezpeleta et al., 1996a))

without polysaccharide addition, and to coat an active material, and these studies involved microparticle preparation (with legumin or vicilin) using simple coacervation. Concerning particle size distribution, the mean particle diameter varied from 200 to 700 nm. Finally, the average sizes of microcapsules based on pea proteins obtained by coacervation, varied from

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 21 about 10, to hundreds of microns, while for spray-dried microspheres, the average size is always less than or near 10 µm.

Pea proteins as an additive for microencapsulating systems

The emulsifying properties of pea proteins (Ducel et al., 2004a) make them potentially useful as an additive to improve emulsion stabilization instead of as a simple main wall material. [START_REF] Gharsallaoui | Utilisation of pectin coating to enhance spray-dry stability of pea protein-stabilised oil-in-water emulsions[END_REF] used a small amount of pea protein (0.5% w/w) as an emulsifier to form oil-in-water emulsion containing small oil droplets. Then pectin and maltodextrin were added, to produce an emulsion containing triglyceride droplets coated with protein-polysaccharide membranes. This study confirmed the interest of combining the properties of polysaccharides with those of proteins. The emulsions with polysaccharides seemed to be less sensitive to pH variations, high ionic strengths and high temperatures, than those with only proteins [START_REF] Dickinson | Hydrocolloids at interfaces and the influence of the properties of dispersed system[END_REF][START_REF] Mcclements | Food emulsions: Principles, practice and techniques[END_REF]. In addition, the hydrophobic polypeptides of proteins, added to polysaccharide based emulsions, have a high capacity to adsorb at the oil-water interfaces, (for example pea globulin at acid pH) and thus to stabilize emulsions [START_REF] Gharsallaoui | Utilisation of pectin coating to enhance spray-dry stability of pea protein-stabilised oil-in-water emulsions[END_REF]. These studies show that the use of small quantities of protein could stabilize emulsions against coalescence, pH and temperature variations.

To sum up, pea extracted proteins show convenient encapsulating properties and are used for active material protection or for emulsion stabilization. Properties of the resulting microparticles were dependent on the microencapsulation technique used, process conditions and the use of additives such as polysaccharides.

Wheat proteins and other cereal proteins

Wheat contains a specific protein: gluten, obtained as a by-product during starch isolation from wheat flour. The latter is a complex material, composed of proteins and a small [START_REF] Bietz | Comparison of peptides from wheat gliadin and glutenin[END_REF].

Gluten represents approximately 80% of wheat seed proteins, plays an important role in wheat flour quality [START_REF] Day | Wheat-gluten uses and industry needs[END_REF], and is used essentially as a human and animal food source. While its insoluble nature is an important property for traditional applications, particularly in bread and baked products, this insolubility in water limits its use in many other applications such as cosmetics and drugs.

Wheat gluten is the cereal protein most studied in the microencapsulation field [START_REF] Ducel | Rheological interfacial properties of plant protein-arabic gum coacervates at the oil-water interface[END_REF]Ducel et al., 2004b;Ezpeleta et al., 1996b;[START_REF] Iwami | Spray-dried gliadin powders inclusive of linoleic acid (microcapsules): their preservability, digestibility and application to bread making[END_REF][START_REF] Mauguet | Gliadin matrices for microencapsulation processes by simple coacervation method[END_REF]Yu and Lee, 1997). Its low water solubility and its viscoelasticity provide this plant polymer with various interesting physico-chemical characteristics, such as gel-and filmforming properties [START_REF] Sun | Rheological behavior of heat-induced wheat gliadin gel[END_REF]. Wheat proteins alone, or in combination with polysaccharides are good for encapsulating active core materials using various techniques.

Some studies have also been made with other cereal proteins as a wall material: barley protein or corn zein [START_REF] Parris | Encapsulation of Essential Oils in Zein Nanospherical Particles[END_REF]Wang et al., 2011a;Wang et al., 2011b;Zhong et al., 2009;[START_REF] Patel | Quercetin loaded biopolymeric colloidal particles prepared by simultaneous precipitation of quercetin with hydrophobic protein in aqueous medium[END_REF]. Barley proteins, studied by Wang et al. (2011b), are composed of two protein fractions: glutelin and hordein. Both these fractions show excellent film-forming and emulsifying properties (Wang et al., 2011a). Corn extracted prolamin -zein is a protein fraction soluble in hydro-alcoholic solutions and well-known for its good filmogenic properties [START_REF] Beck | Physico-chemical characterization of zein as a film polymer. A direct comparison with ethyl cellulose[END_REF].

Microencapsulation by coacervation

Ducel et al. ( 2005) studied the encapsulation of vaseline oil with a gliadin/ gum arabic wall, by complex coacervation, focusing on protein concentration and effect of pH on microcapsule properties. They found that a decrease in medium pH (from 3.5 to 3) gave an increase in viscoelasticity and a decrease in microcapsule average size (from 50 µm to 25 µm). The wall polymer distribution on the droplet surface was more homogeneous so particle aggregation was reduced. Encapsulation conditions and efficiency were improved by the pH decrease. Analogous behavior for a pea protein/ gum arabic encapsulating system, has been

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 24 observed (Ducel et al., 2004), and this result agrees with observations by Lazko et al. (2004a) concerning complex coacervation microencapsulation using soy glycinin. The range of wheat protein microcapsule size using coacervation (simple or complex) can vary from a few to two hundred micrometers [START_REF] Ducel | Rheological interfacial properties of plant protein-arabic gum coacervates at the oil-water interface[END_REF][START_REF] Mauguet | Gliadin matrices for microencapsulation processes by simple coacervation method[END_REF]Yu et al., 2007). These values are in line with those obtained from pea protein and soy protein microcapsules (Ducel et al., 2004b;Lazko et al., 2004a).

Microencapsulation using other processes

Only a few studies deal with wheat proteins for spray drying microencapsulation. [START_REF] Iwami | Spray-dried gliadin powders inclusive of linoleic acid (microcapsules): their preservability, digestibility and application to bread making[END_REF] reported the encapsulation of linoleic acid in a gliadin matrix to improve its stability and digestibility, particularly for bread making applications. Wheat proteins were also used as wall material for microencapsulation with the solvent evaporation method [START_REF] Andreani | Development and characterization of wheat gluten microspheres for use in a controlled release system[END_REF]Ezpeleta et al., 1996b), and proteins extracted from barley seeds were used as carrier material for fish oil microencapsulation by Wang et al. (2011). The spray-drying method was used for microparticle preparation with an inlet temperature of 150°C. The authors demonstrated 97-100% encapsulation efficiency and high oil content in the powder -around 50%. The barley protein microparticles obtained have a spherical shape and porous inner structure with diameters ranging from 1 to 5 µm. These proteins had a good capacity for protecting fish oil against oxidation in food preparations. [START_REF] Andreani | Development and characterization of wheat gluten microspheres for use in a controlled release system[END_REF] worked on wheat gluten microspheres for the controlled release of a model drug (diltiazem), and evaluated the effect of a small amount of poly(ethylene oxide) on microsphere properties. They demonstrated that perfectly spherical porous microspheres could be obtained, with mean particle diameters of between 10 and 20µm, and encapsulation efficiency from 73% to 97%. They showed that the addition of 5% w/w of PEO to the gluten matrix improved the MEE significantly. This is probably due to higher porosity of Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 25 microparticles with PEO, and therefore greater specific surface area favoring better incorporation of the active core material. The effect of nature of solvent was studied by Ezpeleta et al. (1996b), They observed significant variations in microparticle diameter, according to solvent composition, highlighting the influence of physico-chemical interactions between proteins and solvents. Antimicrobial chicken egg white lysozyme, was encapsulated by zein protein using a supercritical anti-solvent process (Zhong et al., 2009), and heterogeneously sized microparticles, ranging from a few to 50 µm and 46.5% MEE were obtained. The active material release kinetics showed very promising microparticle properties for use in food production.

In short, cereal proteins are relevant biomaterials as a matrix for microencapsulation.

They perform well for microencapsulation of hydrophobic and hydrophilic compounds alone, as well as mixed with polysaccharides or synthetic polymers.

Other vegetable proteins potentially useful in microencapsulation

Other proteins have properties making them possible contenders as wall material in microencapsulation, and this is especially true for rice proteins, oat proteins and sunflower proteins. Rice and oat proteins already have a large range of applications in the food sector.

However, the physico-chemical properties of sunflower proteins have been extensively studied, and this natural polymer has no major industrial uses, meaning that it would be interesting to find new applications and develop high added-value products based on them.

Rice proteins

Rice is among the most important cereal crops in the world. It is established as the basic foodstuff for over half the world's population. Containing from 12 to 20% proteins, rice bran, mainly removed from the grain during the milling process to produce white rice, may be a [START_REF] Hamada | Characterization and functional properties of rice bran proteins modified by commercial exoproteases and endoproteases[END_REF]. Compared to rice bran, the protein content in rice grains is slightly lower, varying from 6 to 15% [START_REF] Bienvenido | Le riz dans la nutrition humaine[END_REF].

Rice proteins are generaly prepared by alkali extraction followed by isoelectric precipitation [START_REF] Kaewka | Effect of preparation conditions on composition and sensory aroma characteristics of acid hydrolyzed rice bran protein concentrate[END_REF][START_REF] Pinciroli | Comparison between protein functional properties of two rice cultivars[END_REF] and by subcritical water treatement [START_REF] Hata | Extraction of defatted rice bran by subcritical water treatment[END_REF][START_REF] Sereewatthanawut | Extraction of protein and amino acids from deoiled rice bran by subcritical water hydrolysis[END_REF]. In addition, rice has also been studied for the production of starch, monosodium glutamate, pigments and rice wine; thus rice protein could be an additional by-product to be exploited [START_REF] Cao | Differences in functional properties and biochemical characteristics of congenetic rice proteins[END_REF]. After the sequential extraction of rice protein fractions, the following distribution has been obtained: about 75% glutenin, 15% globulin, 6% albumin and 3% prolamin [START_REF] Agboola | Characterisation and functional properties of Australian rice protein isolates[END_REF]. [START_REF] Chandi | Functional properties of rice bran protein concentrates[END_REF] analyzed the functional properties of rice protein concentrate (55% of the protein fraction). They noticed the excellent foaming stability lasting several days, the high emulsifying capacity in sugar based (5-15% w/w) solutions, and the good stability of emulsions depending on the pH and salt/sugar presence. The physico-chemical properties are similar to those of casein [START_REF] Chandi | Functional properties of rice bran protein concentrates[END_REF].

Rice bran isolate containing approximately 92% protein is prepared from defatted rice bran and its properties have been studied [START_REF] Wang | Preparation and functional properties of rice bran protein isolate[END_REF]. They showed that: the foaming properties of rice protein are similar to those of albumin from egg white; the emulsifying capacities of albumin from bovine serum (BSA) are significantly higher than those of rice proteins; minimum protein solubility is close to the isoelectric point at pH 4 and the maximum at pH 10; the main amino acid content of rice proteins is similar to that of casein and soy proteins; the denaturation temperature of rice protein isolate is about 83.4°C.

Rice proteins also associate well with polysaccharides (alginate and carrageenan) to form complex precipitates with possible new industrial applications [START_REF] Fabian | Precipitation of rice bran protein using carrageenan and alginate[END_REF].

From these results, the physico-chemical properties of rice proteins could provide favorable characteristics for wall material in microencapsulation. However overall, rice protein use

Oat proteins

Oats is one of the most popular cereals for human and animal foods because of its high protein and fatty acid content. Protein content in oat grain is one of the highest, varying from 12 to 24% [START_REF] Chronakis | Complex formation in aqueous medium of partially hydrolysed oat cereal proteins with sodium stearoyl-2 lactylate (SSL) lipid surfactant and implications for bile acids activity[END_REF]. The average amino acid composition of oat proteins is very attractive from a nutritional value point of view, and this is probably related to the higher proportion of albumins and globulins compared to proteins from the other cereal grains.

Globulin represents the major part of oat proteins (around 70-80%). Oat protein concentrate has poor solubility and functional properties. To improve these physico-chemical properties, modifications such as enzymatic hydrolysis [START_REF] Yao | Some functional properties of oat bran protein concentrate modified by trypsin[END_REF], acetylation and succinylation [START_REF] Mohamed | Oats protein isolate: Thermal, rheological, surface and functional properties[END_REF] were carried out, and demonstrated that these chemical modifications could improve the solubility, emulsifying activity and foaming capacity of oat proteins.

In conclusion, oat native proteins do not offer the required properties to be used in microencapsulation, but some specific modifications could allow them to be considered as wall materials.

Sunflower proteins

Sunflowers are mainly cultivated for the production of oil extracted from their seeds, and they are one of the major sources of edible oil. Proteins are the majority constituents in sunflower oil cakes, valued essentially as animal feed. The defatted sunflower flour contains a high quantity of proteins, around 27% in dry weight [START_REF] Ordonez | Obtaining a protein concentrate from integral deffated sunflower flour[END_REF]. The dehulled seed consists of about 20-40% crude protein, this value being highly affected by sunflower variety [START_REF] Gonzalez-Perez | Sunflower proteins: overview of their physicochemical, structural and functional properties[END_REF]. The quantity of proteins extracted from the sunflower, also varies according to used solvent (mainly aqueous solutions) and the extraction conditions (stirring mode, temperature, pH). In the sunflower oil cake, four fractions of proteins are present [START_REF] Linden | Biochimie agro-industrielle: Valorisation alimentaire de la production agricole[END_REF]: globulins constitute the main fraction ranging from 55 to 60%;

albumins account for about 17-23% of total proteins and two minor fractions glutelins and prolamins give 11-17% and 1-4% protein fractions respectively.

In terms of sedimentation coefficients, sunflower proteins show two major fractions:

the 11S globulins (also named helianthinin) and the 2S albumins. Helianthinin has been reported to be present as a globular oligomeric protein with a molecular weight of 300-350 kDa [START_REF] Gonzalez-Perez | Sunflower proteins: overview of their physicochemical, structural and functional properties[END_REF], and this protein mainly exists in the 11S form (hexametric structure). Depending on pH, ionic strength, temperature and protein concentration, helianthinin may also occur in the 15-18S, 7S or 3S forms. In 11S sunflower proteins, different subunits are traditionally processed to give an acidic and a basic polypeptide linked by a single disulfide bond. These basic and acidic polypeptides range in molecular weight from about 21 to 27 kDa and from about 32 to 44 kDa respectively. The solubility of helianthinin with a minimum of 4-5.5 depends strongly on pH and ionic strength.

Albumin proteins from sunflower, with a sedimentation coefficient of approximately 2S and molecular weights ranging from 10 to 18 kDa, show good solubility in aqueous solutions, independent of pH and ionic strength. Contrary to the majority fractions, the functional properties of glutelins and prolamins from sunflower seeds have not been reported in the literature.

The amino acid composition of soy proteins [START_REF] Kovalenko | Determination of amino acid composition of soybeans (Glycine max) by near-infrared spectroscopy[END_REF] and sunflower proteins [START_REF] Conde | Effect of enzymatic treatment of extracted sunflower proteins on solubility, amino acid composition, and surface activity[END_REF] 30 sunflower proteins could lead to an improvement in their functional properties and to new interesting applications [START_REF] Conde | The effect of enzymatic treatment of a sunflower protein isolate on the rate of adsorption at the air-water interface[END_REF]. The presence of phenolic compounds in sunflower proteins, which cause the green-brown color of its powder, limits their development as a source of food proteins for humans. Therefore, there could be very interesting new openings for these proteins in non-food industrial sectors. Microencapsulation could be one possibility for an industrial application of this agricultural by-product.

Industrial applications of microencapsulation by vegetable proteins

Pea proteins show a good properties for their potential application, in particular for the production of adhesives, bioplastics, emulsifiers and wall forming materials for microencapsulation [START_REF] De Graaf | Requirements for non-food applications of pea proteins. A review[END_REF]. However, these proteins are no suggested to be used in technical applications. The functional properties of wheat proteins and corn zein also suggest several potential applications for these natural polymers in the fields of adhesives, matrix materials for microencapsulation, textiles, cosmetics and biodegradable plastics [START_REF] Shukla | Zein: the industrial protein from corn[END_REF]. For both of these proteins, there is still no actual industrial application in microencapsulation, but they are potentially good candidates.

Conversely, soy bean proteins are already used as wall forming materials in the food industry, in particular to mask the undesirable taste of some nutritional additives (bioactive compounds for athletes, such as casein hydrolysate) [START_REF] Favaro-Trindade | The use of spray drying technology to reduce bitter taste of casein hydrolysate[END_REF][START_REF] Mendanha | Microencapsulation of casein hydrolysate by complex coacervation with SPI/pectin[END_REF][START_REF] Ortiz | Production and properties of casein hydrolysate microencapsulated by spray drying with soybean protein isolate[END_REF][START_REF] Sun-Waterhouse | Industry-relevant approaches for minimising the bitterness of bioactive compounds in functional foods: a review[END_REF] or to protect components sensitive to oxidation and\or volatile aromas (orange oil) [START_REF] Gharsallaoui | Applications of spray-drying in microencapsulation of food ingredients: An overview[END_REF][START_REF] Kim | Microencapsulation properties of gum arabic and several food proteins: spray-dried orange oil emulsion particles[END_REF][START_REF] Xiao | Microencapsulation of sweet orange oil by complex coacervation with soybean protein isolate/gum arabic[END_REF].

  .3. Pea proteins as an additive for microencapsulating systems3.3. Wheat and other cereal proteins

Fig. 1 .

 1 Fig. 1. Different morphologies of microparticles obtained by microencapsulation: (a) microcapsule, (b) microsphere, (c) multilayer microcapsule and (d) multishell and multicore microsphere.

Fig. 2 .

 2 Fig. 2. Schematic representation of microencapsulation process by spray-drying (a) and coacervation (b) techniques. (Redrawn from Jyothi et al., 2010).

  are shown in Figure3. Some similarities in total amino acid content for these vegetable proteins can be seen. The physico-chemical properties of sunflower proteins have already been studied[START_REF] Gonzalez-Perez | Physicochemical properties of 2S albumins and the corresponding protein isolate from sunflower (Helianthus annuus)[END_REF][START_REF] Molina | Effect of pH and ionic strength modifications on thermal denaturation of the 11S globulin of sunflower (Helianthus annuus)[END_REF] al., Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 29 2004; Patino et al., 2007). Most authors showed that sunflower preparations have better (or at least similar) emulsifying properties as those of soy protein preparations. The main results of these studies showed that the highest emulsifying capacity is observed in the pH range of 7-8 and the minimum at the isoelectric pH of 4.3; the extraction method and solvent used for protein extraction does not change the emulsifying ability of proteins; heating involving protein denaturation, increases the stability of emulsions but reduces their emulsifying capacities. This latter observation can be explained by the change of protein structure during heating denaturation, favoring chain unfolding and increased conformational flexibility. Thus the surface-active capacity of unfolded sunflower proteins becomes lower during emulsion formation, but after emulsion preparation it stays stable longer.

Fig. 3 .

 3 Fig.3. Amino acid composition of soy[START_REF] Kovalenko | Determination of amino acid composition of soybeans (Glycine max) by near-infrared spectroscopy[END_REF] and sunflower[START_REF] Conde | Effect of enzymatic treatment of extracted sunflower proteins on solubility, amino acid composition, and surface activity[END_REF] proteins, every amino acid fraction is presented in g/100g of protein isolate.

  Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance)(2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 315. Conclusions and future prospectsThe use of vegetable proteins as a wall material for microencapsulation of various sensitive materials, reflects the actual "green" tendency in the food, pharmaceutical and cosmetics industries. The two main techniques used for microencapsulation of different core substances by these natural polymers, are spray-drying and coacervation. Particle morphology is very dependent on the process chosen, mainly because coacervation produces microcapsules, whereas microspheres are generally obtained with spray-drying. Vegetable proteins widely used as encapsulants are pea protein isolate, soy protein isolate, wheat gliadins, corn zein and barley protein. The various studies have proved the ability of proteins to efficiently protect different forms of active materials (hydrophilic or hydrophobic, solid or liquid) as an encapsulating agent, using both spray-drying and coacervation methods. However, microencapsulation efficiency, preparation stability and microparticle size could be affected by different parameters, such as active core and wall material concentrations, temperature and pH of media, encapsulation technique, use of additives or proteins combined with polysaccharides. Other inexpensive proteins extracted from rice, oat or sunflower seeds are known for their interesting functional properties and could be suitable microencapsulation wall forming materials. These natural polymers show good solubility, emulsion forming ability and foaming stability, giving them the appropriate characteristics for potential use as efficient coating materials. Moreover, they can be associated with polysaccharides as is commonly the case in microencapsulation. Thus, the good physico-chemical properties of all these vegetable proteins open a new path for specific applications, the development of innovative delivery systems, and/or functional food products. Some limitations of vegetable protein use for making high added value products could be the extraction cost to obtain high-quality proteins, low solubility of some proteins and Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 32 large polydispersity in the size of naturally occurring protein chains. Compared to other bio based materials for microencapsulation, such as polysaccharides, synthetic polymers or animal-based proteins, plant extracted vegetable proteins represent a very promising source of polymers with interesting functional properties. Their use as a wall material augurs well for the encapsulation of hydrophilic and hydrophobic substances by different techniques, and production of microparticles, with good microencapsulation efficiency and various potential applications.

Table 1 .

 1 Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Microencapsulation with SPI-based wall material.

	11
	Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness.

Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035

Table 2 .

 2 High pressure homogenization influence on the SPI-based emulsions properties.

	Wall/core w/w	Pressure (MPa) Droplet size (µm)	Viscosity	Reference
				(cP)
	2/1	35	1.52	-	Kim et al. (1996)
	2/1	35	0.57±0.1	-	Augustin et al.
					(2006)
	1/1	18 → 35	0.45→0.32	-	Rusli et al. (2006)
	1/1	15 → 55	-	100 → 70	Yu et al. (2007)
	4/1	30	1.05±0.66	-	Rascon et al. (2010)
	2/1	50	1.1±0.02	15	Nesterenko et al.

  and the droplets' coalescence resistance, and thus a decrease in their mean diameter (generally detected by light scattering). On the other hand,Lazko et al. (2004a) also demonstrated that protein concentration does not seem to have a significant influence on the microcapsule wall thickness.

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 15 water interface

Table 3

 3

shows various pea protein/polysaccharide pairs and the different processes used for active core microencapsulation. Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 18

Table 3 .

 3 Microencapsulation based on the complexes pea protein/polysaccharide as a wall material.

	Microencapsulation	Wall material	Core material	Reference
	process	Protein	Polysaccharide
	Complex coacervation	Pea globulins	Gum arabic	Triglyceride	Ducel et al.
			Carboxy-		(2004)
			methylcellulose
			Sodium alginate
	Spray-drying	Pea proteins	Maltodextrin	Ascorbic acid Pierrucci et
					al. (2006)
	Spray-drying	Pea proteins	Maltodextrin	α-tocopherol	Pierrucci et
					al. (2007)
	Spray-drying	Pea proteins	Maltodextrin	Ascorbic acid Pereira et al.
					(2009)
	Spray-drying	Pea proteins	Pectin	Triglyceride Gharsallaoui
		(as additive)	Maltodextrin		et al. (2010)

Table 4 .

 4 The properties of microparticles produced by spray-drying with the pea proteins as a

	wall material (wall/core ratio of 2/1 w/w).
	Wall material	Core material Microparticle	MEE	Core amount	Reference
			size (µm)	(%)	(g/100 g
					powder)
	Pea proteins	α-tocopherol	2.2	86.8	28	Pierrucci
						et al.
	Pea proteins/	α-tocopherol	3.5	77.8	25	(2007)
	maltodextrin (1/1,
	w/w)
	Pea proteins	Ascorbic acid	2.7	101.9	34.6	Pierrucci
						et al.
	Pea proteins/	Ascorbic acid
	maltodextrin (1/1,		8.2	95.9	29
	w/w)

  Its two main components are gliadin and glutenin. Gliadin is composed of single chain polypeptides, with an average molecular weight of 25-100 kDa, linked by intramolecular disulfide bonds, and soluble in neutral 70% ethanol. Glutenin is a hydrosoluble fraction consisting of gliadin-like subunits stabilized by intermolecular disulfide bonds in large aggregates, with a molecular weight greater than 105 kDa

Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 22 polysaccharide fraction.

Table 5 .

 5 Cereal proteins in the microencapsulation

	Microencapsulation	Wall material	Core material	Reference
	process
	Spray-drying	Wheat gliadin, corn zeine	Linoleic acid	Iwami et al. (1987)
	Spray-drying	Barley protein	Fish oil	Wang et al.
				(2011a,b)
	Simple coacervation	Gluten/casein	Pyrrolnitrin	Yu and Lee (1997)
	Simple coacervation	Gliadin	Hexadecane	Mauguet et al.
				(2002)
	Complex	α-Gliadin/arabic gum	Vaselin oil	Ducel et al. (2004,
	coacervation			2005)
	Solvent evaporation	Gliadin	Retinoic acid	Ezpeleta et al.

Table 5 summarizes microencapsulation studies with these biopolymers. Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 23

  Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 26 potential source of inexpensive high quality proteins

  Comment citer ce document : Nesterenko, A., Alric, I., Silvestre, F., Durrieu, V. (Auteur de correspondance) (2013). Vegetable proteins in microencapsulation: A review of recent interventions and their effectiveness. Industrial Crops and Products, 42, 469-479. , DOI : 10.1016/j.indcrop.2012.06.035 27 concerns the food industry, rather than potential low volume, high added value applications of microencapsulation.