Claire Bourlieu-Lacanal

Valérie Guillard
email: valerie.guillard@univ-montp2.fr

Baltasar Vallès-Pàmies
email: baltasar.valles-pamies@rdls.nestle.com

Nathalie Gontard
email: nathalie.gontard@univ-montp2.fr

C Bourlieu
email: claire.bourlieu@univ-montp2.fr

B Vallès-Pamiès

Edible moisture barriers: materials, shaping techniques and promises in food product stabilization

come

Introduction

The reduction of mass transfer between a food product and its surrounding atmosphere by coating the entire product with an edible material, is an extremely old practice, already used in the XII century in China (fruit waxing), and in England during the XVI century (meat larding) [START_REF] Kester | Edible films and coatings: a review[END_REF]. Today, controlling mass, and more specifically moisture transfer, still remains an important challenge to maintain the quality of fresh or processed products, such as fruits, meats and seafood products. In ready-to-eat composite foods, the limitation of internal moisture transfer between components is also of major concern. It is all the more important as consumers' demand for this kind of convenient product has increased. Moisture transfer from the "wet" to the "dry" component of these products affect the physical properties, specially texture, and chemical composition of the food system, and consequently, its quality and shelf-life [START_REF] Katz | Effect of water activity on the sensory crispness and mechanical deformation of snack food-products[END_REF].

The application of edible films and coatings can help reducing internal and external water transfer in slightly modified and processed food products (Debeaufort, Quezada-Gallo and Voilley 2000;[START_REF] Guilbert | Prolongation of the shelf-life of perishable food products using biodegradable films and coatings[END_REF][START_REF] Guillard | Preventing moisture transfer in a composite food using edible films: Experimental and mathematical study[END_REF][START_REF] Koelsch | Edible water vapor barriers: properties and promises[END_REF]. Edible protective films An investigation in international patent databases [START_REF] Anonymous | European Patent Office. Advanced search[END_REF]) for deposited patents dealing with "edible moisture barriers" over the last 25 years, gave more than 50 answers. Of the total, 18 % of the patents deal with the development of pure fat barriers, 58 % with the development of a composite barriers including fat and other components (polysaccharides and protein derivatives or inorganic fillers), 13 % with barriers based on pure hydrocolloids (protein or polysaccharides), 4 % with sugar coatings, 4 % with edible moisture barriers which can be based on any of the three kinds of components, and eventually 2 % based on pure thin inorganic coatings. It is interesting to note that 25 % of the patents directly describe the development of a composite food in which the edible film is supposed to be used.

A new class of barrier materials based on pure, thin (0.05 micron or less) amorphous inorganic coatings have been lately reported [START_REF] Bastiaans | Moisture barrier in foods. Patent NZ530643[END_REF]. Such thin coatings overcome the textural problems associated with the use of organic coatings which have to be applied as a thick layer to be effective. The inorganic substance must be approved to be used in foods. In the United States, according to the section 21 of the Code of Federal Regulations for edible products (Anonymous 1977), authorized inorganic compounds are: silicon dioxide, single silicates, such as sodium silicate, calcium silicate and magnesium silicate, aluminium silicate, magnesium trisilicate, composite silicates such as sodium aluminium silicate, potassium aluminium silicate and calcium aluminium silicate, talc, clay materials such as bentonite, carbon, insoluble carbonates and phosphates. Even though the use of pure thin inorganic coatings as moisture barrier is still marginal, the possibility of using these materials as inorganic fillers in a barrier is also suggested in 8 % of the patents. Edible inorganic compounds can thus be considered as a new category of edible coating materials. Native or hydrogenated palm, palm kernel, rapeseed, soya, peanut, coconut, castor, cotton oils, cocoa butter and their derivatives (obtained by fractionation, esterification, concentration and/or reconstitution: fatty acids and alcohols, mono-, di-and triglycerides, cocoa butter substitutes, margarine, shortenings, acetylated glycerides, lecithins, etc); Carnauba, candelilla, rice bran, and fruit (apple, bamboo, sugar, cane, citrus) waxes, jojoba oil; Wood rosin, tree lacs, citrus terpenes, gum lacs; Camphor, mint and citrus fruit essential oils; Liquorice Animal Chitin, chitosan.

Collagen, gelatin, meat proteins; Keratin; Fish proteins (myofibrillar proteins and elastin); Casein, caseinates; Whey proteins; Ovalbumin Native or fractionated milk , lard, tallow fats and their derivatives (obtained by fractionation, esterification, concentration and/or reconstitution: fatty acids and alcohols, mono-, di-and triglycerides, cocoa butter substitutes, margarine, shortenings, acetylated glycerides, lecithins etc); Beeswax, spermaceti*, chinese wax, shellac

Microbial

Xanthan, dextran, pullulan, gellan Chitosan -Mineral, fossil or synthetic --Paraffin, polyethylene wax, microcrystalline wax; Lignite, peat, montan waxes *Formerly extracted from whale adipose tissue. No longer produced and sold in accordance with international regulations concerning whale capture. Now replaced by synthetic spermaceti made of pure cetyl palmitate or mixtures based on jojoba oil.

Methods of formation of edible barriers

Edible films and coatings are classically made following two main methods [START_REF] Guilbert | Les films et enrobages comestibles[END_REF]:

a) The "thermoplastic method" based on the thermoplastic properties of the film-forming material.

b) The "solvent method" based on a coacervation from a solution or a dispersion of the film-forming material in a solvent phase, followed by the evaporation of the solvent.

An example of these methods illustrated for lipids is shown in Fig. 1. The thermoplastic method consists in shaping film-forming materials using thermal or thermo-mechanical processes in conditions of low hydration and induces structural transitions in the material such as melting of lipids and crystalline parts of polymers or a transition from the glassy metastable state to the rubbery state in the amorphous parts of the polymers [START_REF] Guilbert | Les films et enrobages comestibles[END_REF]. It is the most common procedure of forming lipid films and coatings. However, This method is less commonly used to shape hydrocolloids than the solvent method. The material is melted at an appropriate temperature or following a tempering schedule, and then it is solidified on a surface. Melting-solidification results in a dense crystalline network arrangement of the lipid material. High melting point lipids, such as waxes, require specific care since they solidify quickly. Therefore, they are often applied as emulsions or dispersions, using the solvent method to overcome this issue.

In the solvent method the separation of the solubilised or dispersed material from the solvent phase can be explained by precipitation or phase change induced by solvent evaporation, addition of electrolyte, pH modification or heat treatment [START_REF] Krochta | Protein-based edible films[END_REF]. Such treatments can be adjusted to enhance film formation or specific properties. For composite emulsion-based films or coatings a lipid material and most likely a surfactant, is added to the solution, which is then heated above the lipid melting point and homogenised. The prepared solution is then applied on an appropriate support and the solvent evaporates.

Hydrophilic materials

Due to their hydrophilic nature, pure polysaccharide and protein films exhibit limited water vapour barrier ability. These coatings are thus favoured when other barrier properties are desired. Most of these films present interesting oil and gas (oxygen, carbon dioxide) barrier properties at low relative humidity [START_REF] Albert | Comparative evaluation of edible coatings to reduce fat uptake in a deep-fried cereal product[END_REF]. They are also characterized by good mechanical properties, specially those based on proteins which present a unique structure based on 20 monomers and a high intermolecular binding potential [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF]. Hydrocolloid coatings can be applied in the form of high moisture gelatinous coatings, which delay moisture loss from coated foods by functioning as "sacrificing" agents rather than moisture barriers [START_REF] Kester | Edible films and coatings: a review[END_REF]. They can also be chemically, enzymatically and/or physically treated to improve their moisture resistance [START_REF] Ou | Role of ferulic acid in preparing edible films from soy protein isolate[END_REF][START_REF] Tang | Effect of transglutaminase treatment on the properties of cast films of soy protein isolates[END_REF].

The properties of various film-forming polysaccharides, such as alginates, pectins, starches, dextrins, cellulose, carrageenan, gums, chitosans and their derivatives, have been investigated for a long time and reviewed by [START_REF] Nisperos-Carriedo | Edible coatings and films based on polysaccharides[END_REF]. Their wide uses in the food industry have been favoured by their abundance, variability and low-cost, and are summarized in Tables 2a and2b.

Several studies reviewed formulations, barrier properties and possible application of edible protein-based films (Table 3) [START_REF] Gennadios | Edible coatings and films based on proteins[END_REF]Krochta and Mc Hugh 1997;[START_REF] Torres | Edible films and coatings from proteins[END_REF]. Overall, similarly to polysaccharide films, proteins exhibit relatively low moisture barrier properties, two to four times lower than conventional polymeric packaging materials (McHugh and Krochta 1994d). The limited resistance of protein films to water vapour transmission is attributed to their substantial hydrophilicity and to the amounts of plasticizers, such as glycerol and sorbitol, incorporated into films to impart adequate flexibility. The potential occurrence of adverse reactions to native proteins constitutes a huge limitation in the potential applications of various protein-based films (e.g. allergies to milk, egg white, peanut and soybean proteins, gluten…). Native proteins include two types of molecules: fibrous proteins, with structural water-insoluble groups and biologically active globular proteins. Generally, these globular proteins have to be modified, by heat treatment, pH modification or solvent addition, to obtain extended structures more susceptible to form a film. Flexible yellowish films with low moisture resistance properties formed from soy protein aqueous dispersions upon heating; Alkaline conditions reinforce film functional properties Protection of various food products (nuts, aroma and flavours encapsulation, fresh meat, battered meat, etc) [START_REF] Gennadios | Edible films and coatings from soymilk and soy protein[END_REF]

Hydrophobic materials

The most commonly used hydrophobic film-forming barrier materials include (by decreasing order of efficiency): -waxes -lacs -fatty acids and alcohols -acetylated glycerides -cocoa-based compounds and their derivatives

The classification of lipids by increasing efficiency can be explained by the chemical composition of the molecules (presence of polar components, hydrocarbon chain length, number of unsaturation or acetylation). For components having the same chemical nature, increasing chain length modifies the barrier properties because the polar part of the molecule decreases and does not favour water solubility in the film (McHugh and Krochta 1994d). The barrier efficiency of lipids also depends on their physical state (solid fat content at the temperature of use, crystalline form, etc). Indeed, many lipids exist in a crystalline form and each individual crystal is impervious to water vapour. Water flow permeates mainly between crystals and the intercrystalline packing arrangement has major consequences on the barrier properties of the material [START_REF] Martini | The water vapor permeability of polycrystalline fat barrier films[END_REF]. Consequently, migration rate can be slowed down to a certain extent by a proper tempering, which induces a more efficient structure against moisture migration. In the same way, in a continuous lipid phase, vapour migrates more easily in the liquid fat portion of the product. The solid fat content of the product at the temperature of application clearly influences the migration rate [START_REF] Ghosh | Fat, moisture, and ethanol migration through chocolates and confectionary coatings[END_REF]Kester and Fennema 1989c;Kester and Fennema 1989d).

Paraffin followed by candelilla wax and microcrystalline waxes, and eventually by beeswax, are considered as the most effective moisture barriers derived from edible waxes [START_REF] Morillon | Factors affecting the moisture permeability of lipid-based edible films: A review[END_REF]. There is no satisfactory chemical definition for the term "wax" which is used for a variety of products of mineral, botanical and animal origin that contain various kinds of fatty materials (Table 4). The term "resins" or "lacs" can also be used for plant or insect secretions that take place along resins ducts, often in response to injury or infection, and result in more acidic substances [START_REF] Hernandez | Edible coatings from lipids and resins[END_REF]. However, all waxes tend to contain wax esters as major components, i.e., esters of long-chain fatty alcohols with long chain fatty acids. Depending on their source, they may additionally include hydrocarbons, sterol esters, aliphatic aldehydes, primary and secondary alcohols, diols, ketones, triacylglycerols, etc.

Waxes usually present an orthorhombic system of crystallization, favoured by slow cooling rate, and possibly, a small fraction of hexagonal crystals. They possess variable mechanical properties depending of their composition. Carnauba waxes are harder than all the other waxes to which they are added to improve their strength and gloss. Candelilla waxes solidify slowly and reach an intermediate hardness between carnauba and beeswax. Beeswax is relatively flexible and presents a viscoelastic behaviour [START_REF] Shellhammer | Viscoelastic properties of edible lipids[END_REF].

Table 4. Waxes and lacs: Class, sources and prevalent molecular species in their composition. (Anonymous 2006a;[START_REF] Anonymous | Waxes: Structure, composition, occurrence and analysis[END_REF][START_REF] Hamilton | Waxes: Chemistry, Molecular Biology and Functions[END_REF][START_REF] Spencer | Jojoba Oil Analysis by High Pressure Liquid Chromatography and Gas Chromatography Mass Spectrometry[END_REF]. Wax coatings have been used since the 1930s to control desiccation and ripening of fresh fruits and vegetables by limiting gas diffusion [START_REF] Callegarin | Lipids and biopackaging[END_REF][START_REF] Hernandez | Edible coatings from lipids and resins[END_REF][START_REF] Kester | Edible films and coatings: a review[END_REF]. Such coatings also reduce the surface abrasion of the fruit surface during handling, improve appearance by enhancing surface gloss and were used as carrier for other active components such as fungicides. However, if the gas permeability of the coating is not adequate, waxing can result in the creation of a modified internal anaerobic atmosphere inducing offflavours and deterioration of the product [START_REF] Baldwin | Use of lipids in coatings for food products[END_REF]. Many wax coatings are applied as emulsions (macro-emulsions particles size range 2,000-100, 000 Å or micro-emulsions particles size range 1,000-2, 000 Å) or as wax suspensions. Though most natural waxes have emulsifying properties, the stability of the wax emulsion is reinforced by the use of surface active agents such as fatty acids (palmitic, oleic, stearic etc), glycerol and fatty acids derivatives or lecithins [START_REF] Hernandez | Edible coatings from lipids and resins[END_REF]. Numerous applications of wax coatings on whole citrus fruits have been reported in the scientific literature: emulsion candelilla wax/water on limes (Paredes, Camargor and Gallardo 1974), carnauba wax on lemons [START_REF] Hagenmaier | Wax microemulsions and emulsions as citrus coatings[END_REF][START_REF] Hagenmaier | Layered coatings to control weight-loss and preserve gloss of citrus fruit[END_REF]. Wax coatings have also been applied on slightly processed fruits: carnauba wax on grapefruit pieces [START_REF] Hagenmaier | Edible coatings from morpholine-free wax microemulsions[END_REF], beeswax on orange fruits [START_REF] Baldwin | Use of lipids in coatings for food products[END_REF], other wax coatings on apples and pears [START_REF] Drake | Storage quality of waxed and non waxed 'Delicious' and 'Golden Delicious' apples[END_REF][START_REF] Drake | Quality attributes of d'Anjou Pears after different drying temperatures and refrigerated storage[END_REF], peaches [START_REF] Kraght | Waxing peaches with the consumer in mind[END_REF], tropical fruits and vegetables [START_REF] Baldwin | Edible coatings for fruits and vegetables: past, present, and future[END_REF][START_REF] Baldwin | Effect of two edible coatings with different permeability characteris-tics on mango (Mangifera indica L.) ripening during storage[END_REF][START_REF] Hoa | Effect of different coating treatments on the quality of mango fruit[END_REF][START_REF] Mcguire | Market quality of guavas after hot-water quarantine treatment and application of carnauba wax coating[END_REF]. A limited number of processed food products have also been coated with waxes and lacs, such as shellac: waxing of candies and breakfast cereal mixes [START_REF] Lowe | Continuous raisin coater[END_REF][START_REF] Bolin | Texture and crystallization control in raisins[END_REF][START_REF] Seaborne | Method for preparing a low water permeability edible film[END_REF], application of commercial glazing and anti-sticking blends (waxes/lacs alcoholic dispersions or suspensions) on confectionery and dry fruits (Capol®, Kaul GmbH, Germany). The restricted use of waxes and lacs as edible coatings for processed food products can be explained first by regulatory concerns and secondly because of their high melting point responsible for unappealing sensorial properties (hardness and waxy residues). * GMP: Good Manufacturing Practices; **quantum satis: Amount not to exceed that required to produce the intended effect; ***GRAS: Generally Recognized As Safe for human consumption; #: Additives which may be added to all foodstuffs except for those referred to in the Article 2 (3) of the consolidated directive 95/2/EC.

In the classification of moisture barriers efficiency established by Kester and Fennema, waxes are followed by stearyl alcohol, acetyl acyl glycerols, hexatriacontane, tristearin and stearic acid (Kester and Fennema 1989a). These authors observed that stearic alcohol was seven times more impervious to water transfer (0-100 % RH gradient) than stearic acid. This result can be explained by the lower polarity of the hydroxyl function compared to the carbonyl function but also to the specific sheet structure developed by the stearyl alcohols. Fatty alcohols and fatty acids lack structural integrity to form strong continuous coatings and are used mainly as emulsifying or dispersing agents in combination with other biopolymers. The formulation of multi-component films can affect their properties. Mc Hugh and Krochta [START_REF] Gennadios | Edible coatings and films based on proteins[END_REF] observed that composite films based on fatty alcohols/whey proteins were less effective as moisture barriers than fatty acids/whey proteins. This discrepancy with Kester and Fennema's study could be explained by the influence of the polar support of the fatty alcohol composite film on moisture transfer. Roth and Longin (1984) showed that C16 or C18 fatty alcohols were the most effective to limit water evaporation from the surface of hydrated products. Similarly, [START_REF] Hagenmaier | Edible coatings from morpholine-free wax microemulsions[END_REF] reported that micro-wax emulsions including stearic and palmitic acids were more effective to limit fruit desiccation than those using lauric and oleic acids as emulsifiers. [START_REF] Koelsch | Functional physical and morphological properties of methyl cellulose and fatty acids-based edible barriers[END_REF] showed that the moisture barrier properties of composite films (emulsions: methylcelulose/ fatty acids, 70:30) increased with the degree of saturation and fatty acid chain length up to 18 carbons. The higher efficiency of stearic and palmitic fatty acids and alcohols compared to component of similar chemical nature but of different chain length was reported in various other studies [START_REF] Hagenmaier | Moisture permeability of edible films made with fatty-acid and (hydroxypropyl)methylcellulose[END_REF]McHugh and Krochta 1994c;[START_REF] Park | Fatty-acid concentration-effect on tensile-strength, elongation, and water vapor permeability of laminated edible films[END_REF]. The positive effect of long aliphatic chain up to 18 carbons could be explained by an increased apolar part in the molecule which does not favour water solubility in the film. Above this threshold, the aliphatic long chains induce a more heterogeneous structure.

Similarly to fatty acids and fatty alcohols, acyl glycerols (esters of glycerol and fatty acids) are often used as emulsifying and dispersing agents because of their poor mechanical properties (Table 5). [START_REF] Higuchi | A study of permeability to water vapor of fats, waxes and other enteric coating material[END_REF] could not investigate the moisture barrier properties of pure self-supported films of di-and tri-glyceryl stearates, because of the development of structural defects. However they studied pure glyceryl-monostearate film and blends of glyceryl-stearate with beeswax. The resistance to water transfer of glyceryl-monostearate appeared highly dependent on the relative humidity gradient the film was subjected to. This conclusion was in agreement with another study dealing with a monostearyl-glycerol film formed on a cellophane support (Martin-Polo and Voilley 1990). The film showed moisture barrier properties ten times higher than cellulose triacetate and cellulose acetate propi-onate films but much lower than synthetic plastic films. Mono-stearyl glycerol has been reported as a very effective emulsifier to improve the adherence of an alcane layer on a hydrophile suppport [START_REF] Quezada Gallo | Lipid hydrophobicity, physical state and distribution effects on the properties of emulsion-based edible films[END_REF]. In the classification of lipid materials established by Kester and Fennema (1989a), tristearyl-glycerols were reported as 1.5 times more resistant to water transfer than stearic acid but half as resistant as hexatriacontane (alcane).

Acetyl-acyl-glycerols (acyl glycerols with 1 or 2 acetyl group on the glycerol molecule) commonly called acetylated glycerides, can be prepared through a reaction between glycerides and acetic anhydride or through a catalysed interesterification of edible fats with triacetin. A highly noticeable property of these compounds compared to other lipids is a good flexibility in their α-polymorphic form. The acetylated glycerides are also characterized by a high resistance to oxidative degradation, a non greasy touch and a low melting point resulting from the presence of acetyl groups in the glyceride molecule. Used at concentration from 2 to 10 % (w.b.), they make excellent plasticizers and significantly improve the mechanical properties of high melting point fats or of other fats at low temperature [START_REF] Alfinslater | The Present Status of Acetoglycerides[END_REF]. Their properties depend on the nature of the acyl-glycerols they are based on, and on their acetylation degree. For example, aceto-stearin films have oxidative stability, especially if derived from hydrogenated vegetable oils, while aceto-olein films are less resistant to oxidation. Tailored functional properties of blends can thus be achieved by combining various molecules (Alfin-Slater 1958;Feuge et al. 1953). Distilled acetylated monoglycerides coatings produced under the trade name Myvacet (Eastman Chemical Product., Kingsport, TN) were first used in edible packaging application on fresh products in the late 1950s [START_REF] Woodmansee | Coating subscalded broiler parts in order to afford protection against dehydration and skin darkening in fresh storage[END_REF]. Scientific and patent literature disclose numerous examples of application on meats (Dawson, [START_REF] Davis | Edible moisture barrier coating. Patent GB2242815[END_REF]Schneide 1972;[START_REF] Stemmler | Method for preserving freshly slaughtered meat[END_REF][START_REF] Zabik | The acceptability of cooked poultry protected by an edible acetylated monoglyceride coating during fresh and frozen storage[END_REF], frozen fish (Hirasa 1991;[START_REF] Stuchell | Edible coating on frozen king salmon; effect of whey protein isolate and acetylated monoglycerides on moisture loss and lipid oxidation[END_REF], fresh or dry fruits, and vegetables (AvenaBustillos, [START_REF] Avenabustillos | Water vapor resistance of red delicious apples and celery sticks coated with edible caseinate-acetylated monoglyceride films[END_REF][START_REF] Mate | Whey protein and acetylated monoglyceride edible coatings: Effect on the rancidity process of walnuts[END_REF] .

More recently water-related properties and water barrier properties of acetylated monoglycerides and diglycerides presenting variable chain length and acetylation degrees were evaluated by [START_REF] Guillard | Preventing moisture transfer in a composite food using edible films: Experimental and mathematical study[END_REF]. The extension of shelf-life enabled by such films in a two-component composite bakery food product (sponge-cake/ barrier film/ agar gel) was estimated. Films with the highest acetylation degree (70%) presented significantly lower moisture sorption on the high a w range (0.70-1.00), than other compounds and enabled the best extension of the baked product shelf-life. This observation was in accordance with the decrease in HLB (hydrophilic-lipophilic balance) observed between not acetylated (glycerol monostearate, H.L.B. of 3.8) and acetylated monoglycerides (H.L.B. of 1.5) [START_REF] Hernandez | Edible coatings from lipids and resins[END_REF]. According to model predictions, the most effective barrier (100 µm thick) delayed for more than 20 days the increase of the sponge-cake moisture content from 23 to 40% (w. b.) which was reached in less than 2 days in the product without barrier. Few applications and sensory problems associated with acetylated glycerides edible coatings have been reported, including the tendency of coatings from highly saturated acetylated compounds to crack and flake during refrigerated storage (Hirasa 1991), to pick up foreign flavours [START_REF] Zabik | The acceptability of cooked poultry protected by an edible acetylated monoglyceride coating during fresh and frozen storage[END_REF], and to exhibit acidic or bitter aftertaste attributed to acetylated compounds [START_REF] Morgan | Edible packaging update. Food Product Development[END_REF]. Because of their fluidity, oils exhibit poor moisture barrier properties that can nevertheless be improved by hydrogenation. They are widely used in refrigerated or frozen product, possibly after a winterization (removal of crystalline triacylglycerols).

Among the list of the numerous materials that can be used as moisture barriers, cocoa butters and cocoa-based films are the most widely used in the confectionery (chocolate) and bakery industries [START_REF] Biquet | Evaluation of the Moisture Permeability Characteristics of Chocolate Films as an Edible Moisture Barrier[END_REF][START_REF] Morillon | Factors affecting the moisture permeability of lipid-based edible films: A review[END_REF]. The good sensorial properties of chocolate permit using thick perceptible coatings that will both resist moisture migration and increase the commercial value of the product. The first comprehensive study on chocolate barrier property was done by [START_REF] Biquet | Evaluation of the Moisture Permeability Characteristics of Chocolate Films as an Edible Moisture Barrier[END_REF]. These authors determined the moisture sorption isotherms, effective diffusion coefficient and water vapour permeability of a dark chocolate film (0.6 to 1.2 mm thick). They reported that a 0.6 mm coating of semisweet dark chocolate used as barrier coating on a monocomponent system (agar gel) was a more effective moisture barrier than a 0.025 mm low density polyethylene coating. However, [START_REF] Guillard | Preventing moisture transfer in a composite food using edible films: Experimental and mathematical study[END_REF] pointed out the poor water barrier properties of dark chocolate film used at the interface between two components in the high a w range (a w > 0.8) which could be explained by sugar dissolution phenomena. A comprehensive review and several publications on the barrier properties of chocolate were recently published by Gosh [START_REF] Ghosh | Fat, moisture, and ethanol migration through chocolates and confectionary coatings[END_REF]Ghosh 2003;[START_REF] Ghosh | Diffusion of moisture through chocolate-flavoured confectionery coatings[END_REF][START_REF] Ghosh | Moisture migration through chocolate-flavored confectionery coatings[END_REF].

Formulation and structuring of moisture barrier films

Plasticizer addition and combination of materials (lipids, hydrocolloids or blends of lipids and hydrocolloids) are usually used to formulate moisture barriers and overcome the problems associated with the use of a single film-forming material.

Addition of plasticizer

The cohesiveness and flexibility of edible films are determined by the molecular weight, branching and polarity of their constituents. Molecules with low polarity and high linearity tend to produce films with high degree of cohesiveness and rigidity [START_REF] Morillon | Factors affecting the moisture permeability of lipid-based edible films: A review[END_REF]. Plasticizers, by weakening intermolecular forces between adjacent polymer chains, reduce brittleness, increase flexibility and tear resistance of edible films. This is particularly important when the product is stored at a low RH and/or temperature. The plasticizer must be compatible (miscible) with the polymer and if possible with its solvent. Plasticizers having food applications include: (a) mono-, di-, oligo-saccharides (mainly glucose and fructose-glucose syrups, honey), (b) polyols (mainly sorbitol, glycerol, glyceryl derivatives and polyethylene glycols), (c) lipids and derivatives (mainly fatty acids, monoglycerides and ester derivatives, phospholipids, surfactants). The formulation of films including plasticizers (usually added from 10 to 30 % d.b.) must be conducted carefully, since they tend to migrate, diluting and softening the structure of the film, resulting in lower water resistance [START_REF] Guilbert | Technology and application of edible protective films[END_REF].

Combination of different fat materials

Multilayered pure fat structures were advocated in several patents [START_REF] Nielsen | Composition[END_REF][START_REF] Van Gastel | Moisture barriers[END_REF]. Recently a bilayered barrier which combined a soft spreadable fat (oil continuous spread, Solid fat content of 5-20 % at 20°C) and a high (> 35°C) melting point fat has been patented [START_REF] Van Gastel | Moisture barriers[END_REF]. The soft spread fills up the pores and homogenises the product surface whereas the second layers confers the moisture resistance. Another multilayered lipid-based barrier has been recently patented too [START_REF] Gaonkar | Edible moisture barrier for food products[END_REF][START_REF] Gaonkar | Multilayer edible moisture barrier for food products[END_REF][START_REF] Loh | Physical properties and sensory evaluation of WPI films of varying thickness[END_REF][START_REF] Smith | Microweable grilled cheese and meat sandwiches[END_REF]. It includes a flexible layer (50 µm to 1 mm thick) containing short chain fatty acids crystallized in the αform and a moisture resistant hydrophobic layer composed of a low melting fat (< 35°C) in which have been dispersed micro-particulated high-melting point fat (MP> 70°C). The microparticules can be added up to 35% (w.b.) of the hydrophobic layer and are responsible for fat crystals control and stabilization.

Combination of hydrocolloids

Numerous examples of polysaccharide-protein, polysaccharide-polysaccharide, protein-protein multi-components films have been available since the 1990s. Synergic effects between components, which result from interactions between the macromolecules, such as charge-charge electrostatic linkage, hydrogen bonding and covalent cross-linking have been researched. The resulting multi-components edible barriers can show improved water transfer resistance. For example, film barrier properties are improved by substituting 30 % of gluten by keratin [START_REF] Gennadios | Modification of physical and barrier properties of edible wheat gluten-based films[END_REF], by incorporating non-fat dry milk to acidic polysaccharide films such as alginate and pectin films [START_REF] Parris | Composition factors affecting the water vapor permeability and tensile properties of hydrophilic films[END_REF] and in microcrystal-line cellulose-corn starch-methylcellulose films [START_REF] Psomiadou | Edible films made from natural resources; microcrystalline cellulose (MCC), methylcellulose (MC) and corn starch and polyols-Part 2[END_REF]. Multi-components films may additionally present improved flexibility [START_REF] Garcia | Characterization of composite hydrocolloid films[END_REF][START_REF] Lazaridou | Thermophysical properties of chitosan, chitosanstarch and chitosan-pullulan films near the glass transition[END_REF][START_REF] Park | Biopolymer composite films based on kappa-carrageenan and chitosan[END_REF]) and sensorial properties [START_REF] Longares | Physical properties of edible films made from mixtures of sodium caseinate and WPI[END_REF].

Composite films

Pure lipids can be combined with hydrocolloids such as proteins, starches or celluloses and their derivatives, either by incorporating the lipids in the hydrocolloid filmforming solution (emulsion technique) or by depositing the lipid layer onto the surface of the pre-formed hydrocolloid film to obtain a bilayer [START_REF] Fennema | Edible pre-formed film barrier materials[END_REF][START_REF] Krochta | Edible and biodegradable polymer films: Challenges and opportunities[END_REF]. Multi-components films have been extensively reviewed by [START_REF] Wu | Development and application of multicomponent edible coatings and films: A review[END_REF]. The addition of non lipid-compounds (hydrocolloids, sugar solids…) as dispersed components in fat materials permits forming fat dispersions (e. g. chocolate, Fig. 2).

These composite films take advantage of the distinct functional properties of each class of film-formers: the moisture barrier properties of lipids and the ability to form a resistant matrix of the hydrocolloids. The resulting water barrier efficiency of bilayered films is often of the same order of magnitude than that of pure lipid (Debeaufort, Quezada-Gallo, Delporte and Voilley 2000c) and is much higher than that of emulsion-based films [START_REF] Debeaufort | Effect of Surfactants and Drying Rate on Barrier Properties of Emulsified Edible Films[END_REF]Debeaufort, Martin-Polo and Voilley 1993;[START_REF] Martin-Polo | Hydrophobic films and their efficiency against moisture transfer. 1. Influence of the film preparation technique[END_REF]McHugh and Krochta 1994a;McHugh and Krochta 1994b). There are, however, a number of drawbacks associated with bilayered moisture barriers. The hydrocolloid layer is hydrophilic and tends to absorb water especially when the film is in direct contact with high water activity foods (> 0.75). Furthermore, the additional processing steps (casting and drying) required to form these films, make them difficult to use in high speed commercial production. [START_REF] Debeaufort | Polarity Homogeneity and Structure Affect Water-Vapor Permeability of Model Edible Films[END_REF][START_REF] Debeaufort | Procédés de stabilisation des produits alimentaires par les films barrières[END_REF].

With regard to polysaccharide-lipid films, the most cited in literature are cellulose ether-based, mainly MC and HPMC-based films. Though other trials with pectinate, chitosan, starch, alginate have also been done. The interest towards cellulose derivatives can be explained by their excellent film-forming properties. Composite emulsified films based on HPMC/MC, stearic acid or palmitic/stearic acids blend, and possibly covered by a beeswax layer (Kamper and Fennema 1984a;Kamper and Fennema 1984b;[START_REF] Kamper | Use of an edible film to maintain water-vapor gradients in foods[END_REF][START_REF] Rico-Pena | Edible methylcellulose-based films as moistue impermeable barriers in sundae ice cream cones[END_REF]), but also bilayers of Methylcellulose/waxes (Greener and Fennema 1989a;Greener and Fennema 1989b), presented really good moisture barrier properties and were early patented [START_REF] Fennema | Edible pre-formed film barrier materials[END_REF]. Starch/alginate/lecithin/stearic acid emulsified The possible associations of proteins and lipids in edible films have also been explored but less intensively than for polysaccharides [START_REF] Wu | Development and application of multicomponent edible coatings and films: A review[END_REF]. Their moisture barrier performance is generally lower than that of composite polysaccharide/lipidbased films. Among all the combinations tested and reported in scientific literature, the best moisture barrier properties were attributed to wheat gluten/lipid films (more precisely beeswax/wheat gluten/glycerol/diacetyl tartaric ester of monoglycerides; [START_REF] Gontard | Edible composite films of wheat gluten and lipids -Water-vapor permeability and other physical properties[END_REF] and soy protein isolate/lipid films (more precisely soy protein isolate/glycerol/lecithin/stearic acid; Rhum et al. 1999). A moisture barrier based on a prolamin (10-90%), in combination with a lipid (oil preferably: 0.1 to 50 %) and the salt of a fatty acid has been recently patented (Plijterschuddemat, Plijter, Van Son, Ilhelmus, Don and Noort 2003). The resulting coating combines high mechanical strength, improved moisture barrier property and heat stability. Combination of shellac with prolamins presents similar properties [START_REF] Glasser | Moisture resistant coating for food products[END_REF].

Addition of non-lipid compounds as dispersed components

The addition of non lipid fillers or bulking agents to improve the functional properties of edible moisture barrier (viscosity, adherence on substrate) is more frequent in commercial and patented coatings/films than in scientific literature. Indeed, in commercial references the barriers are often tested on the product to protect. Formulations are thus developed to try to adjust the viscosity of the coating and its mechanical properties in order to obtain a good adherence and protection to the coated product. Conversely, in scientific articles, barriers have been most of the time evaluated as independent self-supported films.

The addition of dispersed saccharides/polysaccharides to enhance the adherence of composite or fat coatings on bakery product has been suggested in various patents [START_REF] Anonymous | Edible moisture barrier compositions[END_REF][START_REF] Haynes | Edible moisture barrier for food products[END_REF][START_REF] Heuvel | Composite food product with moisture barrier[END_REF][START_REF] Youcheff | Composition for improving adherence of fat-based coatings to frozen fat-based confections[END_REF]. The effect of sugars, cocoa powder, emulsifier and fat type on the WVP (3.5-100 % RH, 20°C) of chocolate coatings was investigated by Gosh et al. (2005) and underlined the favourable effect of sucrose on the WVP of the barrier.

The addition of inert filler material in a fat coating or a modified fat coating have been proposed in several patents [START_REF] Bastiaans | Moisture barrier in foods. Patent NZ530643[END_REF][START_REF] Rubenstein | Edible food containers and the method and apparatus for coating said containers[END_REF][START_REF] Rubenstein | Edible food containers and method of coating said containers[END_REF]. Inert fillers increase the viscosity of the fat in the molten state and evidently change its physical properties and enhance the water-occluding action, possibly by improving the coating flexibility and its resistance to external stresses. Inert filler materials must not be chemically reactive, therewith, not too hygroscopic, mechanically dispersible and possess a particle size such that they do not adversely affect the smoothness and sensorial properties of the coating. Typical filler materials include starches, chemically modified starches, dextrins, microcrystalline cellulose and insoluble cellulose derivatives but also inorganic compounds such as food grade talc, titanium dioxide, silicon dioxide, single silicates, clay materials, insoluble carbonates and phosphates. The amount of the filler material (10-25 % w/w of the coating) also depends upon the particular type of filler utilized. If the use of inert fillers such as starch or dextrin in a fat layers improves its mechanical property and facilitate the coating application (application on ice cream cone; [START_REF] Rubenstein | Edible food containers and method of coating said containers[END_REF], inorganic fillers, such as silicates, improve the moisture resistance of the barrier. Such coating permits protecting moisture sensitive food ingredients such as crispy cereals to retain their crispness even in a chilled but not frozen environment for a prolonged period, i.e. four weeks or more [START_REF] Bastiaans | Moisture barrier in foods. Patent NZ530643[END_REF].

Coatings application techniques and critical points

Selection of the techniques of application

Thermoplastic processes used to form edible films and coatings are adapted from techniques developed for synthetic polymers but take into account the specificities of natural polymers (sensibility to heat, chemical or mechanic treatment, high viscosity). These processes include: extrusion [START_REF] Naga | Process for producing edible proteinaceous film[END_REF], injection-moulding, extrusion-blowing [START_REF] Fishman | Extrusion of pectin/starch blends plasticized with glycerol[END_REF][START_REF] Liu | Effect of food ingredients and selected lipids on the physical properties of extruded edible films/casings[END_REF][START_REF] Psomiadou | Edible films made from natural resources; microcrystalline cellulose (MCC), methylcellulose (MC) and corn starch and polyols-Part 2[END_REF][START_REF] Sothornvit | Tensile properties of compression-molded whey protein sheets: Determination of molding condition and glycerol-content effects and comparison with solution-cast films[END_REF] and compression-moulding [START_REF] Cunningham | Tensile properties of soy protein isolate films produced by a thermal compaction technique[END_REF].

Thermoplastic processes are attractive since they avoid the need to add and remove solvent but have not been as much explored as the applications from a solution or dispersion of the film forming material. The trials of extrusion, injectionmoulding, compression-moulding with biopolymers and more specifically with oilseed proteins -soya [START_REF] Choi | Development of biodegradable hot-melt adhesive based on poly-[epsilon]-caprolactone and soy protein isolate for food packaging system[END_REF][START_REF] Foulk | Properties of compression-molded, acetylated soy protein films[END_REF], sunflower [START_REF] Orliac | Effects of additives on the mechanical properties, hydrophobicity and water uptake of thermo-moulded films produced from sunflower protein isolate[END_REF][START_REF] Orliac | Effects of various plasticizers on the mechanical properties, water resistance and aging of thermo-moulded films made from sunflower proteins[END_REF] -and starches [START_REF] Arvanitoyannis | Biodegradable films made from low-density polyethylene (LDPE), rice starch and potato starch for food packaging applications: Part 1[END_REF][START_REF] Fang | Development of biodegradable laminate films derived from naturally occurring carbohydrate polymers[END_REF][START_REF] Fishman | Extrusion of pectin/starch blends plasticized with glycerol[END_REF][START_REF] Psomiadou | Biodegradable films made from low density polyethylene (LDPE), wheat starch and soluble starch for food packaging applications. Part 2[END_REF][START_REF] Suknark | Physical properties of directly expanded extrudates formulated from partially defatted peanut flour and different types of starch[END_REF] targeted biodegradable plastics packaging. They offered strong tensile strength and included non food-grade products in their composition.

The solutions or dispersions obtained by the 'solvent' method can be cast and dry on a flat surface from which they are removed as sheets of material. This technique is the most widely used in laboratories to test the properties of the films. It can also be used by industries (MonoSol Rx®, Indiana, USA) and preformed moisture barrier films obtained by casting were early patented by [START_REF] Fennema | Edible pre-formed film barrier materials[END_REF]. The coating-forming solutions or dispersions are nevertheless more frequently applied directly on the product surface. The techniques of application have been reviewed by Grants et Burns (1994) and include: spraying and spray-coating (drying, cooling and chilling), dipping and draining, falling film, fluidized bed processing, turbines (Table 6).

The dipping method is well adapted to multiple steps applications and to food products that present an irregular surface. After dipping, excess coating is allowed to drain from the product and is then dried or let solidify [START_REF] Greener Donhowe | Edible films and coatings: characteristics, formation, definitions, and testing methods[END_REF]. The thickness of the layer is determined by the viscosity of the coating material and/or by the rate at which the viscosity changes after application. It is widely used in the confectionery industry along with pan coating. This alternative is carried out with the aid of a sugar-coating mill. A smooth, regular and closed surface of coating material is obtained by mutual rotation of centres, on which several layers of coating have been applied. Spraying, unlike dipping, is more suitable for applying a film to only one side of the food to be covered. This is desirable when protection is needed on only one surface, for instance when a pizza crust is exposed to a moist sauce. Air-atomization, which is a common method of micro-encapsulation, can also be considered as a coating technique. It consists in dividing an emulsion of the filmforming material and the material to coat into small drops and dry them in a warm air flow.

The coating systems that can be used to coat an inorganic material on the surface of edible products have been adapted from the biomedical and electronics fields. They include: (a) sputtering or analogous thermal sublimation, (b) electron beam, and (c) plasma deposition but their application in the food industry is still marginal.

Critical factors to consider in barrier coating development

Many known barrier coatings, suffer, from the disadvantage that they are difficult to apply. Furthermore, to be effective, they are often applied in a thick layer which reinforces their detrimental effect on taste and texture. The difficulty in their application can arise from the product itself: very irregular or porous surfaces make the control of the barrier thickness difficult resulting in poor ineffective coatings with defects. When lipids are used, they can impart a waxy or gummy mouth feel. Hence, the sensorial properties of the barrier have to be taken into account since they may interfere with the product characteristics. Surprisingly however, research on the sensory properties of edible film has been limited [START_REF] Kim | Sensory attributes of whey protein isolate and candelilla wax emulsion edible films[END_REF]Longares, Monahan, O'Riordan and O'Sullivan 2004).

The compatibility and resulting adherence between the food surface and the coating can be critical. It is generally the case when a hydrophobic material is used to protect a hydrophilic product. A surface active agent or other kind of material (starch or cocoa powder) compatible with the two products can help improving the coating adherence [START_REF] Nussinovitch | Hydrocolloid coating of food: A review[END_REF]. Most application techniques involve a drying or a solidifying stage of the coating after its application. This stage is critical in the process since it influences the adherence of the coating to the product and its thickness. On the other hand, for economic reasons this stage has to be as short as possible. The parameters of the drying or solidification stage (temperature, air flow, etc), but also the temperature and the state of the receiving surface have to be strictly controlled to avoid irregular coating formation.

In addition, the coating has to resist to the conditions of storage and of preparation of the food product, e.g. storage temperature, oven or microwave heating. Many ready-to-eat composite food products to which edible moisture barriers are applied are intended to be heated before consumption. Common lipid-based coatings tend to melt and flow under normal baking conditions and, thus, loose film integrity and barrier effectiveness. On the other hand, composite coatings can include temperature sensitive compounds, such as proteins that may be denaturated by heat, and result in drastic modifications of the barrier properties. Edible moisture coatings specifically formulated to resist elevated temperature or specific processes, such as microwave heating, have been developed and are disclosed in the patent literature. Regarding microwave heating, complex coatings have been proposed: a bilayer comprising first an hydrophilic layer (dough layer, methylcellulose, carrageenan) which includes a susceptor (glycerine, sucrose ester, and chloride salt) and then a moisture resistant layer [START_REF] Simon | Microwavable fry-like cooking process[END_REF]. This coating allows producing and stabilizing a food with a crisp exterior and a soft, tender interior. [START_REF] Davis | Edible moisture barrier coating. Patent GB2242815[END_REF] proposed using a barrier coating (comprising fats and a milk protein) in chilled compos-ite products which are to be eaten hot. The coating softens on heating without phase separation, is compatible with the product both visually and organoleptically, and results in an extended shelf-life.

Conclusions and future trends

An extremely wide range of edible moisture barriers has been explored in the scientific and patent literature since the 1950s. The use of such barriers on fresh and slightly modified fruits and vegetables, meats, fish and seafood, mimicking or complementing naturally present protective layers, is now generalized. The combination of various types of film-forming agents (polysaccharides, proteins, lipids) along with the improvement in the film forming methods, the possible modifications of the film-forming materials (denaturation, cross-linking, acetylation, grafting, etc) has allowed the improvement and tailoring of the water vapour resistance of some barrier films. The necessity of adopting an integrated approach in the development of edible moisture barriers to combine regulatory, nutritional, organoleptic and technical requirements (Fig. 3) is well illustrated in the recent patent literature. Indeed, most of the patents not only disclosed the barrier composition but also the food product in which it has to be applied and the technique of application of the barrier. This integrative approach should also be adopted in the scientific papers dealing with the determination of moisture barriers efficiency, to determine accurately the promises of a specific barrier in a given food product. The field of multidomain ready-to-eat food products is still developing and remains nowadays the more challenging in terms of moisture transfer control. Edible moisture barriers appear as an interesting answer to consumers' demand for composite product with good nutritional value and stable organoleptic properties. However, the development of coatings including inorganic compounds seems to be a new trend, interesting to reinforce the barrier property of the film but which may not be well accepted by the consumers. Hence, the necessity of developing and characterizing, simply formulated, flexible, easy to apply moisture resistant barriers is still of prime importance. These edible moisture barriers are all the more attractive as they could limit the use of highly water-resistant multilayered synthetic packaging film, generally non-recyclable.

Fig. 1 .

 1 Fig. 1. Methods of formation of edible lipid-based films by a) "thermoplastic" method, and b) the "solvent method".

Fig. 2 .

 2 Fig.2. Schematic representation of the different types of composite edible films and their mechanism of resistance to transfer (Adapted from[START_REF] Debeaufort | Polarity Homogeneity and Structure Affect Water-Vapor Permeability of Model Edible Films[END_REF][START_REF] Debeaufort | Procédés de stabilisation des produits alimentaires par les films barrières[END_REF].

  Resistance of lipid layer R 2 = Resistance of Proteic or polysaccharidic layer Resistance of bilayered film = R1+ R2 1/ Resistance of emulsion-based or dispersion film = 1/R 1 +1/R 2 film, tested by Wu et al. (2001) on a 50-100 % RH difference also presented high water barrier resistance.

Fig. 3 .Formulation

 3 Fig. 3. Promises and limits of edible moisture barriers.

Table 1 .

 1 Polysaccharides, proteins, lipids and derivatives potentially used as film-former or barrier compound and their origin.

	Origin	Polysaccharides	Proteins	Lipids
		Cellulose and derivatives
		(CMC, MC, HPC, HPMC);	Corn zein;
		Starch and derivatives	Wheat gluten and
		(fractionated:	amylose,	derivatives (gliadins,
		amylopectin,	modified:	glutenin);
		propylated, acetylated…,	Soya proteins and
		hydrolysed starch: dex-	derivatives (globulin
	Botanical	trins, maltodextrins, glu-cose syrups, pregelatinized	7S, globulin 11S); Rice and manioc
		starch);			proteins;
		Pectin and pectinates;	Pea proteins;
		Alginate,	agar,	carra-	Peanut (conarachin),
		geenan, furcellaran; Gums	pistachio,	cotton,
		(arabic, guar, locust bean,	sunflower,	and
		carob, karaya, adragant,	rapeseed proteins
		tara, sterculia, tamarind,
		ghatti)

Table 2a .

 2a Origin, film-characteristics and uses of polysaccharide-based edible films.

	Compound/ Origin	Functional molecules	Film characteristics	Uses
				Few applications of native starch;
			Native starch: high	High amylose starch film widely
	Starch (native or modified) /	Amylose: linear chain of D-glucose with α-	susceptibility hydration and low to mechanical resis-tance;	used: extruded wraps, deep fried potato products, meat products (Gennadios, Hanna and Kurth 1997), refrigerated strawberries
	Various ubiquitous natural sources: tubers and cereals	1-4 links; Amylopectin: ramified chain with α-1-4 and α-1-6 links	Amylose-based films: coherent, relatively strong, free-standing films; Amylopectin-based films: brittle and non-continuous	(Garcia, Martino and Zaritzky 1998a; Garcia, Martino and Zaritzky 1998b); Current tendency: alternative sources of starch with better physico-chemical and func-tional properties (Mali, Grossmann, Garcia, Martino and Zaritzky 2004;
			(Zobel 1988)	Mali, Grossmann, Garcia, Martino
				and Zaritzky 2005)
	Cellulose (modified) / Structural polysaccha-ride of plants	D-glucose units β-1-4 glycosidic linkage; Native cellulose: crystalline water insoluble; Ethers: anionic (car-boxymethyl cellulose or non-ionic (methyl, hydroxypropyl and hydroxypropyl me-thylcellulose). Relative hydrophilicity: HPC < MC < HPMC < CMC. line cellulose. rivative: microcrystal-Water insoluble de-	Cellulose based films: flexible ether-1995) Park and Chinnan and transparent, moderately strong, resistant to oil and fat migration and moderately barriers to moisture and oxygen (Arvanitoyannis and Yamamoto 1996; Arvanitoyannis and Biliaderis 1999;	Cellulose ethers-based films widely used: on pharmaceutical tablets, confectionery (Porter and Woznicki 1989; Woznicki and Grillo 1989), starchy fried products (Mallikarjunan, Chinnan, Balasubramaniam and Phillips 1997; Williams and Mittal 1999); Microcrystalline cellulose: filler in some coatings
	Sodium alginate/ Brown seaweeds	Linear (1→4) linked polyuronic acid with 3 types of polymer segments: poly-β-D-mannuronic acid, poly-α-L-guluronic acid, blocks consisting of alternating D-mannuronic and L-guluronic acid residues (King 1983)	Reaction several polyvalent with cations to form gel; Films of increased water resistance obtained by immer-sion in CaCl 2 solu-tions after formation (Rhim 2004)	Patented gelatinous coatings limit-ing moisture loss and oxidation (Earle 1968; Earle and McKee 1976); application to various meats (Allen, Nelson, Steinberg and McGill 1963a; Earle and McKee 1987; Lazarus, West, Oblinger and Palmer 1976); Carriers of antim-icrobial agents: potassium sorbate and ascorbate or sorbic acid (Wong, Gregorski, Hudson and Pavlath 1996)

Table 2b .

 2b Origin, film-characteristics and uses of polysaccharide-based edible films

	Compound/ Origin	Functional molecules	Film characteristics	Uses
						Low-methoxyl pectins	Few uses, reduction of
	Pectin /					(esterification degree <	stickiness and improved
			D-galacturonic	acid	50 %) capable of	appearance of dry fruits and
	By product of	polymers (α-1,4) with	forming	gel	with	dates (Schultz, Owens and
	citrus	and	varying	degrees	of	calcium ions;	Maclay 1948; Schultz, Miers,
	apple produc-	methyl esterification		Films with low mois-	Owens and Maclay 1949;
	tions					ture resistance ob-	Swenson, Miers, Schultz and
						tained after drying	Owens 1953);
								Widely used: on fresh and
								frozen meat and fish to
			Sulphated polysaccha-			prevent superficial dehydra-
			rides of D-galactose and			tion (Shaw, Secrist and
	Carrageenan / Red seaweeds	3,6-anhydro-D-galactose. Number and position of sulphate groups on the disaccha-ride repeating unit determine classification in three major types: κ, ι, and λ. (Yuguchi, Thuy,	Thermoreversible gels produced from heated aqueous solutions; gelation promoted by the presence of cations (potassium, calcium and sodium)	Tuomy 1980), casing (Macquarrie 2002), sausage granulation-coated powder, dry solids foods, oily foods (Ninomiya, Suzuki and Ishii 1997), soft non gelatine capsules (Bartkowiak and Hunkeler 2001; Fonkwe,
			Urakawa and Kajiwara			Archibald and Gennadios
			2002)					2003; Tanner, Getz, Burnett,
								Youngblood and Draper
								2002)
	Gums/		Arabic gum most used:
	Botanical (arabic, guar) or Microbial (xanthan,	complex mixture com-posed of arabinogalactan oligosaccharides and polysaccharides with a	Arabic gum solutions present good adher-ence properties and form film upon drying.	Arabic gum: limitation of flavour evaporation (Nisperos-Carriedo 1994)
	gellan, etc)	proteic part.

Table 3 .

 3 Origin, film-characteristics and uses of the main kinds of protein-based edible films.

	Compound/ Origin	Functional molecules		Film characteristics		Uses
	Collagen and derivatives (Gelatin) / Skin, tendon, and animal connective tissues	fibrous proteins constituted of fibril sub-units	Collagen films formed by reticu-lations of amine and carboxyl groups; Gelatin-based films: flexible, clear, with good oxygen barrier properties, but poor moisture resistance; Classical formulations: 20-30 % gelatin, 10-30 % plasticizer and water.	Traditionally used in the meat industry: sausage casing and meat preservation early proposed in patents disclosures (Harvard and Harmony 1869); Various pharmaceutical and other food industry applications: ingredients micro-encapsulation, tablets and capsules (Gennadios et al. 1994)
						Sodium caseinates films tested as
				Casein films formed from aque-	wrapping on bread: preservation
		Casein (80 %	ous solution without further	of bread texture for 6 hours
		of the total of	treatment;	compared to	control (Schou,
		milk	pro-	Casein/glycerol (1:2) films:	Longares, Montesinos-Herrero,
	Milk proteins/	teins);		transparent, flavourless, flexible,	Monahan,	O'Riordan	and
		Whey protein	highly permeable to moisture and	O'Sullivan 2005); Whey protein-
	By product of	(20 % re-	very water soluble (Avena-	based coatings widely used:
	the milk and	maining	of	Bustillos and Krochta 1993; Chen	breakfast cereals, raisins, frozen
	cheese manu-	milk protein,	1995; McHugh and Krochta	peas cheese pieces, micro-
	facture	solubility at	1994); Whey protein films:	encapsulation of food addi-
		pH 4.5);		similar properties to casein films,	tives…; Reduction of the textural
		Total	milk	but	water insoluble. Heating	perceptibility of whey protein
		proteins		required for their formation	film by mixing with sodium
				(disulfide bonds)	caseinate (Longares, Monahan,
						O'Riordan and O'Sullivan 2005)
	Cereal pro-teins/ Corn, Wheat, Sorghum…	Non (gluten) and water alcohol soluble (prolamin) fractions from cereal pro-teins	Films based on corn zein and wheat gluten extensively studied; homogeneous, yellowish, rela-tively strong and water resistant wheat gluten film (ethanol disper-sions/partial denaturation); Other film-forming cereal proteins bran) studied (sorghum kafirin, rice	Corn zein-based edible coatings widely used to extend the shelf-life of nuts by retarding, rancid-ity, staling and sogginess; Commercial uses for confection-ery glaze and pharmaceutical tablets
	Oilseed proteins/	Soy proteins most studied: globulin
	Oilseed producing plants	protein fractions (2S, 7S, 11S and 15S)

Table 5 .

 5 Summary of the U.S Code of Federal Regulations (Title 21, Food and Drug Administration) and directive 95/2/EC concerning the use of the main kinds lipid film-forming materials different from waxes, lacs and their derivatives, as coating or components of coatings in food products(Anonymous 1977;Anonymous 2004b).

	Substances	Authorized applications in the U.S. Code of Federal Regulations [CFR section]	Authorized applications in the con-solidated directive 95/2/EC [E No]
			Lubricant, binder, and defoaming agent;
	Fatty acids	Limit: GMP*; [172.860] Stearic acid: GRAS*** substance; Limit:	#; Limit: quantum satis**; [E 570]
			GMP; [184.109]
	Mono diglycerides and	GRAS [184.1505]	substance;	Limit:	GMP;	#; Limit: quantum satis; [E 471]
	Acetylated glycerides	Monoglycerides, multipurpose additive; Limit: quantum satis; [172.828]	#; Acetic acid ester of mono and diglycerides; Limit: quantum satis; [E 472 a]
	Lactic	acid	Emulsifiers, plasticizers, or surface-active
	esters mono	of and	agents for bakery products, desserts and shortenings; Limit: quantum satis;	#; Limit: quantum satis; [E 472 b]
	diglycerides	[172.848]
	Acetylated or					#; Mono and diacetylated DATEM
	none Tartaric	DATEM, GRAS substance; Limit: GMP;	[472 e];
	Acid Esters of	[184.1101]				DATEM [472 d]; mixed acetic and
	Mono	and	Tartaric acid, GRAS substance; Limit:	tartaric acid esters of mono and
	Diglycerides	GMP; [184.1099]			diglycerides [E 472 f];
	(DATEM)					Limit: quantum satis
			Emulsifiers or stabilizers in various goods;
	Sucrose fatty esters	texturizers in various goods; components of protective coatings applied to a re-stricted number of fruits to retard ripening	Colours and fat soluble antioxidant; [E 473]
			and spoiling; Limit: GMP; [172.859]
							Fine bakery, granola breakfast (10 g /
							kg), emulsified liqueur (5 g/l), egg
	Polyglycerol	Cloud inhibitor in vegetable and salad oils,	products (1g/kg), beverage whiteners
	esters of fatty	emulsifiers in dry or whipped topping	(0.5g /kg), chewing gum, fat emul-
	acids		base; [172.854]			sions, milk and cream analogues (5
							g/kg); sugar confectionery, desserts (2
							g/kg); [E 475]
							#; Sodium, potassium and calcium
	Salts of fatty	Binder, emulsifier, and anticaking agent in	salts of fatty acids; [E 470 a]; magne-
	acids		food; Limit: GMP; [172.863]		sium salts of fatty acids; [E 470 b];
							Limit: quantum satis
	Lecithins		GRAS [184.1400]. substance;	Limit:	GMP;	#; Limit: quantum satis; [E 322]

Table 6 .

 6 Techniques of application of edible moisture barriers films depending on the systems characteristics (shape of product to be coated and targeted thickness of the barrier) and their critical points.

				Substrate shape/characteristics
			Flat or with flat surfaces	Irregular	Spherical
			Brushing:	substrate	Fluidized thermoregulated air bed: low density
			with flat and smooth	small size particle/ heat resistant, batch process,
			surfaces, brush rigidity,	limited weight of centres, sensibility to
		Thin	drying stage following continuous process,	heat.
			application			Screw coating: flexibility on substrate shape/
							quite resistant, continuous process
							Dipping:	Flexibility on substrate shape/
			Spraying:		flat	smooth surface of substrate, viscosity of the
			system unless associated	barrier, adhesion, cooling rate
			to other technologies,
			continuous	process	Pan and Drum coating:
		Thin to	viscosity the barrier at	spherical	substrate/
	Barrier	Thick	the	temperature	of	quite resistant, hard
	targeted		spraying, tempering of	shelf on soft product
	thickness		the barrier, pressuriza-	(sugars coatings on jelly
			tion and nozzle pattern,	beans/candied	fruit),
			target thickness			batch process, heat
							balance control, adhe-
							sion,	avoid	cluster
							formation
							Enrobing:	Flexibility on substrate shape/
							maximal temperature of the falling film, viscos-
		Thick	Casing: samples (e.g. confec-large	ity, temperature balance substrate/barrier
			tionery bars), contrac-
			tion of the barrier,	Coextrusion:	cold
			cooling rate and tem-	flow of the filling,
			perature control		minimal thickness of
							barrier wall (1 mm),
							barrier formulation,
							splitting of the rope

Bourlieu, C. et. al.