Agro-polymers for edible and biodegradable films: Review of agricultural polymeric materials, physical and mechanical characteristics

Agro-polymers for edible and biodegradable films: review o f agricultural polymeric materials, physical and mechanical Sttfphane Cuilbert and Nathalie Contard

introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Agro-polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 Processing ............................................................. 266 Properties and applications o f edible and biodegradable films . . . . . . . . . . . . . . . . . . . . . 267 Applications o f agro-polymer based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 Market opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

Introduction Various polymers obtained from products or by-products of agricultural origin are proposed for the formulation of biodegradable materials or edible films. These polymers (polysaccharides, proteins and lipids, or polyesters) can be used in various forms Innovations in Food Packaging ISBN:0-12-31 1632-5

Copyright O 2005 Elsevier Ltd All rights of reproduction in any form reserved

264 Innovations in Food Packaging

(coatings, simple or multi-layer films, three-dimensional items, simple materials, mixtures, blends, and composites). The materials obtained from these agro-polymers are fully renewable and biodegradable (except when severe chemical modifications are applied). They are non-toxic to the soil and the environment, and when food-grade ingredients are used to formulate the materials, they can be edible. In addition, edible and biodegradable coatings or films produced from agropolymers provide a supplementary and sometimes essential means of controlling physiological, microbiological, and physicochemical changes in the food products. This is achieved by controlling mass transfers between food products and ambient atmospheres, or between the components in heterogeneous food products. Furthermore, due to some original material properties, they can also be used as an "active bio-packaging" to modify and control food surface conditions (for example, gasselective materials, controlled release of specific functional agents, of flavor compounds, etc.).

The formulation of bio-plastic or edible films implies the use of at least one component able to form a matrix, having sufficient cohesion and continuity. They are polymers which, under preparation conditions, have the property to form crystalline or amorphous continuous structures. Only polyesters, polysaccharides or proteins are used for making "materials". Natural lipid compounds are also used, primarily in the field of edible film and coating applications. Generally they are applied in thin layers, or as a composite with a polymeric matrix. Three different techniques using agricultural raw materials (fully renewable raw materials) are possible to make agro-polymers (Guilbert, 1999): 1. Agricultural polymers (polysaccharides or proteins) can be extracted and eventually purified. They can be used alone or in a mixture with synthetic biodegradable polymers such as polycaprolactone or other synthetic biodegradable polyesters. 2. Agricultural products can be used as fermentation substrates to produce microbial polymers (e.g. polyhydroxyalcanoates). 3. Agricultural products (or by-products) can be used as fermentation substrates to produce monomers or oligomers which will be polymerized by conventional chemical processes (e.g. polylactic acid obtained by polymerization of natural lactic acid produced by fermentation of corn).

Edible films or coatings Agro-polymers that have been proposed to formulate edible films or coatings are numerous (Cuq et al., 1995; Guilbert and Cuq, 1998). Polysaccharide, protein or lipid

Agro-polymers for edible and biodegradable films 265

materials are used in various forms (simple or composite materials, single-layer or multi-layer films). Polysaccharides used for material formulations are generally the same ones as those used as stabilizing, thickening, and gelling agents. These polysaccharides are of various origin: plant polysaccharides such as cellulose and derivatives; starches and derivatives; pectin or arabinoxylanes; algae gums such as alginates or carrageenans; and microbial gums, pullulan, xanthan and gellan. Many plant and animal proteins have been studied as raw materials for films and coatings. These proteins are generally characterized by having interesting functional properties (Guilbert and Cuq, 2002). Lipids and derivatives are used due to their good water-barrier properties. The use of lipids (or derivatives) with a polysaccharide or protein-based matrix or support is generally advised. A few examples of applications of edible films or coatings, used to improve product appearance or conservation, include sugar and chocolate coatings for candies or icecreams, wax coatings for fresh h i t s , and oil or fat coatings for raisins and dry h i t s . Edible films become a part of the whole food product; their composition is dependent on the product nature, and must conform to regulations that apply to the respective food product. Therefore, edible film and coating technologies are regarded as a "problem" for food formulations (Kester and Fennema, 1986; Krochta et al., 1994; Guilbert and Cuq, 1998).

Biodegradable materials As far as biodegradable materials are concerned, starch is the most commonly used agricultural raw material. Starch is inexpensive, widely available, and relatively easy to handle. "All-starch" bio-plastics are made from thermoplastic starches, which are produced by standard techniques of synthetic polymer films, such as extrusion or injection molding (Colonna, 1992). The use of thermoplastic proteins has also been investigated (Gontard and Guilbert, 1994; Guilbert and Cuq, 2002), but its commercial applications are still expected. Among the proteins, milk proteins (casein, whey proteins), soy proteins and cereal proteins (wheat gluten, zein) have been more extensively studied (Genadios et al., 1994; Redl et al., 1996; Guilbert et al., 2001). Materials based on hydrocolloids constitute effective oil or fat barriers, but they are generally not very resistant to water and their moisture-barrier properties are poor. However, in some cases, water solubility or the sensitivity to water is a functional advantage - for example, in the formulation of soluble sachets of edible films or coatings (less perceptible in the mouth) or in the formulation of "active" materials, where the water swelling is used to induce a drastic change in the properties. However, in general, improving water resistance and water-barrier properties is of most importance. Therefore, chemical modifications of the biopolymers and the development of specific additives (cross-linking agents or plasticizers) adapted to the polymer structures are proposed. Regarding these developments, proteins that have a "nonmonotonous" complex structure and many potential functional properties are promising (Cuq et al., 1998).

Commercial water-resistant starch-based bio-plastics (for non-edible applications) are produced by using fine molecular blends of biodegradable synthetic polymers and starches. These composite materials are made with gelatinized starch (up to 60-85%), hydrophilic synthetic polymers (e.g. ethylene vinyl alcohol copolymer) or hydrophobic synthetic polymers (e.g. polycaprolactone), and with compatibility agents (Fritz et al., 1994). The most important starch-based material on the market is ~ a t e r b i @ proposed , by Novamont. Microbial polymers (e.g. poly(3)-hydroxybutyrate-hydroxyvalerate) are excreted or stored by micro-organisms cultivated on starch hydrolysates or lipid mediums. The isolation and purification costs of these products, which are obtained from complex mixtures, can be high. Monsanto stopped the commercialization of their product "~iopol@"in 1999. Coopeazucar, in Brazil, has built new facilities for pilot plant production of these polyhydroxyalkanoates. Polylactic and polyglycolic acids are mainly produced by chemical polymerization of lactic acid and glycolic acid, which are obtained by Lactobacillus fermentation. Commercial applications of these materials are rapidly increasing under the tradefrom Mitsui. marks of copl la@ from CargilVDow Chemical, or ~ a c e a @

Processing Two general process pathways for agro-materials are distinguished: 1. The "dry process", such as thermoplastic extrusion, is based on the biopolymers' thermoplastic properties when plasticized and heated above their glass transition temperature under low water content conditions 2. The "solvent process" or "casting" is based on the drying or dispersion of the filmforming solution. The casting process is used to form edible preformed films, or to apply coatings directly onto the products (Guilbert and Cuq, 1998). This process is generally adapted for coating seeds and foods, for making cosmetic masks or varnishes, and for making pharmaceutical capsules. Heat processing of agro-polymer based materials, using techniques usually used for synthetic thermoplastic polymers (e.g. extrusion, injection, molding etc.) is more cost effective. This process is often used for making flexible films (e.g. films for agricultural applications, packaging films, and cardboard coatings) or objects (e.g. biodegradable materials) that are sometimes reinforced with fibers (composite bio-plastics for construction, automobile parts etc.). The material characteristics (e.g. polysaccharide, protein, polyester or lipid; plasticized or not; chemically modified or not; used alone or in combination) and the fabrication procedures (e.g. casting of a film-forming solution, thermoforming) must be adapted to each specific food product and the conditions in which it will be used (relative humidity, temperature).

Agro-polymers for edible and biodegradable films

Edible and biodegradable films must meet a number of specific functional requirements (e.g. moisture barrier; solute andlor gas barrier; water or lipid solubility; color and appearance; mechanical and rheological characteristics; non-toxicity, etc.). These properties are dependent on the type of material used, its formation, and its application. Plasticizers, cross-linking agents, antimicrobials, anti-oxygen agents, texture agents etc. can be added to enhance the functional properties of the films. The properties of edible or biodegradable films depend on the type of film-forming materials used, and especially on their structural cohesion characteristics. Cohesion depends on the structure of the polymer, its molecular length, geometry, and molecular weight distribution, and on the type and position of its lateral groups. Film properties are also linked to the film-forming conditions (e.g. type of process and process parameters). The properties of amorphous or semi-crystalline materials are seriously modified when the temperature of the compounds rises above the glass transition temperature (T,). The glass transition phenomenon separates materials into two domains, according to clear structural and property differences, thus dictating their processing conditions and potential applications (temperature and water resistance). Generally, fully arnorphous bio-plastic applications are limited by the fact that a polymer's Tg is highly affected by the relative humidity (especially for hydrophilic polymers). Below the T, the material is rigid, and above the T, it becomes visco-elastic or even liquid. Below this critical threshold, only weak, non-cooperative local vibration and rotation movements are possible. Film relaxation in relation to temperature follows an Arrhenius time course. Above the T, threshold, strong, cooperative movements of whole molecules and polymer segments can be observed.

Organoleptic properties Edible films and coatings must have organoleptic properties that are as neutral as possible (clear, transparent, odorless, tasteless etc.) so as not to be detected when eaten. Enhancing the surface appearances (e.g. brilliance) and the tactile characteristics (e.g. reduced stickiness) can be required. Hydrocolloid-based films are generally more neutral than those formed from lipids or derivatives and waxes, which are often opaque, slippery, and waxy tasting. It is possible to obtain materials with ideal organoleptic properties, but they must also be compatible with the food's filling - for example, sugar coatings, chocolate layers (or chocolate analogs), and starch films for candies, biscuits, some cakes and icecream products (wafer coatings) etc. Films and coatings can also help to maintain desirable concentrations of coloring, flavor, spiciness, acidity, sweetness, saltiness etc. Some commercial films, especially Japanese pullulan-based films, are available in several colors, or with spices and seasonings included. This procedure could be used to provide nutritional improvement without destroying the integrity of the food product - for example, by using edible films and coatings enriched with vitamins and various nutrients.

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The optical properties of films depend on the film formulation and fabrication procedures.

Biodegradability Biodegradation kinetics are dependent on the type of polymer used (molecular weight, structure, crystallinity) and on the additives used (e.g. plasticizers, fillers etc.). The methods used to evaluate biodegradability are generally based on Sturrn's procedure (Sturm, 1973) - i.e. international standard IS0 14852 - which measures the ultimate aerobic biodegradability of materials. Another approach consists of the evaluation of the biodegradability in soil. The agro-polymer based materials are generally fully biodegradable (apart from when some very severe chemical modifications are applied). They are non-ecotoxic. Polylactic acid is biodegradable, but at temperatures above its Tg(>45-55OC) - that is, in a composting medium. It is often interesting to adjust biodegradation rates to each desired type of application. This can be achieved by applying formulation andlor chemical modifications.

Mechanical properties Mechanical properties are also dependent on the type of polymer and the additives used. For many biodegradable polyesters, the tensile strength is often similar to that of polyethylene or PET, but the elongation is often much lower (without external plasticization). Hydrocolloids, which generally require plasticization (e.g. using low molecular weight hydrophilic molecules such as glycerol or with amphipolar molecules such as fatty acid derivatives) show lower tensile strength, but their elongation is mainly dependent on the plasticizer's content. The mechanical properties of various biodegradable, edible or conventional materials are illustrated in Figure 16.1. It is clear from this figure that biodegradable synthetic and conventional materials have very similar properties (high tensile strength and high elongation), while microbial bio-plastics and agricultural bio-plastics are either highly deformable or highly resistant, but not both simultaneously.

Water vapor transfer Owing to their relatively low water-vapor barrier properties, agro-polymer based materials and edible films can only be used as protective barriers to limit moisture exchange for short-term applications. However, they can be of considerable interest for numerous applications where very high water vapor permeability is required, such as in the case of modified atmosphere packaging of fresh, minimally processed or fermented foods (fish, meat, fruits, vegetables, and cheeses). Biodegradable polyesters (lipids or waxes for edible applications) show better water-vapor barrier properties compared to starch or protein-based materials, but still have significantly lower watervapor barrier properties than most conventional materials.As far as edible applications

Agro-polymers f o r edible a n d biodegradable f i l m s

0

200

400

600

800

1000

Elongation at break (%)

Figure 16.1 Mechanical properties ofvarious biodegradable, edible and conventional materials (adapted from Guilbert et a/., 2001). (a) Syntheticmaterials (closedymbols): thermoplastic polyurethane elastomer (Dow Chemical); Polyvinylchloride; WC plasticizedwith Di-2-ethylheylphtalate; Polypropylene; low-density polyethylene and biodegmdablesynthetic materials (open ymbols): Bak 1095, polyester amide (Bayer, Germany); Ecoflex: 1,4 Butandiol adipinicdicarbonic and terephtalate copolyester (Basf, Germany); Eastar 14766: poly (tetramethylene adipatecoter6phtalate) (Eastman, USA); Bionolle 3000: polybutylene succinateladipate(Showa, Japan). (b) Biodegmdablematerialsfmm micmbialorigin: Biopol: polyhydroxybutyrate(Monsanto, Italy); Lacea: Polylactic acid (Mitsui, Japan); Ecopla, Polylactic acid (CargillIDow, USA). (c) Biodegradable materialsfrom apculturalorigin: Biotec: starchlpolyester (Biotec, Germany); Materbi: starch/polycaprolactone (Novamont, Italy) and edible films: wheat gluten films; molded soy protein isolate films, alginate based films; chitosan based film; pullulan based films.

are concerned, lipid compounds such as animal and vegetable fats (natural waxes and derivatives, acetoglycerides, surfactants, etc.) are generally proposed for their excellent moisture-barrier properties, but can cause textural and organoleptical problems due to oxidation and a waxy taste.

Control of gas exchange Agro-polymer based materials have impressive gas-barrier properties in dry conditions, especially against oxygen. For instance, the oxygen permeability of a wheat gluten film was 800 times lower than that of low-density polyethylene, and two times lower than that of polyamide 6, a well-known high oxygen-barrierpolymer. Increasing the water activity promotes both the gas difisivity (due to the increased mobility in the hydrophilic macromolecule chains) and the gas solubility (due to the water swelling in the matrix), leading to a sharp increase in the gas permeability. With carbon dioxide, a sharp increase in the permeability is more important than with the oxygen permeability. The selectivity coefficient between carbon dioxide and oxygen (Gontard et al., 1996;Mujica Paz and Gontard, 1997)is very sensitive to moisture and temperature (for example, fi-om 4 up to 35 for gluten films), whereas the selectivity coefficient for synthetic polymers remains relatively constant, at 4 4 . This could be explained by the

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270 Innovations in Food Packaging

differences in the water solubility of these gases (e.g. carbon dioxide is very soluble), but also by specific interactions between carbon dioxide and the water-plasticized polymer. Very high gas selectivity is particularly interesting for modified atmosphere packaging of cheeses (to control the proliferation of the microflora) and fresh fruit and vegetables (to control the respiration rates). Some selective gluten-based films have been shown to lead to the production of new modified atmospheres (e.g. low oxygen and carbon dioxide concentrations at equilibrium) when used to wrap fresh vegetables (Guilbert et al., 1996, 1997; Barron et al., 200 1).

Modification of surface conditions and controlled release of additives "Active" edible or biodegradable films can be applied to foods to modify and control the atmosphere of food surface conditions (Cuq et al., 1995). Improvement in food microbial stability can be obtained using edible active layers as surface retention agents to limit the diffusion of food additives into the food core, or they can be used in combination with treatments such as refrigeration and a controlled atmosphere. Maintaining a high concentration of an effective preservative in a local area may allow a reduction in the total amount of the preservative while sustaining the same effectiveness. Improvement of food microbial stability can also be obtained by reducing the surface pH. This can be achieved using films that immobilize specific acids or charged macromolecules (Torres and Karel, 1985). It is important to be able to predict and control preservative release (Torres et al., 1985; Red1 et al., 1996). Microbiological analysis generally confirms the efficiency of the preservative retention within surface coatings that are able to increase the shelf life of intermediate moisture foods (Torres and Karel, 1985; Guilbert, 1988).

Applications of agro-polymer based materials Applications of biodegradable plastics Biodegradable plastics can be classified into three categories (Guilbert, 1999): 1. Plastics to be composted or recycled (i.e. where reuse or fine recovery is difficult) 2. Plastics to be used in the natural environment (i.e. where recovery is not econornically or practically feasible) 3. Specialty plastics (with specific features where bio-plastics possess preferential properties). Since agro-polymers are relatively costly to produce, actual applications are limited to special niches with environmental considerations. Loose-fill packaging and compost bags are the two major end uses, constituting nearly 90% of the demand in 1996 (Bohlmann and Takei, 1998). However, new applications in agriculture and food packaging are emerging. In the domain of plastics for agriculture, the demand for

Agro-polymers for edible and biodegradable films 271

biodegradable mulching plastics is increasing because biodegradability is a real functional advantage. For food packaging products such as plastic films, nets and small containers or trays are of concern, even if they are more expensive than the traditional products. The principal applications of agro-polymers are listed in Table 16.1.

Applications Plastics to be composted or recycled Loose-fill packaging Waste and carrier bags Dishes and cutlery Hygiene disposables

Miscellaneous short-life goods Food packaging

Paper or cardboard

Plastia used in natuml environments (no recovery) Biodegradable/soluble/controlled-release materials for agriculture and fisheries

Civil engineering, car industry and construction materials

Disposable leisure goods Speciality ingredients or materials Functionalizednanoparticles Medical goods Edible films/coatings

O2 barrier, selective 02/C02barrier, aroma barrier Matrix for controlled-release systems Super-absorbents Adhesives Paints Dyes and pigments

2 .

.

Shock absorbers Compost bags Trays, spoons, cups Diapers, sanitary napkins, sticks for cotton swabs, razors, toothbrushes Pens, tees, toys, gadgets, keyrings Dried foods, short lifecycle food packaging(e.g. foast-food packaging, egg boxes Fresh or minimally processed fruits and vegetables, dairy products, organically grown products Accessories or windows for paper envelopes or carc board packaging, coating for paper or cardboarc

Mulching plastic, films for banana culture, twine, nursery pots Materials for controlled-release fertilizers or agrochemicals High water-retention materials for planting Soluble sachets, biodegradablecontainers for fertilizers or agrochemicals Fishinglines and nets Heat insulators, noise insulators, form wares Car interior door casings Retainingwalls or bags for mountain areas or sea, protective sheets and nets for tree planting Golf tees, miscellaneousgoods for marine or mountain sports Nanoparticles for rubber reinforcement Bone fixation, sutures, films, non-woven tissues Barrier internal layers, surface coatings, "active" superficial layers Soluble sachets for instant dry food and beverages, for food additives Component o f simple or multi-layer packaging

Slow release o f fertilizers, agrochemicals, pharmaceuticals, food additives

.&

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Application of edible films Edible films and coatings provide an easy means of structurally strengthening certain foods, reducing particle clustering and improving visual and tactile features on product surfaces (Cuq et al., 1995). Edible films can also be used to package components or additives that are to be dissolved in hot water or food mixes, and also act as an additional parameter for improving overall food quality and stability. They represent one way to apply hurdle technology to solid foods without affecting their structural integrity (Guilbert, 1986; Guilbert et al., 1997). Many functions of edible films are the same as those of synthetic packaging, but they must be chosen according to a specific application and type of food product, and its main deterioration mechanisms. Films that have a significant gas- and moisture-barrier properties are required for many applications. For example, they are used to control gas exchange for fresh or oxidable foods, and to reduce moisture exchange with the external atmosphere, etc. The retention of specific additives in edible films can lead to a functional response. This is generally confined to the surface of the product, where they are used to modify and control "surface conditions7'.Edible coatings can also limit oil and solute penetration into foods during processing. Figure 16.2 is a schematic representation of a

Food additives (e.g. antioxygen and antifungic agents)

\

Diffusion of food additives

........ ..............

32

, ............... (a) Without active layer. Food additives in Surface retention of food additives

Storage

Control of water transfer

(b) With active layer.

Figure 16.2 Schematic representation o f food preservation with or without edible films and coatings as active layers, when the first mode o f deterioration results from respiration (a), from dehydration or moisture uptake (b), or from surfice microbial development or oxidation (c), adapted from Cuq etal. (1 995).

Agro-polymers for edible and biodegradable films 273

food preservation method using agro-polymers as ediblehiodegradable active layers when the food product undergoes three types of deterioration, due to (a) respiration, (b) dehydration or moisture uptake, or (c) surface microbial development or oxidation. The protective feature of the film is dependent on gas- and water-vapor barrier properties, on surface condition modifications, and on its own antimicrobial properties.

Market opportunities Edible films and coating are generally not commercialized as preformed self-supporting materials, but as pre-mix ingredients. This makes it very difficult to estimate the demand in the market, especially when this is not significant compared to demand for other specialty ingredients and additives. Companies in this type of market are usually suppliers of cellulose and derivatives, gums, food-texturing agents, food-grade surfactants, waxes, and lipid derivatives. Recently, lipid materials specifically designed to be used as edible water barriers have been proposed for commercial use. Most of them are composed of specifically designed acetoglycerides, either alone or mixed with waxes. The market for biodegradable plastic materials is currently expanding, but it remains a niche market. The total world production capacity of "modern thermoplastic bio-plastics" is approximately 300 000 tons (estimation for 2003). However, the 2003 world consumption is estimated to be only between 70 000 and 90 000 tons. Eighty-seven percent of biodegradable materials will result from renewable resources. This market still has great diversity in producers and materials, but attempts are being made to decrease this variety. The major companies involved are Cargill Dow, Novamont, Basf, Eastman, Mitsui, and Solvay. These corporations have a double fimction, acting both as purchasers and as suppliers. The total target market is 500000-900000 tons in 2005-2007. Today, these film applications account for about 30% of the sales, and this percentage is increasing, with estimates in the range of 30-50% by 2010. They are not meant to replace the traditional plastics. The applications that require biodegradability as a fimctional characteristic currently account for about 75% of sales, and it is expected that this proportion will decrease to 60-50% in the future. Currently, the largest market share for bio-plastics is provided by blends of thermoplastic starch and synthetic biodegradable polyesters, representing about 65% of the total market. They contain an average starch content of about 60-70%. Novamont is the leader in this market, with the ~ a t e r bproducts. i~ Polylactic acid is the other major bio-plastic produced commercially at the moment (mainly by Cargill Dow with ~ c o ~ l a @ ) . The industrial fhture for biodegradable plastics is still under discussion, owing to numerous problems. One of the main obstacles to worldwide use of bio-plastics is the cost of commercially available products (which is similar to that of speciality plastics, e.g. 1.5 to 10eurolkg). Bio-plastics are still unknown to the processing industry, and their performances (e.g. the water sensitivity of agricultural-based plastics) and processibility often remain a problem. In terms of the marketing point of view, the major

274 Innovations in Food Packaging

hurdles are the absence of well-identified demands, the lack of eligibility of the offer, the risk in terms of the image, consumer perception, and the complexity of the whole system. Other major obstacles are the absence of a recovery chain, and of the infiastructure of nationwide disposal in most developed countries. New legislations in Western Europe and in Japan, which are in favor of using biodegradable plastics, has helped to increase the demand, but developing international standards for biodegradable polymers is difficult and the competition regarding the recycling of plastics is still an obstacle. In addition to being biodegradable, the plastics must now also be non-ecotoxic. However, with the development of a new range of functionalitiesand the benefits to the environment, topics that are in line with international objectives, a dynamic market is predicted.

After a long period of latency, biodegradable plastics have now become credible. Major polymer manufacturers are entering the market, material costs are quickly falling, and performances and processibility are improving significantly. Important niche markets are opening due to consumer demands, which are considered important. In addition, new legislations and standards in favor of the use of biodegradable plastic are helping to encourage market growth. More significant markets are expected to be located in Western Europe and in Japan. Among the different categories of biodegradableplastics obtained h m agro-polymers, the starchlpolyester blends and the "microbial" biodegradable plastics satisfy the majority of the requirements proposed by plastic packaging industries (material qualities, processibility,performance, etc.). Interest has been shown in other bio-plastics based on natural polysaccharides or proteins, due to their low costs; however, their nonreproductive quality and their lower performance are still problems. Edible films and coatings provide an added and at times necessary means to control physiological, microbiological, and physicochemical changes in food products. Recently, the concept of "edible active layers" (Cuq et al., 1995) has been introduced. These films or coatings contribute to food preservation, for instance by controlling mass transfer of water vapor, oxygen, carbon dioxide, ethylene, etc., or by modifying and controlling the food surface conditions (e.g. pH, level of functional agents, slow release of additives etc.). Edible and/or biodegradable packages formed with several compounds (composite or complex materials) have been developed to take advantage of the complementary functional properties of the different constituent materials, and to overcome their respective drawbacks. Most composite films studied to date combine one or several polyester (or lipid) compounds with a hydrocolloid-based one. The future of edible and/or biodegradable materials is therefore probably dependent on the development of applications where some of their preferential properties (such as gas selectivity) are enhanced, or on the development of composite materials.

Agro-polymers for edible and biodegradable films 275

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