Edible films and coatings help in encompassing the shelf lives of various food products. Increasing consciousness about ecological problems that are primarily related to the disposal of solid waste and decreasing the amount of waste generated are among the major factors behind food producers’ increasing focus on recyclable packaging solutions. Some successful outcome of similar efforts on fruits, vegetables, and other food products are reviewed here by Preeti Shukla, Suresh Bhise and S. S. Thind.
Edible Films and Coatings are defined as primary packaging made from edible components. Edible films are distinguished from coatings by their method of manufacture and application to the food product. Edible Films and Coatings is a growing demand due to food and pharmaceutical industry, which are expected to be one of the major drivers of the market over the next several years. In the first part of this two-part-series different methods and materials used for edible films and coatings were discussed. In this concluding part in properties of different edible films/coatings and their coating techniques are explained here.
Properties of edible films and coatings
Moisture barrier properties
Effective moisture barrier properties or resistance to water vapour transmission is frequently quantified by permeability. The simple edible film based on milk protein can significantly retard moisture loss. The addition of lipid or its derivative improves the moisture barrier properties of edible films or coatings.
Gas barrier properties
The higher protein content of edible film results in a better oxygen barrier. The addition of lipids and increased concentration of plasticizer increases oxygen permeability. These facts indicate the advantage of the complementary effect of protein-lipidid films. Protein provides a good gas barrier and lipid makes the film more resistant to moisture transport.
Mechanical properties are important for edible films and coatings since they indicate the durability of films and the ability of the coating to enhance the mechanical integrity of foods. Protein-based films are viscoelastic materials that possess characteristics of solids and liquids. Interactions between proteins and small molecules including water, plasticizers, lipids, and other dispersed additives also contribute to the mechanical behaviour of the film. Tensile and yield strength, elastic modulus, and elongation help relate the mechanical properties of edible films to their chemical structures. Whey protein edible films possess excellent resistance to tension and puncture.
The mechanical properties of edible films prepared with heat-denatured or native WPI. Since films made from native whey protein isolate (WPI) had lower elastic modulus, tensile strength, and elongation than heat-denatured WPI films, they would be less stiff, weaker and less extensible than denatured WPI. This has been attributed to the covalent cross-linking reactions, such as disulfide/ sulfhydryl interchange, thiol/thiol oxidation, and intermolecular disulfide bond formation, due to heat denaturation. The microstructure of WPI edible films was found to depend on WPI concentration, the plasticizer used, and the pH. At increased WPI concentration, a highly aggregated structure with larger pores was formed, and these pores tended to be more and smaller when sorbitol was used as plasticizer instead of glycerol. When the pH increased from 7 to 9, a denser protein structure was formed and the strain at break increased while the oxygen permeability decreased
Tensile strength and elongation properties of edible film
Tensile strength is defined as the maximum tensile stress that a film can sustain. Elongation is usually taken at the point of break and is expressed as the percentage of change of original gauge length of the specimen. A whey protein concentrate (WPC) protein emulsion film with the lipid of higher melting point showed better mechanical strength than that of lipid of lower melting point. The type and concentration of plasticizer also affect the film strength. The addition of plasticizer reduces the mechanical strength of the edible film based on protein. Mechanical strength increased as the protein plasticizer ratio increase.
Puncture strength of the edible film
Puncture strength (PS) is defined as the peak force divided by the cross-sectional area of the probe and reported in mega Pascal. The low protein concentration of WPC resulted in films with less resistance to puncture.
Characteristics, solubility, and hydrolysis of edible films
Edible films based on milk proteins are generally flavorless, tasteless and flexible. The film varied from transparent to translucent depending on the formulation. It was reported that the covalent cross-linking of whey protein by transglutaminase cross-linked whey protein film was found to be greatly affected by plasticizer concentration, i.e. glycerol, pH and length of the incubation period. Hydrolysis of the film or coating is also important in foods containing proteases on the surface. The hydrolysis of cross-linked whey protein film by protease greatly affects the stability of the film during storage (Kester and Fennnema, 1986).
Coating application consists of applying a liquid or a powder ingredient on to a base product. Application of coating generally requires a four-step process:
- Deposition of coating material (solution, suspension, emulsion or powder) on the surface of the product to be coated through spraying, brushing, spreading or casting.
- Adhesion of coating material (solution, suspension, emulsion or powder) to the food surface.
- Coalescence (film forming step) of the coating on the food surface.
- Stabilization of the continuous coating layer on its support or food product through co-acervation by drying, cooling, heating or coagulation.
The choice of a coating system or apparatus depends mainly on the size, shape, and characteristics of the product to be coated.
Mode of Application of Coatings on Food Products
Enrobing involves the application of a thick coating layer by dipping the product to be coated in solution batter or molten lipid (e.g., a chocolate-based coating). Coating of fresh or frozen products with a batter and/or breading can enhance palatability, add flavour to an otherwise bland product and reduce moisture loss and oil absorption during frying.
Pan coating is used to apply either thin or thick layers onto hard, almost spherical particles in a batch process. For example, coating confectionery centres (peanuts and almonds) with gum arabic provides a uniform base layer for further coating and a hydrophilic/ lipophilic surface, prevents moisture and fat migration and allows incorporation of additional flavour.
Drum coating is often the best technique for applying either a thin or a thick layer onto hard or solid foods in a continuous process (e.g., nuts). Oiling and salting of nuts enhance palatability, adds flavour, and delays lipid oxidation in peanuts. Adding a chocolate coating to cornflake cereals enhances palatability, adds flavour and delays moisture absorption.
A screw coater allows the application of a thin layer of coating onto a solid and firm food material in a continuous process. For example, it is often used to deposit thin coatings on sticky particles, such as cheese shreds and pieces, to improve anti-caking properties and prevent agglomeration of particles when the product is stored in flexible packaging.
Fluidized-bed coating is a technique used to apply a very thin layer onto dry particles of very low density and/or small size. Agglomeration of the powder promoted by the fluidized-bed coating technique enhances the dispersion and solubility of the coating material. The powder is fluidized with hot air and sprayed simultaneously with a binder liquid. This process causes particle adhesion, agglomeration, and drying of agglomerates. It applies to both batch and continuous operations. The process of coating seeds with pesticide slurry is often done by this technique. Seeds are treated by coating them with a polymer containing the desired pesticide/fungicide combination needed for a successful germination period. The process increases the resistance of seeds to both mechanical breakage and biological attack.
The spray-coating technique can be used alone or in combination with pan, drum, screw and fluidized-bed coaters. Spraying makes it possible to deposit either thin or thick layers of aqueous solution or suspensions and molten lipids or chocolate. It is the most commonly used technique for applying food coatings. The spraying nozzle plays a critical role in the coating process. Spraying efficiency depends on the pressure, fluid viscosity, temperature and surface tension of the coating liquid, as well as nozzle shape or design. This, in turn, affects the flow rate, the size of the droplets, spraying distance and angle, and overlap rate.
Application of edible coating in the food industry
According to Martins et al. (2003) alginate and gelatin-coating at different concentrations with plasticizers such as glycerol and carboxymethyl-cellulose (CMC) and sucroesters coating plasticized with mono/diglycerides were tested. The effects of those coating on the storage stability were followed by measurements of peel and pulp firmness. The 2 percent alginate and 5 percent gelatin-coating significantly reduced weight loss, thus maintaining fruit firmness and thereby preserving fruit freshness. The effect of these coatings includes improving the appearance and imparting an attractive natural looking sheen to the fruit.
Whey proteins are soluble proteins present in milk serum after caseinate is coagulated during cheese processing. Whey proteins represent about 20 percent of total milk proteins (Brunner, 1997). Dangaran et al. (2006) worked on whey protein-sucrose coating and reported that whey protein coating protects foods from deterioration and can extend product shelf life. Whey protein coatings may also change over a period of time if not properly formulated. Whey protein isolates, sucrose, high gloss coatings, with and without crystallization inhibitors, were formed on chocolate-covered peanuts. WPI coating containing raffinose had significantly higher gloss values. Edible coating of WPI can be used to improve the effectiveness of water-based high gloss edible coatings.
According to Kim (2002) the shelf life of grape was increased by edible coating material such as methyl cellulose (MC) which antimicrobial substances such as N-Capric acid isopropyl ester (Ci) and sodium nitrate (Sn) added and coating applied on grapes by spraying method. The quality changes of packaged grapes with wrapping PE film on EPS tray were investigated for 16 days at 30ºC. The reduction in the rate of firmness of grape, MCsn, and MCCi after 16 days at 30ºC were 42.2, 26.5 and 23.2 percent respectively. The addition of Ci and Sn had much affected the reduction of bacteria and yeast counts especially at an early stage of storage.
According to Bravin et al. (2006), edible coating of polysaccharide-lipid film composed of corn starch, methylcellulose (MC) and soybean oil had effective moisture transfer control in moisture-sensitive products. It was evaluated by coating crackers, a low water activity type cereal food. The spread film gave better water vapour barrier and mechanical properties than sprayed film. Coated crackers had a longer shelf life and higher reference at all storage conditions. A semi-solid edible coating may inhibit lipid migration in chocolate enrobed products. The coating containing hydrocolloids and sweeteners were tested for lipid barrier and sensory properties in addition to chocolate viscosity and water activity. A coating containing high methoxy pectin, acacia gum, high fructose corn syrup, dextrose, fructose, and sucrose was most effective.
Mes (1986) described a method for preparing cheese-based food in which a cheese received an edible coating such as beaten egg and bread crumbs and is optionally deep-fried, deep-frozen (at below –30ºC) and reheated before use. The product can be based on cheese alone or a combination of a slice of cheese with a slice of meat. Kimura (2006) found that an edible coating with excellent adhesion, pleasant mouthfeel and good moisture transfer resistance, is based on a macromolecular polysaccharide derived from seaweed and edible vegetable fiber which could be effective for meat products. The coating containing a mixture of the macromolecular polysaccharide and vegetable fiber with the food and an agent containing an appropriate metal ion to cause gelation was applied to meat products.
Lee et al. (2002) studied consumer acceptance of whey protein coated chocolate as compared with shellac coated chocolate. The WPI formulations without lipid varied in native as compared with the heat-denatured WPI amount. The WPI formulations with lipid varied in the lipid amount. The shellac formulation consisted of 30 percent solids of which 90 percent was shellac and 10 percent was propylene glycol. The results strongly indicated that water-based WPI lipid coating can be used as an alternative glaze with higher consumer acceptance than alcohol-based shellac.
Lee et al. (2002) studied the effect of an edible coating combined with a modified atmosphere (MA) (60 percent O2, 30 percent CO2 and 10 percent N2) packaging and gamma irradiation on the microbial stability and physicochemical quality of minimally processed carrots. A coating based on calcium caseinate and whey protein isolate were used. Samples were evaluated periodically for aerobic plate counts (APCS) and physicochemical properties (firmness, white discoloration, and whiteness index). The coating was able to protect carrots against dehydration during storage under ambient conditions. Coating and irradiation at 1 KGy were also able to protect carrots firmness during storage under ambient conditions. Modified atmospheric packaging retarded whitening of uncoated carrot but had a detrimental effect on firmness. The edible coating used in this study did not significantly inhibit microbial growth on carrots.
Consumer related issues
Since edible films and coatings are consumable parts of food products, the potential uses of edible materials may be significantly affected by consumer acceptance. Consumer acceptance is an integrated index of the subjective preferences of consumers for the products and includes organoleptic properties, safety, marketing, and cultural hesitation regarding the use of new materials. Organoleptic properties may include favorable flavor, tastelessness, sensory compatibility with coated packaged foods, texture, and appearance. Safety issues related to the potential toxicity or allergenicity of the new edible film-forming materials, and microflora changes of the packaged coated food products. Marketing factors include the price of the final products, consumers’ reluctance to use of the new materials, and special attention of consumers to the use of the new packages if special instructions are required for opening the packages, consuming the packaged located foods, and disposing of the used packaging materials.
Besides consumer acceptance, there are many limiting factors for the commercial use of edible films and coatings. They may include the complexity of the production process, large investment for the installation of new film-production or coating equipment, potential conflict with conventional food packaging systems, manufacturers’ resistance to the use of new materials, and regulatory issues.
Since edible films and coatings are an integral part of the edible portion of products, they should follow all required regulations regarding food products. All the ingredients of film-forming materials, as well as any functional additives in the film-forming materials, should be food-grade, non-toxic materials. Most GRAS status ingredients have specific restrictions. For example, when the GRAS notification document describes the intended use of certain material as a surface treatment for poultry meat, the use of the same material for red meats, cheese, or other food products would not be consistent with the GRAS notification. From a regulatory standpoint, edible films and coatings could be classified as food products, food ingredients, food additives, food contact substances, or food packaging materials. In the case of pharmaceutical and nutraceutical applications, there may be other regulations regarding their use.
If there is a different viewpoint of categorization between manufacturers and regulatory agencies, critical mislabeling problems may occur, resulting in mandatory product recall situations. When food manufacturers formulate film-forming materials and apply the materials on their food products, they should include all the film-forming ingredients on the labels of their final products. However, if they use edible film or coating materials that have been produced by different suppliers, there may be an opportunity to categorize them as food contact substances or food packaging materials. It is recommended that the edible film and coating material suppliers obtain no-objection notifications from related authorizing agencies for the use of their film and coating products as food ingredients, with careful considerations of proper labelling, including nutritional information and possible allergenicity.
Edible films and coatings are promising systems for the improvement of food quality, shelf life, safety, and functionality. They can be used as individual packaging materials, food coating materials, active ingredient carriers, and to separate the compartments of heterogeneous ingredients within foods. The efficiency and functional properties of edible film and coating materials are highly dependent on the inherent characteristics of film-forming materials, namely biopolymers (such as proteins, carbohydrates, and lipids), plasticizers, and other additives. Most biopolymers are relatively hydrophilic compared to commercial plastic materials. For industrial use, it is necessary to conduct scientific research to identify the film-forming mechanisms of biopolymers to optimize their properties. It is also suggested that feasibility studies be performed regarding the commercial uses of edible films and coatings by extending the results of research and development studies to commercialization studies, such as new process evaluation, safety and toxicity determination, regulatory assessment, and consumer studies. Much more research is needed as there is no universal edible film that is applicable to every problem.
Obviously, specific barrier requirements and food product specifications will determine the type of layer that is best for a given situation. Products with high moisture content need fatty layers to prevent loss of water. To prevent possible discoloration, an oxygen barrier component is needed. Unsaturated fats that are easily oxidized require similar protective layers. As stated above, water and oxygen permeability generally are inversely related; thus, many films will need to be composite materials with multiple properties (not unlike Mother Nature’s own protection). The ideal edible film should have the following characteristics:
- Contain no toxic, allergic and non-digestible components
- Provide structural stability and prevent mechanical damage during transportation, handling, and display
- Have good adhesion to the surface of the food to be protected providing uniform coverage
- Control water migration both in and out of protected food to maintain desired moisture content
- Provide semi-permeability to maintain the internal equilibrium of gases involved in aerobic and anaerobic respiration, thus retarding senescence
- Prevent loss or uptake of components that stabilize aroma, flavor, nutritional and organoleptic characteristics necessary for consumer acceptance while not adversely altering the taste or appearance
- Provide biochemical and microbial surface stability while protecting against contamination, pest infestation, microbe proliferation, and other types of decay
- Maintain or enhance aesthetics and sensory attributes (appearance, taste etc.) of product
- Serve as a carrier for desirable additives such as flavor, fragrance, coloring, nutrients, and vitamins. Incorporation of antioxidants and antimicrobial agents can be limited to the surface through use of edible films, thus minimizing cost and intrusive taste.
- Last but not least-be easily manufactured and economically viable.
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