Edible and Biodegradable Films

Biodegradable and edible films mainly comprise one or more proteins, lipids, or polysaccharides; each of these base materials has unique strengths and weaknesses as packaging materials and vehicles for antimicrobial compounds, and is thus usually employed in combinations. Polysaccharide (cellulose, gums, starch) or protein (gelatin, corn zein, soy protein, whey, etc.) films are highly sensitive to moisture and are poor barriers to water vapor; however, they exhibit suitable mechanical and optical properties. Films composed of lipids (waxes, lipids) have good water vapor barrier characteristics, but do not exhibit suitable mechanical and optical properties (are opaque and may be brittle). Wheat gluten and soy protein isolate films are effective O2 barriers at low relative humidity, but have limited vapor barrier ability. Addition of lipid components to protein-based films improves the characteristics of both materials by optimizing both permeability to moisture and structural strength. Polarity of these natural films will determine compatibility with a particular antimicrobial and application or incorporation method.

Use of antimicrobials in edible films and coatings concentrates active compounds at the produce surface where protection is needed; thus small levels of additive are needed. This type of treatment is attractive to the increasing population of consumers who desire minimally preserved fruits and vegetables. Antimicrobials as edible film or coating components must be approved for food use. Waxes, with incorporated antimicrobials such as imazalil and benomyl, have been successfully used to minimize water loss and to inhibit microbial growth through gas exclusion and wound protection on fruit; however, benomyl is not an FDA GRAS substance and cannot be directly applied to food, and imazalil has limited FDA approval.

Some biodegradable film components such as chitosan naturally exhibit antimicrobial properties. Chitosan, a polysaccharide derived from shellfish and some fungi, has been found to exhibit broad antimicrobial activity towards a range of yeasts, molds, and bacteria and thus shows potential for application in MAP systems. Lee and others [25] found that chitosan, applied as a paper packaging coating, inhibited growth of E. coli O157:H7 in orange juice. Srinivasa and others [26] found that a chitosan film employed as a lid on a cardboard container for MAP storage of whole mango fruit inhibited fungal growth and extended shelf life of the product from 9 to 18 days at 27° C as compared to use of LDPE as an overwrap.

As with synthetic films, O2 permeabilities of edible films and coatings generally can be very low and CO2/O2 permselectivities can be quite high. In a selection of edible films [27] including pectin, wheat gluten, chitosan and bilayer gluten, and beeswax films, at 25°C the CO2/O2 permselectivity ranged between 6 and 28.4, and O2 permeability ranged between 2 and 258.8 pO2 ml mm/(m2 day atm). Thus the same risk for development of anoxic conditions and reduced food safety conditions exists for edible natural films as for synthetic films. Appropriate combinations of coatings and antimicrobial compounds may compensate for these effects. For example, a wax-based coating may incorporate nisin to reduce the risk of growth of Clostridium botulinum and/or L. monocytogenes [15].

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