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Everywhere you look, in malls, on the street, in cars, people are eating and drinking. Sometimes the food is partially wrapped, sometimes the food is extracted from a container, and sometimes the food and its package are one—consider the banana and hard-boiled eggs still in their shell. And some of the containers are edible, such as the ice cream cone and the edible hot dog casing.
Food packagers devote their careers to protecting the edibility of foods while minimizing the mass of external materials needed for this indispensable objective. Much of food packaging technologists’ resources have been invested in obviating environmentalists’ concerns about solid waste insulting the earth and depleting its resources. The bottom line is that edible packaging of food might make much more energy, economic, and technical sense than mainstream materials, especially when it can be combined in conventional package structures in ways that might result in reduced package mass, simpler packaging (for easier recycling), and lower costs.
Edible coatings have been applied to foods for centuries or perhaps millennia. Waxes, polishes, shellac, sugar, salt, and their derivatives and analogs have all been used to coat foods to reduce moisture, oxygen, or flavor gains or losses. Coatings also enhance the appearance of fruits and vegetables and frequently contain other functional entities to retard insects, microorganisms, oxidation, and other intruders that would diminish the product.
Some edible coatings are engineered to prohibit the transfer of materials from one to another food component. For example, nuts might be coated to prevent moisture migration from nearby caramel, which would reduce their crunch. Some coatings help to retain the masticatory integrity of cereal flakes and similar products. And some are intended to impart flavor to the products, such as seasonings on cured meats.
Most of the current research on edible films aims at facilitating formation of films using new materials with improved properties. Usually the vision is that the films might be formed into stand-alone packaging or coatings on foods. Edible coatings applied to surfaces are not generally intended to replace the need for non-edible packaging. Rather, they provide some level of protection and thus may reduce the complexity—and cost—of conventional packaging, which is still indispensable.
Are edible coatings integral to foods or a component of packaging? Or both? One might postulate that coatings are a beautiful marriage of packaging science and food science.
Although somewhat akin to edible films, biodegradable package materials are engineered to degrade by natural biological elements and thus disappear after they have performed their basic function of protection in distribution channels. Issues such as activation of the biodegradation process and safety and final disposition of the end products have not yet been resolved.
Compostable materials are designed to be attacked in the ground by natural biological creatures and converted into useful garden contributors. Easy efficient sorting of biodegradable packaging would be essential to avoid contamination of recycling and biodegradation processes. In addition, designing a package to biodegrade precludes its being used as a resource for other products or for energy generation. Enormous investments have been made in the development and marketing of biodegradable package materials and disposable utensils for quick-service food retailing, but commercial results have been limited.
Related to biodegradability is photodegradablity, the ability to decompose into constituent elements upon exposure to visible and/or ultraviolet radiation after fulfilling the functionality of containing product. Satisfying to some environmentalist factions, photodegradable thermoplastic sheet and film have not proven to meet the dual criteria of functionality and disappearance soon after functioning in distribution channels.
Edible Film Composition, Fabrication, and Properties
Edible films are made of various materials, are formed by various processes, and have various properties.
• Composition. Edible film formers include polysaccharides, proteins, and lipids.
Polysaccharides may include cellulose derivatives; starches and their derivatives; seaweed extracts such as carageenan and alginates; pectinates; and chitosan. Protein film formers include collagen, gelatin, whey protein, corn zein, soy protein, and wheat gluten. Polysaccharide and protein film materials are characterized by high moisture permeability, low oxygen and lipid permeability at lower relative humidities, and compromised barrier and mechanical properties at high relative humidities.
Edible lipids include waxes, triglycerides, shellac, acetylated monoglycerides, fatty acids, fatty acid alcohols, and sucrose fatty acid esters. Lipid materials are not generally polymers and so cannot form standalone films. They can, however, provide gloss or moisture barrier or composite with other edible structures (analogous to thermoplastic coextrusion or lamination) to deliver barrier properties.
• Plasticizers. Like thermoplastic films, edible films often require incorporation of low-molecular-weight plasticizers to improve film flexibility and durability. By interrupting the chain interactions and lowering the glass transition temperatures, greater flexibility may be achieved at the expense of permeability. Edible plasticizers include sucrose, glycerol, propylene glycol, fatty acids, and monoglycerides.
Even more than thermoplastic materials, edible films may have the potential for incorporation of further functional entities such as antimicrobials, antioxidants, flavors, and nutrients.
• Fabrication. Technologies for producing edible films are similar to those for thermoplastic structures: solvent casting and extrusion. Obviously, the conditions are different, but the principles are allied.
In solvent casting, water or water–ethanol solutions or dispersions of the edible materials are spread on smooth surfaces. When the solvent is evaporated, the film can be stripped from the surface. Except for collagen, corn zein, and wheat gluten, most other edible films are water soluble. Corn zein and wheat gluten films may be formed from water–ethanol solution or from water dispersion, resulting in somewhat better moisture barrier than other protein-based films.
Insoluble whey protein and other protein films result from denaturation and subsequent cross-linking due to heat treatment. Methyl cellulose and hydroxypropyl cellulose films applied for water-soluble pouches are cast commercially from solvent.
Polysaccharide–lipid multilayer films with water vapor barrier have been produced by casting from water–ethanol solutions. Lipid layers may be formed directly from ethanol solution or a melt. Stable emulsions of lipids in proteins or polysaccharides produce well-dispersed composite films with enhanced moisture barrier.
In processes very similar to extrusion casting of thermoplastic films or coextrusions, lipid films, laminants, and coatings may be produced by cooling melts to form solid structures. Certain polysaccharides and proteins (often with plasticizer required) can be extruded into flexible transparent films, again similar to extrusion casting of thermoplastic films. Water-insoluble collagen film for sausage casings is made by regeneration of collagen from animal hides. Formation of collagen film involves extrusion of a low-solids-content collagen into a coagulation bath (Krochta, 1997).
• Properties. Operating conditions greatly influence the properties of edible films. Elevated relative humidities increase the moisture content of polysaccharide and protein films, resulting in increased permeability of water vapor, oxygen, and flavor and decreased physical characteristics.
Polysaccharide and protein films are not good moisture barriers alone, but when they are combined with waxes and/or fatty acids, good water vapor barriers result. Some edible films, not surprisingly, offer better oxygen barrier, approaching that of ethylene vinyl alcohol, at low relative humidities than do polyethylenes. Research to date suggests that edible films are good aroma barriers—a definite asset when seeking to avoid flavor scalping (Krochta et al., 1994).
Water-soluble pouches derived from polysaccharides have been commercial for years, and their superior oxygen barriers have been known if not applied commercially. Obviously, collagen has been a major sausage casing for decades, but its progress in other areas has been limited, perhaps because of marginal sensory characteristics. More recently, other materials, such as whey protein, have been extruded into transparent films that can be converted into pouches (Krochta and De Mulder, 1997).
One major objective of many academic researchers today is to develop new materials into edible films and coatings that offer properties that would permit commercial interests to reduce their reliance on “traditional” thermoplastic structures, or at least reduce the quantities required. With the price of hydrocarbon resources soaring, the notion of renewable resources is intriguing. Additional analysis, however, is required to factor in the energy cost of edible films.
Further serious questions that must be answered include how much of the research activity is due to the desire of raw material suppliers to identify new uses for their commodities, how much is driven by environmentalist interests, and how much is motivated by actual needs of package converters and packagers.
It is evident that universities performing the research have a wonderful teaching tool in translating edible film packaging concepts into mainstream packaging, developing new knowledge along the way. Funding provided by raw material suppliers is supporting the education of many graduate students. Are the objectives of edible films and coatings, however meritorious, sufficient to warrant the ongoing investment? Or should industry and government seek to redouble their efforts, since we are so close to breakthroughs in edible films? Or might we attempt the crossover—no physical law prohibits the marriage with thermoplastic or even paperboard of edible materials to achieve a goal of better packaging. Perhaps food packaging technologists should contemplate incorporating edible packaging into their mainstream conventional materials and structures the next time they lick the ice cream from their (edible) cones?
The input of John M. Krochta, Professor, Dept. of Food Science & Technology, University of California, Davis, in the preparation of this article is appreciated.
by AARON L. BRODY
President and CEO,
Packaging/Brody, Inc., Duluth, Ga.
Krochta, J. M. 1997. Film, edible. In “Encyclopedia of Packaging Technology,” 2nd ed., ed. A. Brody. Wiley Interscience, New York.
Krochta, J.M. and De Mulder, C.L.C. 1997. Edible and biodegradable polymer films—Challenges and opportunities. An IFT Scientific Staus Summary. Food Technol. 51(2): 61-74.
Krochta, J.M., Baldwin, E.A., and Nisperos-Carriedo, M. 1994. “Edible Coatings and Films to Improve Food Quality.” Technomic, Boca Raton, Fla.