Credible Edible Films

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Food Product Design

Credible Edible Films

September 1999 -- Applications

By: Suanne J. Klahorst
Contributing Editor

  In the beginning, each food came in its own natural, biodegradable, edible film. But ever since the biblical Adam took his first bite of the apple and noticed that the uneaten portion turned brown after a few minutes, humankind has sought effective substitutes for nature's coating in order to keep those uneaten portions fresh for future consumption. Sometime after Adam and Eve got kicked out of the Garden of Eden, plastic wrap was invented, and like everything else outside of the garden, it was less than perfect.

  Allegorically, this story describes the evolution of man-made packaging. While a little packaging is a wonderful thing, a lot of packaging is a nightmare, particularly when landfills around the world threaten to engulf our living space. European countries have devised an incentive to reduce packaging in the form of a "packaging tax." Meanwhile, the land-a-plenty United States is cranking out more varieties of packaging than ever before. The ratio of packaging to food is approaching unity as we enter the "single-serving" century. Fortunately, the topic of edible packaging is still of interest to the food industry and other organizations funding research to solve packaging dilemmas.

  Edible films have potential in a number of different areas. They can coat food surfaces, separate different components, or act as casings, pouches or wraps. They can preserve product quality by forming oxygen, aroma, oil or moisture barriers; carrying functional ingredients, such as antioxidants or antimicrobials; and improving appearance, structure and handling.

Film school

  Edible films are defined by two principles. First, edible implies that it must be safe to eat or that it is generally recognized as safe (GRAS) by the FDA. Second, it must be composed of a film-forming material, typically a polymer. You can dip, and you can coat, but the term edible film refers to a continuous barrier that is formed as the film adheres to the surface of the foodstuff. The key is to maintain that barrier for some period of time, in the same way that an inedible package might.

  An important term associated with all films is permeability, which measures the transfer of gas - water vapor, oxygen, carbon dioxide or ethylene - from one side of the film to the other. Barriers are either water-soluble or solvent-soluble. The more water-soluble a film, the more permeable to water vapor it usually is, while the solvent-soluble barriers are the best moisture barriers because water vapor does not pass through them as readily.

  Polymers that form films can be composed of carbohydrate, protein, solid lipid/wax, or resin. Examples of carbohydrate polymers include various forms of cellulose, such as carboxymethylcellulose (CMC) and hydroxypropyl cellulose (HPC); starch and dextrins; pectin; and alginates. Proteins currently used include albumen, corn zein, soy protein isolate, collagen and whey. Waxes include beeswax and carnuba wax, while shellac is the only food-grade resin. Combinations of these materials can offer some of the benefits of each component.

An almost perfect coating
  Shellac is one of the oldest substances collected for the purpose of coating and adhesion. This resin is secreted by a female, mite-sized beetle - about the size of a pinhead - as a means to hold its eggs to the bark of a tree, usually a Ficus benjamina cultivated for this purpose in India or Thailand. Most people think of shellac as something that was used to finish violins before polyurethane was invented. Indeed, it is still used for this purpose by those who follow the older traditions of this meticulous craft. However, suppliers of food-grade shellac prefer to compare it to beeswax, another well-accepted food-grade insect secretion.

  After shellac is harvested from the trees, it is used to formulate a product known among confectioners as "confectioners glaze." The most common use for confectioners glaze is to provide a very durable, glossy finish for a pan-coated candy, such as a chocolate-covered almond. Chocolate that is poured into a smooth mold and allowed to harden has a very shiny appearance, but chocolate that is pan-coated onto a nut or a raisin does not develop this shiny coating without some assistance from food technology. Stephen Santos, technical director, Mantrose Bradshaw Zinsser Group (MBZ), Westport, CT, recounts the unique properties of shellac that make it a favorite with the confectionery industry: "No other finish holds its gloss like shellac, even in conditions of high humidity when starch-based coatings lose their shine. It never gets sticky or dull, and it will adhere to almost any surface, even slick surfaces like glass and metal."

  The application of shellac to candy is a simple operation. It is added to a conventional candy pan and tumbled with the candy. Once the glaze has been applied, it offers improved shelf life, scuff resistance, anti-sticking properties and serves as a moisture barrier. It's even kosher according to most authorities.

  For some operations however, shellac is no longer perfect. The primary disadvantage of "lac resin glaze," as it is also known to confectionery professionals, is that it requires a solvent, such as food-grade ethanol to apply it. The ethanol is exhausted from the surface of the candy by airflow. In some regions of the United States, this ethanol exhaust is regulated, since it contributes to total volatile organic compounds (VOCs). In urban areas, VOCs are more tightly regulated by the state and the Environmental Protection Agency. While the ethanol required is minimal, processors under the strictest regulations avoid any VOCs whenever possible. "Although water-soluble alternatives for confectioners glaze have been developed, it is not without the loss of some of the resin's superior characteristics," explains Santos.

Films for fresh cuts
  MBZ has also developed edible films for whole and fresh-cut fruits and vegetables. Shellac has been used to impart shine to whole citrus fruits for a number of years, and it also helps slow down respiration of the fruit, retarding ripening and increasing shelf life. New products are now available for this purpose based on sucrose-ester chemistry. For cut fruit, a different type of system has been developed. Nature Seal™, developed for apples, pears, bananas and avocados, is composed of a blend of vitamins and minerals. Ascorbic acid and calcium are two that are well known for their benefits in fruit processing for prevention of browning and softening. This coating extends shelf life by maintaining color, firmness and texture for up to two or three weeks. Some formulations also contain amino acids, which inhibit the natural enzyme activity that results in produce degradation.

  For some types of produce, edible coatings can eliminate the need for expensive modified-atmosphere packaging. A cellulosic film-former, such as CMC or hydroxypropyl methylcellulose (HPMC), can be used as a moisture barrier for cut vegetables that tend to turn white as they dry out. Says Santos: "Each fruit requires a custom blend, which is where MBZ can offer the benefits of our expertise. There are even differences between varieties of apples; so far we have studied coating applications for 40 different varieties."

Synergistic films
  Edible film can protect cooked foods as well as raw foods; carbohydrate-based ingredients are used in a number of these types of applications.

  Dorothy Gentile, technical service supervisor with National Starch & Chemical Company, Bridgewater, NJ, advises product developers for foodservice on the uses of starch coatings to enhance the almost perfect frozen French fry. The objective, she says, is to create layered functionality that will keep the starch film in place from the initial through to the final phases of the frying process. She prefers a combination of cornstarch, tapioca dextrin, high-amylose starch, rice flour and salt. In fact, some french fry product developers will specify up to five different types of starch to get the optimum performance from the film.

  Unlike other starch applications, where building viscosity is the primary goal, starch films require high levels of solids without viscosity. Gentile has seen clear coats used that are up to 42% to 43% solids. An important function of these films is to control water evaporation during frying. If the water cooks out too fast, the film coating tears and the benefit is lost. When the coating functions properly, the cooked fry will have excellent freeze/thaw stability. Once it is refried at the retail outlet, it will retain heat longer and have a "shelf life" of up to 15 minutes. It will also be crisp, but not brittle.

  While starch does not contribute to flavor or color, ingredients that add these attributes are also frequently added to the final coating formulation. The development process of putting together the right formulation often results in a proprietary film that differentiates the final product from the other french fries in this competitive market.

  Methylcellulose, HPC, gellan gum and calcium-reactive pectin all have been used as film coatings in fried foods to maintain moisture and limit fat uptake. This not only results in a lower-fat finished product, but also reduces cost on a per-serving basis by lowering the amount of fat that must be replaced after product is fried. Using these types of films can reduce moisture migration into the oil and conceivably extend the frying oil's shelf life.

Sans package
  John Watson, president of Watson Foods Company, West Haven, CT, demonstrated entrepreneurial innovation when he introduced an edible film that completely replaces packaging. This preformed HPC film is clear, food-grade, heat-sealable and cold-water soluble. It provides a nearly error-free delivery mechanism for adding expensive ingredients to food formulations in small quantities. Vitamins, minerals, enzymes, colors and flavors are a few of the ingredients that Watson Foods can offer in this type of packaging. Pre-weighed ingredients in soluble packaging eliminate the need for additional measuring and handling. The whole package goes into the batch, the free water dissolves the package, and further mixing distributes the contents evenly.

  The package dissolves best within a temperature range of 40°F to 125°F. Unlike some forms of cellulose that dissolve better in hot water, this edible package dissolves more easily as the temperature gets lower. At temperatures over 125°F, the film forms a gel and is insoluble. Determining the point of addition to a food formulation requires a minimal amount of testing to ensure that enough free water and mixing time are available to obtain the desired results for the size of the package.

  For those interested in just the film, it is also available as packaging for a variety of precision-conscious settings, from foodservice to pharmaceutical and medical. The film is also available colored, and when broken into fine particles it makes an eye-catching edible glitter for icings. It can be purchased by the bobbin, or as a preformed bag. Widths vary according to customer requirements, and thicknesses range from 1.5 mils (millionths of an inch) to 3.2 mils.

Potential Applications of Whey-Protein Films

  • Oxygen- or aroma-barrier coatings for low-moisture food products vulnerable to oxidation or aroma loss in conjunction with a simple, moisture-barrier packaging film bag (e.g., LDPE)
  • Respiration-reducing coatings for fresh fruits and vegetables that are exposed to low relative humidity during storage and transportation
  • Lipid barrier films or coatings separating lipid-rich ingredients from other components of heterogeneous foods
  • Structure-enhancing coatings on friable food such as freeze-dried foods
Courtesy of Dairy Management Inc.TM

Meat films
  Films for seasoned, precooked meats are gaining popularity as meat processors develop a variety of ready-to-heat products for quick meals. Additional convenience is now available from instant starch coatings. Sometimes referred to as "meat glazes," these films offer a significant advantage since they do not require cooking before application.

  R. Daniel Putnam, technical manager with Grain Processing Corporation, Muscatine, IA, draws on his meat-industry background to give an insider's appreciation for what an edible meat coating can offer as a seasoned glaze. Such a film is formed by the application of a solution made up of 15% to 30% hydrated food starch. Once hydrated, the modified starch offers several beneficial properties. It can reduce purge (moisture "weeping"); promote weight gain; adhere flavor, particulates and color; and improve freezer stability. According to Putnam, expertise is required to "determine the optimum glaze and moisture balance. The glaze will tie up moisture and reduce moisture loss. However, if pushed too close to capacity, the purge may dissolve the coating. Moisture is very important to the delivering flavor in meat products, and starch coatings are an economical way to make sure meats stay juicy, tender and flavorful during cooking or reheating."

  Other ingredients commonly added to the glaze are spices, flavors, and heat-developed color. The antimicrobial agent sodium lactate can also be added. The film performs well even with the addition of low-pH flavor components such as lemon.

Protein edibles
  Another hydrophobic coating that makes an excellent moisture barrier is corn zein. Corn zein is extracted from corn gluten, the protein component of corn. It is hydrophobic because it is richer in hydrophobic amino acids - the ones with aromatic side chains - than hydrophilic amino acids. Like shellac, it is soluble only in ethanol or other solvents, but alternative, water-dispersible types of corn zein have been developed.

  Freeman Industries, L.L.C., Tuckahoe, NY, is the largest supplier of both solvent- and water-soluble corn zein in the United States. Paul Freeman, principal of this family business, serves as the technical advisor for applications of this GRAS, kosher ingredient. "Corn zein is currently used to coat candy, flavors, dried fruits, nuts, rice, seeds, and pharmaceutical tablets - especially where time-release benefits are required," he says. An added benefit is zein's natural resistance to bacterial attack and even some insects. As a film, it forms a tasteless coating that is clear, hard and nearly invisible. And, like other hydrophobic coatings, it is prized for its stability in conditions of high humidity and high heat.

  Researchers at the Agricultural Research Service's Eastern Regional Research facility in Wyndmoor, PA have developed experimental films possessing good mechanical and moisture barrier properties from emulsions of pectin, zein and palmitic acid, which acts as a moisture barrier and plasticizer. The resulting films might be used to protect fresh or minimally processed foods from dehydration or serve as edible wrappings.

  Scientists are also developing other types of hydrophilic protein films. An active area of research is using whey proteins to make edible and biodegradable films. Made from cheese whey, whey protein is separated from lactose using ultrafiltration, after which it is evaporated and spray dried to a dry white powder. The California Dairy Foods Research Center, University of California, Davis, is one of the research centers working on the development of these whey-protein-based films. Funded by Dairy Management Inc., Rosemont, IL, University of California-Davis professor John Krochta invents whey films for a variety of applications. Potential future applications include the protection of oxygen-sensitive foods (such as nuts); films to block oil migration into surrounding food components; and alternatives to collagen for meat casings.

  Whey-protein isolate makes an excellent film, and has the advantage of being very water soluble across a wide pH range. Whey-protein films can provide barriers to oxygen, aroma and oil at low-to-intermediate relative humidity. To make a film, whey-protein isolate is added to water and is heat-denatured. After cooling and removal of trapped gas, it can be poured into a glossy, bland film. A plasticizer, such as glycerol, may be added to improve the film's mechanical properties. Films are evaluated by several standard tests, including tensile strength, which is measured by recording the pulling stress the film can withstand before it breaks. The addition of a plasticizer can show significant improvement in a protein film's tensile strength.

  Likewise, there's also interest in the use of soluble soy proteins as barriers on bakery products. Bakery coatings are frequently used to add shine, and as adhesives to hold poppy seeds, nuts, or whole grains to the crusts of specialty breads. While egg albumin, an egg-white protein that imparts a shine to bakery products, is effective, it can't be used in vegan applications. Protein Technologies International, a DuPont business based in St. Louis, has developed a bakery wash with isolated soy protein that gives bakeries a vegetable alternative for developing a glossy crust. The wash is made from about 9% soy protein, 91% water, and an optional smidgin of lecithin to control foaming. After 15 minutes of hydration in hot water, the coating is ready to apply as a pre- or post-bake application.

  Other proteins from sources as diverse as animals (gelatin, collagen), peanuts and wheat also have potential. No matter what the protein, there is always a use for protein as a film because of the superior coating properties of these amazing chemical structures. Research is continuing on improving the functional properties of protein films through physical, chemical or enzymatic means. For example, researchers M. S. Zutara, H. Chen and J. Li at the department of nutrition and food sciences of the University of Vermont, Burlington, have shown that the water solubility of whey-protein films can be modified by varying the extent of denaturation, adding lipids, using different plasticizers, or exposing the films to UV irradiation. These options would allow the development of whey-protein-based films with different degrees of water solubility that might be required for a range of applications.

  Is there such a thing as the perfect package? From an environmental point of view, it's the edible variety that fits the bill. Long term, edible films show potential in eliminating synthetic oxygen and gas barriers now used in multi-layer packages, minimizing damage to the foods and extending their shelf life. Using edible films in place of some of the current packaging might also make the packaging more recyclable. While it is improbable that edible films will ever completely replace plastics, city landfills are grateful for any fraction of the tons of plastic that they might replace today or in the future. And the people who live in the city, or near a city landfill, are grateful too.


  Suanne Klahorst is associate director of the California Institute of Food and Agricultural Research (CIFAR) at the University of California, Davis. CIFAR conducts applied, industry-relevant research leading to the utilization of food and agricultural by-products and their chemical constituents. She can be contacted at sjklahorst@ucdavis.edu.


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