Edible Films and Coatings for Food Applications

Edible films and coatings, such as wax on various fruits, have been used for centuries to prevent loss of moisture and to create a shiny fruit surface for aesthetic purposes. These practices were accepted long before their associated chemistries were understood, and are still carried out in the present day. The term, edible film, has been related to food applications only in the past 50 years. One semi-sarcastic tale was that spies’ instructions were written on edible films, so that in the off-chance they were captured, they could easily destroy their secrets by eating them. In most cases, the terms film and coating are used interchangeably to indicate that the surface of a food is covered by relatively thin layer of material of certain composition. However, a film is occasionally differentiated from a coating by the notion that it is a stand-alone wrapping material, whereas a coating is applied and formed directly on food surface itself. As recently as 1967, edible films had very little commercial use, and were limited mostly to wax layers on fruits. During intervening years, a significant business grew out of this concept (i.e., in 1986, there were little more than ten companies offering such products, while by 1996, numbers grew to 600 companies). Today, edible film use has expanded rapidly for retaining quality of a wide variety of foods, with total annual revenue exceeding $100 million.

Why do we need edible films? Most food consumed comes directly from nature, where many of them can be eaten immediately as we take them from the tree, vine or ground. However, with increased transportation distribution systems, storage needs, and advent of ever larger supermarkets and warehouse stores, foods are not consumed just in the orchard, on the field, in the farmhouse, or close to processing facilities. It takes considerable time for a food product to reach the table of the consumer. During time-consuming steps involved in handling, storage and transportation, products start to dehydrate, deteriorate, and lose appearance, flavor and nutritional value. If no special protection is provided, damage can occur within hours or days, even if this damage is not immediately visible.

Research and development on films and coatings made from various agricultural proteins has been conducted over the past 20 years, but is of heightened interest, due to the demand for environmentally-friendly, renewable replacements for petroleum-based polymeric materials and plastics. To address this demand, films and coatings have been made from renewable resources, such as casein, whey, soy, corn zein, collagen, wheat gluten, keratin and egg albumen. Those made from agricultural proteins create new outlets for agricultural products, byproducts and waste streams, all of which can positively impact the economics of food processes.

Due to casein’s ability to form water-resistant films, it was used for hundreds of years in paints and coatings (Gettens and Stout 1984). In the late nineteenth century, casein was converted into a hard plastic material by cross-linking it with formaldehyde. A patent for this technology was issued to Adolf Spitteler in Bavaria (Brother 1940), and it was used for the manufacture of products, such as buttons, umbrella handles, small boxes and pen cases.

Polysaccharide gums are hydrocolloids of considerable molecular weight, and are water-soluble. They dissolve in and form intensive hydrogen bonds with water. Because of the size and configuration of their molecules, these polysaccharides have the ability to thicken and/or gel aqueous solutions as a result of both hydrogen bonding between polymer chains and intermolecular friction when subjected to shear. Gums dissolve in water through the formation of solvent–polymer hydrogen bonds; in solution, polymer molecules may arrange themselves into an ordered structure, called a micelle that is stabilized or fortified by intermolecular hydrogen bonds (Fig. 3.1). The micelle traps and immobilizes water and, depending on the extent of the intermolecular association, the water is either thickened, as measured by a parameter called viscosity, or converted into a gel that possesses both liquid- and solid-like characteristics or viscoelasticity.

Edible films and coatings satisfy a variety of needs and meet specific product challenges for a large number of food applications. There is a general lack of agreement as to what constitutes a coating. A layer of seasoning on a snack or an oil spray applied to a cracker or a baked product, are examples of edible coatings. Further examples include soft, hard and chocolate panning in confectionery; application of carnauba wax to a gummy candy to preserve individual piece identity; application of icings or glazes to baked goods, use of caramel coatings for popcorn, and enrobing or dipping items in chocolate. Layers of barbeque sauce or fruit glaze on meats are coatings. Seasonings added to a chip, extruded corn collet or a rub applied to a chicken wing are often referred to as coatings. Tempura, battered and breaded fried appetizers are dependent upon coatings for their crunchy texture and eating quality. Egg wash layer added to yeast-leavened baked items for gloss is a protein-based aqueous edible coating. Liquid between air cells in foam could be described as a solute-stabilized edible film. The early Apollo astronauts ate foods coated with starch-based films to prevent the crumbs from becoming airborne and floating around the weightless environment of the cabin.

There are also examples of less traditional coatings and films, which are often freestanding or self-supporting. Edible packaging has been a topic of interest for many years, though few if any commercial examples exist. An invisible, edible coating would likely be more acceptable to consumers than petroleum-based plastic packaging. Edible coatings can make excessive packaging unnecessary, which is also perceived as a positive consumer benefit.

The quality of food products depends on their organoleptic, nutritional, and microbiological properties, all of which are subject to dynamic changes during storage and distribution. Such changes are mainly due to interactions between foods and their surrounding environment or to migration between different components within a composite food.

In the last 20 years, there have been over 45 edible packaging patents. During 2006 alone, there were 174 scientific papers focused on edible packaging. Most work covered in these papers deals with water vapour transfer. However, there are other potential applications. For example, edible packaging can be used to encapsulate flavour and aroma compounds, antioxidants, antimicrobial agents, pigments, ions that prevent browning reactions, or nutritional substances such as vitamins.

Starch-based and composite edible films and coatings can enhance food quality, safety and stability. They can control mass transfer between components within a product, as well as between product and environment. They can improve performance of the product through the addition of antioxidants, antimicrobial agents, and other food additives. Unique advantages of edible films and coatings can lead to the development of new products, such as individual packaging for particular foods, carriers for various food additives, and nutrient supplements. Film materials and their properties have been reviewed extensively in this book and previously (Guilbert 1986; Kester and Fennema 1986; Krochta and De Mulder-Johnson 1997).

Composite films can be formulated to combine the advantages of each component. Biopolymers, such as proteins and polysaccharides, provide the supporting matrix for most composite films, and generally offer good barrier properties to gases, with hydrocolloid components providing a selective barrier to oxygen and carbon dioxide (Guilbert 1986; Kester and Fennema 1986; Drake et al. 1987, 1991; Baldwin 1994; Wong et al. 1992; Baldwin et al. 1997). Lipids provide a good barrier to water vapour (Nisperos-Carriedo 1994; Baldwin et al. 1997), while plasticizers are necessary to enhance flexibility and improve film’s mechanical properties.

There is a growing trend toward increased consumption of fresh fruits and vegetables. According to the USDA, fresh fruit consumption in the United States in 2000 was 28% above average annual fruit consumption of the 1970s, and fresh vegetable consumption was 26% above average annual vegetable consumption for the same period (USDA 2001–2002).

Higher consumption of fruits and vegetables has been associated with growing interest in a healthier diet, and is expected to increase over time. International organizations (World Health Organization/WHO 2002; Food and Agriculture Organization/FAO 2003) have been urging nations everywhere to promote consumption of fruits and vegetables, as a diet high in such foods has been found to be associated with decreased incidences of birth defects, mental and physical retardation, weakened immune systems, blindness, cardio-vascular diseases, and some forms of cancer and diabetes (Ford and Mookdad 2001; Genkinger et al. 2004; Hung et al. 2004; FAO 2003). WHO (2002) has estimated that low fruit and vegetable intake is among the top ten risk factors contributing to mortality (2.7 million deaths each year). They also reported that 19% of gastrointestinal cancer, 31% of ischemic heart disease, and 11% of stroke occurrences are caused by low intake of fruits and vegetables.

Edible films and coatings are defined as continuous matrices that can be prepared from proteins, polysaccharides and/or lipids to alter the surface characteristics of a food. Although the terms films and coatings are used interchangeably, films in general are preformed and are freestanding, whereas, coatings are formed directly on the food product. Proteins used in edible films include wheat gluten, collagen, corn zein, casein and whey protein. Alginate, dextrin, pectin chitosan, starch and cellulose derivatives are commonly used in polysaccharide films. Suitable lipids for use in films and coatings include waxes, acylglycerol, and fatty acids (Kester and Fennema 1986). Composite films containing both lipid and hydrocolloid components have also been developed.

Plasticizers are often added to film-forming solutions to enhance the properties of the final film. These film additives are typically small molecules of low molecular weight and high boiling point which are highly compatible with the polymer. Common food-grade plasticizers such as sorbitol, glycerol, mannitol, sucrose and polyethylene glycol decrease brittleness and increase flexibility of the film, which are important attributes in packaging applications. Plasticizers used for protein-based edible films decrease protein interactions and increase both polymer chain mobility and intermolecular spacing (Lieberman and Guilbert 1973). The type and concentration of plasticizer influence properties of protein films (Cuq et al. 1997); mechanical strength, barrier properties, and elasticity decrease when high levels of plasticizers are used (Cherian et al.1995; Galietta et al. 1998; Gontard et al. 1993). Water is another important plasticizer for protein films (Krochta 2002). Similar to other plasticizers, water content impacts film properties.

Edible films and coatings are promising systems for improvement of food quality, shelf life, safety, and functionality. They can be used as individual packaging materials, food coating materials, and active ingredient carriers. They can also be used to separate the compartments of heterogeneous ingredients within foods. In fact, edible films and coatings can incorporate food additives, such as anti-browning agents, antimicrobials, flavors, colorants, and other functional substances. Enhanced sensory properties of a food can be achieved by adding flavoring agents to an edible film or coating, leading to development of new flavor delivery systems that improve food quality and utility. Currently, there are numerous applications of edible films and coatings whose main purpose is to impart desirable mouthfeel to the coated product; this is especially true for snacks (e.g., popcorn, corn chips, and potato chips), nuts, meat, fish, and poultry. Further, incorporation of active ingredients can enhance functionality of edible films and coatings, thereby providing health benefits to consumers. For example, addition of probiotic organisms to films and coatings could open up opportunities to develop new, health-enhancing products. Edible films and coatings are promising delivery systems for flavor and active ingredients that improve food quality and functionality.

This chapter will focus on use of edible films and coatings as carriers of flavor and active ingredients in foods. It will also discuss incorporation of some flavoring and active substances into edible films and coatings, which can improve quality and functionality of foods. It will also provide insights about recent advances in this area.

Functional efficiency of edible films and coatings strongly depends on the nature of film components and physical structure. Choice of a film-forming substance and/or active additive depends on the desired objective, nature of the food product, and specific application. Thus, lipids or hydrophobic substances such as resins, waxes or some insoluble proteins are most efficient for retarding moisture transfer. On the contrary, water-soluble hydrocolloids, like polysaccharides and proteins, are not very efficient barriers against water transfer. However their permeability to gases is often lower than those of plastic films. Moreover, hydrocolloids usually provide better mechanical properties to edible films and coatings than lipids and hydrophobic substances. Therefore, combinations of desirable properties can be obtained through collective use of hydrocolloids and lipids to create composite films. Film-forming substances, particularly proteins, require film additives such as plasticizers to improve film resistance and elasticity, or emulsifiers, to increase hydrophobic particle distribution in composite emulsion-based edible films (Debeaufort et al. 1998).

In the modern analytical laboratory today, one will find an assortment of analytical instrumentation such as gas chromatography for the analysis of volatile compounds and liquid chromatography for the analysis of non-volatile chemicals. Many other computer-assisted instruments are used for the analysis of materials such as foods or other consumer products, as well as industrial materials. For many of these instruments, talent and experience is required to prepare the sample prior to analysis and this can be challenging. These procedures require time and even more time is needed to develop them as useful analytical methods.

Use of natural polymers, such as proteins and polysaccharides, as coating or film materials for protection of food has grown extensively in recent years. These natural polymers can prevent deterioration of food by extending shelf life of the product and maintaining sensory quality and safety of various types of foods (Robertson 1993). Generally, film and coating systems are designed to take advantage of barrier properties of polymers and other molecules to guard against physical/mechanical impacts, chemical reactions and microbiological invasion. In addition, the use of natural polymers presents added advantages due to their edible nature, availability, low cost and biodegradability. The latter particularly is of paramount interest due to demand for reducing the amount of non-biodegradable synthetic packaging. Furthermore, these polymers can be easily modified in order to improve their physicochemical properties for filming and coating applications.

Edible films are defined by two essential elements. First, edible implies that the film, including all of its components, must be safe to eat or that they are “generally recognized as safe” (GRAS), as defined by the U.S. Food and Drug Administration (FDA). Second, edible films must be composed of a film-forming material, typically a water-soluble hydrocolloid or polymer. Further, these films must be manufactured on equipment and in a facility suitable for processing food products.

Edible coatings applied to fruit, vegetables, meat and other food products for improved protection or appearance are often referred to as edible films. For the purposes of this chapter in the book, these applications are not included in this chapter. Edible films can be consumed directly (e.g., a breath freshener film) or can be used to wrap food products, form a pouch or become a component layer in other food products. In most cases, these films are water-soluble and dissolve rapidly in water or in the mouth. However, some components of edible films (e.g., shellac and soy protein) can be insoluble in water, but are digestible when consumed.