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Proteins

Proteins have been used for their film-forming abilities for nonfood applications since ancient

times as a component of glue, paint leather finishes, paper coatings, and inks. More recently,

protein materials, such as the milk protein casein and com protein zein, have been used as edible

coatings for extruded meats as well as nuts and confectionery items, respectively [112]. A list of

proteins commonly used in coatings and films, along with thlir status according to the U.S. FDA

21 CFR [95], is given in Table 21.3. Filmforming proteins of plant origin include com zein,

wheat gluten, soy -protein, peanut protein, and cottonseed protein, of which all but the latter are

considered GRAS [95]. Keratin, collagen, gelatin, casein, and milk whey proteins are film

formers derived from animal sources, of which casein and whey proteins are GRAS [95].

Adjustment of protein film pH can alter film formation and permeability properties, as was

demonstrated for soy protein, wheat gluten [113,114], and casein [70]. Most protein films are

hydrophilic and, therefore, do not present good barriers to moisture. However, dry protein films

such as zein, wheat gluten, and soy present relatively low permeabilities to O2 [113]. Milk

protein products include casein (80% of total milk protein) and whey (20% of total milk protein),

and combinations of both [65,115]. They can result in films of different properties depending on

the commercial source and method of extraction [70,115].

21.3.3.1.1 Casein

Caseins are soluble in aqueous solutions and form flexible colorless films. The addition of lipid

compounds and adjustment of pH reduced the water vapor permeability of casein films [116],

while the addition of whole milk, sodium caseinate, and nonfat dry milk or whey into

polysaccharide films.

TABLE 21.3

Commonly Used Protein Materials in Coatings

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Protein Materials

Casein/sodium caseinate

collagen

Cottonseed (modified

products)

Gelatin

Fish protein concentrate

Fish protein isolate keratin

Peptones

Soy protein isolate

Wheat gluten

Whey

Zein

Classification

GRAS,GMP

Food additive

Microcapsules for flavorings

(succinylated gelatin)

Food supplement

Food supplement

GRAS, nutrient supplements

Migrating to food from paper

products

GRAS, stabilizer, thickener,

surface

finishing agent

GRAS,GMP

GRAS, surface finishing agent

u.s. FDA 21 CFR #

182.90, 182.1748

172.894

172.230

172.385

172.340

184.1553

182.90

184.1322

184.1979

184.1984

Note: GRAS = generally recognized as safe by the U.S. Food and Drug Administration; GMP =

good manufacturing practices.

decreased the water vapor permeability of these hydrophilic films [117]. Lactic acid-treated

casein films retained more sorbic acid preservative, improving the microbial stability of

dehydrated apricot and papaya in intermediate-moisture food test systems [118].

21.3.3.1.2 Whey

Whey proteins produce films similar to those produced from caseinates. Heating is required to

form intermolecular disulfide bonds, which produces films that are water insoluble and brittle,

requiring plasticizers [112,115].

21.3.3.2 Col/agen and Gelatin

Collagen is the major component of skin, tendon, and connective tissues in animals [112]. This

material is partially digested with acid or enzymes to produce edible collagen casings. Collagen

casing for meat products was one of the first examples of edible film application in modem

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times. Gelatin is formed from the partial hydrolysis of collagen [112] and is also allowed as a

component of microcapsules for flavorings and for soft capsules in the pharmaceutical industry

[95]. It is soluble in aqueous solutions, forming a flexible, clear, oxygen-permeable film.

21.3.3.3 Wheat Gluten

The gluten complex is a combination of gliadin and glutenin polypeptides with some lipid and

carbohydrate components [112,119]. It is soluble in aqueous alcohol, but alkaline or acidic

conditions are required for the formation of homogeneous film-forming solutions [113]. This

material also requires plasticizers to increase flexibility, for the films are excessively brittle [112,

119]. These films have high water permeability but are good barriers to 02 and CO2 [119].

21.3.3.4 Corn Zein

Zein is a prolamine derived from com gluten and is insoluble in water except at very low or high

pH. This is due to its high content of nonpolar amino acids. It is soluble in aqueous alcohol and

dries to a glossy grease-resistant surface. The film is, however, brittle and plasticizers are

required to add plasticity [112,119]. It has been used as a substitute for shellac because of its

high gloss appearance, faster drying rate, and increased stability during storage [119].

21.3.3.5 Soy Protein

Soy protein is available as concentrate (70% protein) or isolate (90% protein). Film formation is

enhanced by heating, which partially denatures the protein, allowing formation of disulfide

bonds. This was shown to lower water vapor permeability. Enzymatic digestion can increase

cross-linking [120]. The pH must be adjusted away from the isoelectric point of the soy protein

(~4.6) to form films. In Asia, films are formed from heated soy milk and are used for wrapping

food products [112].

Peanut Protein

Peanut protein films can be formed by two methods. The first is surface film formation, using

protein/lipid solutions derived from roasted peanut, partially defatted peanut flour, and protein

concentrate. This produces films with rough surface texture and poor mechanical properties.

Films can also be produced by deposition method at pH 9.0 from peanut protein concentrate.

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This method showed promise for development as an edible film [121]. Zein, casein, and soy

proteins have been used on confectionery, fruit, and vegetable products as well as on eggs. Self-

supporting sheets of edible proteins have been developed that dissolve in water for

microencapsulation of flavorings. ~asein, soy [123], peanut, com protein, and collagen have been

used to form freestanding films or sheets for wrapping of foodstuffs [112]. Films made of

proteins can add a nutritional component to coated foods especially if formulated to include diet-

limiting amino acids [112,119]. Allergies to food proteins can be a concern when developing

coatings and films from these materials. Gluten intolerance and allergic reactions to milk

proteins (casein and whey) and associated lactose are common and may require labeling.