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Edible Coatings

Figure 1:
Emulsion based (top) and bilayer coating

An edible coating is a thin layer that is deposited on the surface of a food and is co-consumed. This layer is not self-supporting. Traditionally it is used to improve handling properties (M&Ms melt in your mouth, not in your hand) or to prevent moisture loss (wax coatings on fruits and vegetables). It can also be used as an adhesive to seasonings on snack foods (e.g. salt on dry roasted peanuts), as a glaze on baked goods (instead of egg-based coatings, avoiding microbial problems associated with raw eggs) and to increase shelf life and reduce the need of packaging material. The shelf life is extended, because the film acts as a barrier to moisture and/or oil and/or vapor transmission.[22] Edible coatings make good oxygen and lipid barriers at low to intermediate RH, because the polymers can effectively make hydrogen bonds.[7] An edible coating has to have good eating properties: acceptable color, odor, taste, flavor and texture or be undetectable. An other requirement is that it must adhere to the food, but (in most cases) does not stick to packaging material.[22] Edible coatings can be divided in two groups: biopolymers (proteins and polysaccharides) and lipids. Examples of biopolymers are gluten, milkproteins, gelatin, starch, pectinates and cellulose-ethers. Biopolymers are usually hydrophilic and are, therefore, good barriers against hydrophobic compounds like lipids, oxygen and certain flavors.[12] Lipids, like fats and waxes are hydrophobic and form good barriers against water and water soluble compounds.[12]

Often a barrier against hydrophilic as well as hydrophobic compounds is wanted, for example a barrier against water and oxygen. In that case lipids and biopolymers can be combined. Coatings from a combination of hydrophobic and hydrophilic substances basically can have two forms: A bilayer coating or an emulsion based coating. (figure 1) [12]

Most research on edible coatings has been done with cellulose ethers, starch, hydroxypropylated starch, corn zein, wheat gluten, soy protein and milk proteins.[7]

source: Wageningen University, Department of Agrotechnology and Food Sciences. Processing of Agricultural Raw Materials for Non-Food Products. P050-217.  http://www.ftns.wau.nl/agridata/EdibCoat.htm 27oct01

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