Deleterious changes in the quality of food products can occur with inadvertent alteration of the water content of the food product. The driving force for water transfer in food products is primarily water partial pressure gradients existing between the food product and the environment or between components of a multicomponent food product. However, even in the absence of water partial pressure gradients, water potentially can migrate via liquid diffusion driven by such forces as water concentration gradients or capillary forces. Hereinafter, "vapor pressure" shall be understood as "partial pressure of water," unless expressly indicated otherwise. The partial pressures involved can be expressed in terms of water activity, (a.sub.w), which equals the partial pressure of water exerted by the food material divided by the partial pressure of pure water at the same temperature. Prevention of water transfer between the food product and the environment can be accomplished by using a water vapor impermeable packaging material, either edible or inedible. Prevention of water transfer among individual components of a multicomponent food product is more difficult.
Edible food coatings have been used to retard water transfer within foods. However, the ability of such coatings to maintain vapor pressure gradients among food components for extended periods has not been proven. Those skilled in the art are generally cognizant of the use of carbohydrate, protein, and lipid coatings to retard water transfer in foods. Lipids include hydrophobic compounds such as fats, oils, and waxes and are especially effective in retarding water transfer within foods. However, when lipids such as waxes are applied to food products, the coatings are not continuous. Instead, they contain pinholes and cracks that provide routes for water transfer. Carbohydrate compounds are capable of forming structurally continuous coatings, but these compounds are generally hydrophilic and hence are less effective in retarding water transfer in foods. Therefore, in the past, lipids have been used in conjunction with carbohydrates in order to form a continuous coating to prevent water transfer in foods.
In U.S. Pat. No. 2,909,435 to Watters, et al., a double layer coating was applied sequentially on foods such as raisins. The first layer consisted of a polysaccharide, upon which was applied a second layer of a melted wax composition. The layers were applied to the food by brushing, spraying, or dipping, and they were dried after application. In U.S. Pat. No. 3,323,922 to Durst, an aqueous solution was prepared from starch or a carboxymethyl cellulose and a plasticizer, and a lipid was suspended in the solution. The coating was then applied to a food product by brushing, dipping, or spraying, and the coating was dried after application to the food product.
U.S. Pat. Nos. 3,471,303 and 3,471,304 to Hamdy, et al., disclosed two types of coating compositions having as one constituent a cellulose ether. The cellulose ether was plasticized by various fatty acid derivatives such as (1) an ester of a linear polysaccharide having alpha-glucoside linkages and a fatty acid having between 8 to 26 carbon atoms, or (2) a glyceride containing a fatty acid having between 6 to 20 carbon atoms, together with a fatty acid metal salt wherein the fatty acid contains between 16 and 22 carbon atoms. The coating composition in the Hamdy patents could be extruded as a non-layered, molten sheet over the food to be coated, and then dried.
U.S. Pat. No. 3,997,674 to Ukai, et al., disclosed an aqueous solution used to coat foods such as fresh fruits. The coating solution contained a water soluble high polymer such as methylcellulose and also hydrophobic substances such as waxes or oils. In Ukai, the food product was coated and then dried.
Drying a food coating after application on a food has several disadvantages. It is difficult to control the thickness of such coatings. Furthermore, drying coated foods adds time and hence cost to the manufacture of such foods. Therefore, it would be advantageous to provide off-the-shelf, pre-formed films that could effectively retard or prevent water transfer between components having different a.sub.w values or different moisture contents in a multicomponent food product.