Highly pure soy protein yields strong films.

The film-forming ability of soy proteins has been used in the Far East to make soy protein-lipid films called yuba films. The process used to make yuba films consists of boiling soy milk in shallow pans, collecting the films formed as a result of surface dehydration and hanging the films so that they dry in air. Others have prepared soy films by spreading soy protein isolate solutions on Teflon-coated baking pans and then baking the pans at 100 C for an hour.

But depositing and drying soy protein solutions is a more promising way to make commercially useful films since this approach allows for greater consistency and control during film formation. Others have produced homogeneous free-standing edible films from commercial soy protein using glycerol as a plasticizer.

Scientists at the University of Nebraska (Department of Food Science and Technology, 352 Food Industry Complex, Lincoln, NE 68583) compared the tensile strength, elongation at break, water vapor permeability, solubility in water and color properties of films formed from commercial soy protein isolate with those of films formed from laboratory-prepared crude 7S and 11S fractions, and soy protein isolate.

The investigators found that soy protein isolate of increased purity--about 96% to 98%--can yield stronger films than commercially available soy protein isolate. Also, using 11S soy globulin fractions alone yields stronger films than commercial soy protein isolate. But the improvement in film strength would come at the additional cost of further purifying soy protein isolate--to nearly 100% purity--or separating the 11S protein fraction.

Although soy protein films are poor water vapor barriers, they are effective oxygen barriers. The good oxygen barrier capability of the films could be utilized in the manufacture of multilayer packaging systems. In these packages, soy protein films could function as the oxygen barrier layer. In another possible use, soy protein coatings on precooked meats might control lipid oxidation and limit loss of surface moisture.

Incorporating antioxidants and flavoring agents into the coatings could improve the overall quality characteristics of different products. The high hydrophilicity and substantial solubility of soy protein films in water might make the materials appropriate for making hot water-soluble pouches, similar to the cellulose ether-based water-soluble pouches on the market.

Further information. Susan Cuppett; phone: [phone]; fax: [phone]; email: [e-mail].