Abstract: Pepsin and 2,4,6-trinitrobenzenesulphonic acid were used to test in vitro the soy protein isolate digestibility and the content of reactive lysine of film proteins respectively. The results showed that in vitro protein digestibility and content of reactive lysine of film proteins decreased with increase in pH of film forming solution and addition of cross-linking agents such as tannin, ferulic acid and hydrogen peroxide, also with increase of mechanical properties of films. However, cornstarch seemed an exclusive, which increased the mechanical properties of films but almost kept unchanged in vitro protein digestibility and the content of reactive lysine of film proteins. It can be concluded that the nutritional value of soy protein isolate decreased after it was used to prepare film. Thus, it appeared that it was more reasonable to make films prepared from soy protein isolate as biodegradable films rather than as edible films.

Key words: Soy protein isolate , Film , Nutrition