Among the film forming proteins, soy protein has been studied for its excellent film forming abilities and excellent barrier properties against oxygen and oil movement in low relative humidities (Gennadios, McHugh, Weller, & Krochta, 1994). Potential application, mechanical properties, and barrier properties of soy protein films have been comprehensively reported (Stuchell & Krochta, 1994; Kunte, Gennadios, Cuppett, Hanna, & Weller, 1997; Were, Hettiarachchy, & Coleman, 1999; Rhim, Gennadios, Handa, Weller, & Hanna, 2000).
Soy protein isolate (SPI) is a complex mixture of proteins with widely different molecular properties. The major soybean proteins have molecular weights ranging from 200 to 600 kDa. Most soy proteins (~90%) are globulins, which can be fractionated into 2S, 7S, 11S and 15S according to their sedimentation coefficients. The 7S and11S fractions, the main fractions making up about 37% and31% of the total extractable protein, have the capability of polymerization (Wolf, 1972).
The formation of films from soy proteins has been described as a two-step process involving the heat denaturation of the proteins followed by surface dehydration. Upon drying, the unfolded proteins link through intermolecular interactions, such as disulfide bonds and hydrophobic interactions, leading to the formation of a network (Gennadios & Weller, 1991). In addition, Subirade, Kelly, Gueguen, and Pezolet (1998) reportedthat intermolecular hydrogen-bonded β-sheet structure is essential for the network formation in soy protein film.