Formation of edible soybean and soybean-complex protein films by a cross-linking treatment with a new Streptomyces transglutaminase

Soybean protein isolate (SPI) was used in the investigation of the formation of edible protein films through an enzymatic cross-linking method with a purified microbial transglutaminase (MTG) produced from a new effective strain Streptomyces sp. WZFF.L-M1 preserved in our laboratory, followed by the addition of glycerol and suitable heating and drying treatments. Cheaper partially-purified skimmed soybean protein powder (SSP) and whey protein isolates (WPI) were used as the substitutes partially replacing the expensive SPI products, and purified beta-lactoglobulin was taken as the positive control of WPI. As a result, the three alternatives could also form highly efficient edible films under the optimal operation conditions. The films made with SPI alternatives, about 50 mu m thin, had homogenous network structures, without any holes by direct observation with the naked eye. The tests of the properties of these films showed that they had high water-keeping capacity and strong elasticity, that the ultimate tensile strength (TS) and the elongation at break (Eb) had been remarkably increased (TS>5 MPa, Eb>50%), and that the prevention rates against the permeability of water vapour and oxygen in the air were also upgraded more than 85 and 70%, respectively.