Mechanical properties and sorption characteristics of stearic acid-cysteine-soy protein isolate blend films

The soy protein isolate (SPI) blend films which have good mechanical properties and better barrier to moisture were prepared by using stearic acid as plasticizers and cysteine as cross-linking reagent. The tensile strength (TS) and percentage elongation (E%) of SPI blend films were determined after conditioning blend film specimens at 25°C and 50% relative humidity (RH) for 2 days by using Texture Analyzer. Moisture sorption characteristics of the SPI blend films at 25°C were studied for water activities ranging from 0.10-0.90. The moisture sorption rate and sorption isotherm data were mathematically fitted to Peleg's Equation and GAB (Guggenheim-Anderson-de Boer) model, respectively. The results indicated the TS, E% and moisture sorption rate of SPI blend films changed remarkably with the ratio of stearic acid to cysteine in blend films. The optimum ratio of stearic acid to cysteine was 40:60 (w/w) (10 g blend additive/L protein solution) in SPI blend films. The experimental results showed the TS value of the SPI blend films with ratio of 40:60 was 2 times higher than that of the original SPI films. In addition, the blend films with ratio of 40:60 had the optimum sorption rate. The moisture sorption data of the SPI blend films were well fitted to GAB model. GAB model had the highest R2(0.99).