Optimization of the conditions for producing soy protein isolate films
The soy protein isolate (SPI) is a biopolymer highlighted as a raw material for producing films and demands the investigation of the processing conditions that make it possible to obtain a film with adequate functional properties. The objective of this work was to evaluate the effect of the pH and concentration of SPI on the mechanical properties, water vapor permeability, solubility, and color of the SPI films produced by casting using a central composite rotational design (CCRD). In general, the films presented a yellow coloration with a continuous aspect, good handling, and no apparent glycerol migration. The linear effect of pH positively influenced the tensile strength (TS) and elongation at break (ELO) of the film (p <0.05) but negatively influence the solubility (SOL) and color parameter L* for which we obtained lower values of SOL and L* at a high pH. This possibly occurred because of the denaturation of the soy proteins at an alkaline pH, far from the isoelectric point, resulting in their unfolding and solubilization which facilitated the interaction between the chains, forming a more compact structure. Considering the optimization by the desirability function, to obtain a film with high TS and ELO and low SOL, it is necessary to use 7.56 g/100 g of SPI film-forming solution and to adjust the film-producing solution to pH 10.54.