Structure-modifying alkaline and acidic pH-shifting processes promote film formation of soy proteins

The effect of pH-shifting, a process that induces the molten globule state in proteins, on the film-forming potential of soy protein isolate (SPI) at different temperatures was investigated. Partial unfolding at pH 1.5 or 12, followed by refolding at pH 7.0, was performed to alter the protein structure. Glycerin-plasticised films were prepared from pH-treated SPI at ambient temperature (20 degrees C), or by heating at 50, 60, 70, or 80 degrees C (30 min). Tensile strength (TS). elongation at break (EAB), water vapour permeability (WVP), protein solubility (pH 3-7), and non-participating proteins of films were analysed, and the film microstructures were examined. The pH(12)-treated SPI spontaneously formed a transparent, slightly yellowish film at 20 degrees C, which had the greatest EAB, while pH(1.5)-treated and native SPIs required preheating at 50 and 70 degrees C, respectively, to form a film. Heating generally decreased solubility and WVP but increased TS. Films formed from both pH(12)- and pH(1.5)-treated SPIs were more elastic (up to 2-fold greater in EAB, P < 0.05) than the film formed from untreated SPI despite slightly reduced TS and WVP. Electrophoresis revealed disulphide bonds between A and B subunits of glycinin being a dominant force in pH(12)- and pH(1.5)-treated SPI films, while noncovalent forces were abundant in untreated SPI films. The pH(12)-treated SPI film consisted of more interactive protein strands than other SPI films, which seemed to explain its superior elastic properties. (C) 2011 Elsevier Ltd. All rights reserved.