Structural and Gel Textural Properties of Soy Protein Isolate When Subjected to Extreme Acid pH-Shifting and Mild Heating Processes

Changes in the structural and gel textural properties were investigated in soy protein isolate (SPI) that was subjected to extreme acid pH shifting and mild heating processes. The SPI was incubated up to 5 h in pH 1.5, solutions at room temperature or in a heated water bath (50 or 60 degrees C) to lead to protein structural unfolding, followed by refolding at pH 7:0 for 1 h. The combination of pH-shifting and heating treatments resulted in drastic increases in the SPI gel penetration force (p < 005). These treatments also significantly enforced the conversion of sulphydryl groups into disulfides, increased the particle size. and hydrophobicity values, reduced the protein solubility (p < 0.05), and strengthened the disulfide-mediated aggregation of SPI. The intrinsic fluorescence spectroscopy results indicated structural unravelling when protein was subjected to acidic pH-shifting in combination with heating processes. The slight loss of secondary structure was observed by circular dichroism. These results suggested that pH-shifting combined with heating treatments provide great potential for the production of functionality-improved SPI, with the improved gelling property highly related to changes in the protein structure and hydrophobic aggregation.