Soy protein isolate (SPI) was subjected to enzymatic hydrolysis with protease and phytase, and the solubilities of the modified SPIs (MSPIs) were characterized and compared with those of a commercial modified protein (SUPRO 670 IP from the Solae Company, St. Louis, MO). The solubilities of MSPIs showed a typical pH-dependent curve, shifting upward with an increase of hydrolysis time and enzyme dose. In particular, the solubility at pH 4.5 of the MSPI with degree of hydrolysis of 9.8% increased 9.7-fold more than SPI. Furthermore, the combined enzymatic treatment (protease and phytase) led to a significantly increased solubility of SPI at pH 4.5, which improved by 11.5-fold and 1.8-fold more than SPI and SUPRO 670 IP, respectively. In conclusion, the use of hydrolytic enzymes to synergistically improve solubility in the acidic range of pH may provide a positive impact on the MSPIs as a multifunctional food ingredient.

Practical Applications

Even though soy protein isolates (SPIs) are used in food industry because of their beneficial health effects as well as specific functional properties, the applications of SPIs to processed foods under acidic conditions are very limited. In this study, SPIs were modified by each or combined enzymatic treatments and their solubilities were compared in a wide range of pH. The solubility of the modified SPI (MSPI) was significantly improved compared with the commercial product. Therefore, the MSPI with enhanced solubility at acidic pH could be used in an extended range of processed foods.