Solubility of Isolated Soy Protein in Ionic Environments and an Approach to Improve Its Profile

Abstract

The solubility characteristics of isolated soy protein (ISP) in high ionic environments were measured in accordance with an NSI (nitrogen solubility index) method in order to study the effect of the thermal process in its commercial production. When the acid precipitated protein (APP) dispersions were heated to above 60°C and subsequently spray dried, the solubility of the resultant ISP in 0.5 M (molar) NaCl was lowered with increasing temperature, ranging from 91% at 60°C to 43% at 90°C, while the solubility in H₂O remained at almost the 100% level regardless of heating condition. Solubility changes in 90°C-heated ISP in various 0.5m (molal) salt solutions indicated that salting-out of this protein preparation can be represented as a function of molal surface tension increment of the salt used ; the solubility of ISP in ionic environments can be determined by its hydrophobicity, which seems to increase with a rise in the heat treatment temperature of APP. Therefore, there was a significant difference in salting-out behavior depending upon the order of addition of ingredients to the system ; when salt was added to the slurry after the ISP was thoroughly rehydrated, no salting-out was observed in 0.5m ionic environments. Ultrasonication of 90C-heated APP prior to spray drying was tested to examine the possibility of improving the solubility profiles of resultant ISP, with the result that a remarkable increase of solubility in 0.5 M NaCl was attained. The effect of such ultrasonication was probably due to its ability to increase the hydrophilic nature of proteins.