Physicochemical and functional properties of soy protein isolate as a function of water activity and storage

The effect of storage temperature on the physicochemical characteristics, solubility and gelling properties of soy protein isolate (SPI) with different water activities (a(w)) was investigated. SPI with a(w) of 0.19 (SPI-0.19) was placed in environments with relative humidity (RH) of 33% and 74%. After reaching equilibrium, in 20 days, the samples were named SPI-0.33 and SPI-0.74. For SPI-0.74, modifications in the protein characteristics started during the equilibrium period, with a decrease in the solubility and alterations in both the electrophoretic profile of the soluble proteins and in the gelling characteristics. During the 180-day storage period, SPI-0.19 and SPI-0.33 showed similar behaviours: decrease in protein solubility and alteration in hardness, cohesiveness and microstructure of the gels. These alterations were more pronounced during storage at 45 degrees C than at 25 degrees C, and in SPI-0.33 more than in SPI-0.19. Results suggest that storage conditions - temperature and RH - affect the functional properties of the proteins and the use of the isolates as a functional ingredient. (c) 2005 Elsevier Ltd. All rights reserved.