The effect of pH on the gelling behaviour of canola and soy protein isolates

Abstract

The present study investigates the gelation mechanisms of a canola protein isolate (CPI) as a function of a pH (3.0–9.0), and compares it to that of a commercial soy protein isolate (SPI). A rheological investigation found that CPI was non-gelling at pH3.0, and then formed a gel with increasing strength as pH was raised from pH5.0 to 9.0. In contrast, the commercial SPI ingredient was found to be non-gelling at pH9.0, but formed the strongest networks at pH5.0 near its isoelectric point (pI=4.6). Denaturation temperature as determined by differential scanning calorimetry were found to occur at ~78°C for CPI at pH5.0, then shifted to higher temperatures (~87°C) at pH7.0/9.0, whereas detection of SPI denaturation could not be obtained due to instrument sensitivity. Gelling temperatures were similar for both CPI and SPI (~82–86°C) at all pHs, with the exception of SPI at pH5.0 (~46°C). Overall CPI networks were stronger than SPI, since the latter had weaker inter- and intramolecular junction zones. Confocal laser scanning microscopy images indicated that CPI gels became denser with lower lacunarity values as pH increased from 3.0 to 9.0. Moreover, the fractal dimension of CPI gels was found to increase from ~1.5-1.6 to ~1.8 as pH increased from 5.0/7.0 to 9.0, respectively suggesting diffusion-limited cluster-cluster aggregation. Images of SPI networks were not concurrent with fractal analysis under the conditions examined. Despite CPI having excellent gelling properties that are comparable to SPI, its need for alkaline pH conditions will limit its applicability in foods.