Viscoelastic properties of soy protein isolate - pectin blends: Richer than those of a simple composite material

Concentrated soy protein isolate (SPI) - pectin blends acquire fibrous textures by shear-induced structuring while heating. The objective of this study was to determine the viscoelastic properties of concentrated SPI-pectin blends under similar conditions as during shear-induced structuring, and after cooling. A closed cavity rheometer was used to measure these properties under these conditions. At 140 °C, SPI and pectin had both a lower G* than the blend of the two and also showed a different behavior in time. Hence, the viscoelastic properties of the blend are richer than those of a simple composite material with stable physical phase properties. In addition, the G'_pectin was much lower compared with the G'_SPI and G'_SPI-pectin upon cooling, confirming that pectin formed a weak dispersed phase. The results can be explained by considering that the viscoelastic properties of the blend are influenced by thermal degradation of the pectin phase. This degradation leads to: i) release of galacturonic acid, ii) lowering of the pH, and iii) water redistribution from the SPI towards the pectin phase. The relative importance of those effects are evaluated.