Effect of ionic strength and pH on the thermal and rheological properties of soy protein–amylopectin blend

The effects of amylopectin (AP), moisture content, pH, and buffer molarity on the thermal properties of milled defatted soy flour (MDF) and soy protein isolate (SPI) were studied by differential scanning calorimetry (DSC), and the rheological properties of effect of AP on MDF were also investigated. Native soy protein, which is found in the MDF sample, showed higher stability at alkaline pH, as indicated by higher onset and peak temperature when compared with SPI. The effect of manufacturing isolation process on soy proteins is apparent in the differences between MDF and SPI thermal properties and SDS–PAGE profiles. The onset temperature and ΔH values of the 7S showed significant difference between MDF and SPI at 20% moisture content, 0.3 M and pH 4. The 11S of the MDF and SPI showed that the peak temperature and ΔH to be significantly different at the same buffer amount and pH mentioned earlier. The 30% moisture content generated significant differences between the MDF and SPI under all experimental conditions, i.e. pH and molarity. The increase in the moisture content to 40% produced similar changes as at 30%. The 11S of the SPI sample displayed ΔH values higher than MDF, which may indicate aggregation as a result of the manufacturing process. The amylopectin suspension had strong viscoelastic solid properties and the properties were stable during heating or cooling between 25 and 55 °C. MDF also displayed viscoelastic solid properties, but not as strong as those of AP. The viscoelastic properties of MDF suspensions were not stable and were damaged during heating and cooling processes. Blending MDF and AP exhibited similar properties as MDF alone, but they were reversible during the heating/cooling process, indicating that the networks were not damaged. The strong gel properties of AP in the blend were reduced.