Effects of Heat Treatment and Drying Methods during Soy Protein Isolate Preparation on Their Thermal Aggregation Behavior and Gelation Properties
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Abstract:
Soy proteins, isolated from low-temperature de-fatted soy flakes via alkaline dissolution and acidic precipitation, were subjected to dispersion, heat treatment, and spray drying or freeze drying to prepare soy protein isolates (SPIs). The thermal aggregation behavior of the obtained products, as well as the mechanical properties of their gels, were observed to investigate the effects of heat treatment and different drying methods on the gelation properties of SPIs. The results indicate that heat treatment caused the SPIs to exist predominantly as aggregates. The solubility (at pH value 7.0) of the spray dried and freeze dried SPIs was reduced from 79.9% and 69.8% to 35.2% and 42.0%, respectively. Unlike SPIs prepared with heat treatment, the differential scanning calorimetry results of SPIs prepared without heat treatment showed heat absorption peaks at 78 ℃ and 98 ℃. After heating the dispersions again, the volume of the protein fraction in the dispersions decreased from 30% to 20%, showing a trend of further aggregation and the formation of ordered structures. The fracture stresses of gels prepared from spray dried-SPI and freeze dried-SPI sample dispersions with mass fractions of 16% were 10.80 kPa and 12.50 kPa, respectively, which are significantly higher than those of the heated spray dried-SPI (3.69 kPa) and heated freeze dried-SPI (4.36 kPa) gels. Nevertheless, the heated spray dried-SPI and heated freeze dried-SPI gels were harder than their unheated counterparts. In contrast, the drying methods did not have a significant impact on the solubility, thermal aggregation behavior, or gelation properties of the resultant SPI products. Therefore, the dispersion heat treatment used during SPI preparation is the key step that significantly affects the functional properties of SPI, including solubility, thermal aggregation behavior, and gelation properties. By seeking alternative solutions for heating during preparation, the quality and functional properties of SPIs could be improved. They could potentially be used as a functional ingredient in food processing, especially for plant-based foods.
