Influence of Pre-/Postultrasound on Forming a Molten Globule-Like Conformation and Improving the Emulsifying Properties of Thermally Induced Soybean Protein Aggregates

The study compared the effects of applying ultrasound (20 kHz, 200/600W, 10 min) before or after heating (100 °C, 20 min) on the physicochemical and emulsifying properties of soy protein isolate (SPI) under the same conditions. The results showed that ultrasound generally reduced the particle size of thermally induced soluble protein aggregates and increased protein solubility as the content of surface sulfhydryl groups increased. Ultrasound also increased the surface hydrophobicity, absolute value of the ζ-potential, and the change of protein tertiary structure and facilitated the reduction or restoration of changes in the secondary structures of protein caused by heating, possibly producing the better emulsifying properties observed in the samples treated by ultrasound. Most importantly, ultrasound treatment before heating led to the soluble protein aggregates showing the most molten globule-like conformation and having a homogeneous particle size, which was significantly different from the particle size distribution profile of the samples sonicated after heat treatment. Protein subjected to this combination treatment at a high ultrasonic power displayed the best emulsifying activity and emulsion stability. The findings suggest that the ultrasonic modification of protein molecules before heating is better than attempting remediation after heat treatment.