Abstract: The effect of ultrasound on the freeze-thaw stability of soy protein isolate (SPI) gel induced by transglutaminase (TG) was explored. Changes in the water-holding capacity (WHC), texture, microstructure and soluble protein content of SPI gel during freeze-thaw cycles were analyzed. The structure of SPI was measured by particle size, surface hydrophobicity, endogenous fluorescence spectroscopy and Fourier transform infrared spectroscopy. The results showed that as the number of freeze-thaw cycles increased, the WHC and soluble protein content of SPI gel without ultrasound treatment decreased, while the gel hardness and elasticity increased, and a honeycomb-like microstructure appeared. These changes indicated that the quality of the gel was deteriorated. Ultrasonic treatment could increase water retention, hardness and elasticity of SPI gel, and make the gel microstructure more uniform and dense. After freeze-thaw cycles, the changes in WHC, hardness, elasticity and soluble protein content of ultrasonic-treated SPI gel were significantly smaller than those of native SPI gel, indicating that ultrasonic treatment could improve the freeze-thaw stability of the gel. After 5 freeze-thaw cycles, the WHC and soluble protein content of SPI gel ultrasonicated for 40 min decreased by 38.59% and 3.65%, respectively, and the hardness and elasticity increased by 510.23 g and 0.07 mm, respectively as compared to those without freeze-thaw treatment. Accordingly, ultrasonic treatment is an effective way to enhance the freeze-thaw stability of TG-induced SPI gel.

Key words: soy protein isolate, ultrasound, glutamine transaminase, gel, freeze-thaw stability