Effects of low-frequency and high-intensity ultrasonic treatment combined with curdlan gels on the thermal gelling properties and structural properties of soy protein isolate

The effects of low-frequency and high-intensity ultrasonic treatments and curdlan gels on the thermal gelling and structural properties of soy protein isolate (SPI) were investigated. Low-frequency (20 kHz), high-intensity ultrasonic treatment improved the solubility and rheological properties of the SPI and promoted gelling, especially with 400 W treatment for 10 min. The addition of thermo-reversible curdlan gel (TRC) or thermo-irreversible curdlan gel (TIRC) further improved gel strength and reduced cooking loss from SPI gels pretreated with ultrasound. Fourier transform infrared spectroscopy (FTIR) of the SPI composite gel showed that the peak at 3268 cm−1 for the O–H vibration exhibited a blue shift compared to the control SPI and a red shift compared to pure TRC or TIRC, which indicated formation of hydrogen bonding interactions between SPI and TRC or TIRC. Circular dichroism spectroscopy (CD) showed that the addition of TRC or TIRC significantly decreased the α-helix content but increased the β-sheet contents of SPI gels due to denaturation and unfolding of SPI. Scanning electron microscopy (SEM) showed that the addition of TRC or TIRC led SPI gel to form a more regular and homogeneous network. These results revealed that the ultrasonic treatment and curdlan gels had obvious synergistic effects that improved the quality of SPI gels. Moreover, compared with TIRC, TRC addition promoted expansion and interactions of SPI molecules and formed a more stable network. Therefore, ultrasonic treatment combined with curdlan addition significantly improved the thermal gelation and structural properties of SPI.