Ultrasound heat treatment effects on structure and acid-induced cold set gel properties of soybean protein isolate

Soybean protein isolate (SPI) was treated using ultrasound at 400 W for 5 and 15 min at 80 °C. The effects of ultrasound heat treatment on the structure of SPI and properties of glucono-δ-lactone induced SPI cold set gels (GSCG) were investigated. The results showed that ultrasound heat treatment of SPI reduced α-helixes and β-sheets and increased random coils, leading to a less compact tertiary conformation. The increase in surface hydrophobicity and free sulfhydryls changed the particle size of SPI and its gelation properties. However, the formation of GSCG mainly involved hydrophobic interactions rather than disulfide bonds due to the limitation of sulfhydryl group oxidation during acidification. Ultrasound heat treatment of SPI produced GSCG faster and increased the storage/elastic modulus (G′), gel strength and water holding capacity of GSCG, resulting in a more compact and homogeneous microstructure with smaller pores. The 5 min ultrasound heat treated GSCG had higher G′ and gel strength than 15 min, suggesting the synergistic effect of ultrasound and heat treatment. The results suggested that the combination of ultrasound and heat treatment might be an effective method for modifying the structure of SPI and improving the properties of GSCG.