Influence of transglutaminase-assisted ultrasound treatment on the structure and functional properties of soy protein isolate

In this study, the effects of transglutaminase (TG) cross-linking on physicochemical, conformational, and functional properties of soybean protein isolate (SPI) with or without ultrasound pretreatment were investigated. Electrophoresis and free amino group analysis confirmed that the SPI was cross-linked by TG treatment. The cross-linked SPI showed a visible decrease in solubility and increase in apparent viscosity compared with the untreated SPI. TG catalyzed the SPI to unfold the constitutive polypeptides, resulting in a more random secondary structure. With ultrasound pretreatment, the cross-linked SPI exhibited higher solubility (from 41.80% to 54.42%), emulsifying activity index (from 45.82 to 64.10 m(2)/g) and better foaming capacity (from 31.81% to 44.23%) than the untreated SPI, and the sulfhydryl group contents and surface hydrophobicity of the SPI were increased. Practical applications This study aimed to explore the effect of TG cross-linking with or without ultrasound pretreatment on the physicochemical structure and functional properties of commercial SPI. The results demonstrated that a combination of ultrasound and TG treatments may be a valid way for the functional modification of SPI, which would be conducive to enhance its application as a novel functional ingredient in a broad range of food systems.