*3.1. Enzyme Treatment*

Enzyme treatment has been an effective method to modify the functional properties of proteins due to the advantages of high specificity and tunable properties of the hydrolyzed peptides. Enzymatic hydrolysis could reduce the molecular weight and expose the groups (hydrophobic or hydrophilic) buried in the globular structure of the soy protein, which might elevate the emulsifying properties. The tunable functional properties make the hydrolyzed soy proteins important ingredients in the food industry [10]. In a recent report, three different enzymes were reported namely flavorzyme, alcalase, and protamex to hydrolyze SPI to obtain hydrolysate with degrees of hydrolysis of 3%, 7%, and 11% [12]. Flavorzyme was found to induce soy protein nanoparticles (SPNPs) at all DH, whereas protamex showed a limited effect on the formation of SPNPs. The SPNPs attached more rapidly at the oil–water interface as compared to native SPI, indicating the good emulsifying potential of the emulsifier. The peptides with decreased molecules show the advantage of higher mobility when absorbing at the interface, which might also be a disadvantage in maintaining the emulsion stability. Therefore, even though most of the previous studies indicated that that enzymatic hydrolysis improved some of the functional properties of soy proteins, the degree of hydrolysis is the critical parameter for enzyme modification [13]. Enzyme treatment could not only hydrolyze the protein, but also crosslink protein to modify the emulsifying properties. Zang et al. [14] hydrolyzed soy protein by papain with a hydrolysis degree of 6% and then reacted with transglutaminase (TGase) to partial crosslink soy protein hydrolysate. Compared with the raw SPI, transglutaminase treated SPI and SPI hydrolysate, emulsion stabilized by transglutaminase treated SPI hydrolysate exhibited the lowest particle size, creaming index, flocculation degree, coalescence degree, and highest freeze–thaw stability.
