Linear and non-linear rheology of heat-set soy protein gels: Effects of selective proteolysis of β-conglycinin and glycinin

Abstract

Soy protein is widely used in the food industry as a gelling agent in many food products. Selectively hydrolyzing one of the two main components (β-conglycinin and glycinin) of soy protein is expected to modify its gelling ability in a novel way. To explore these modifications, selective proteolysis was applied on native soy protein isolate (SPI). Degraded β-conglycinin hydrolysate (DβH) and degraded glycinin hydrolysate (DGH) were obtained with a similar degree of hydrolysis (DH), as verified by SDS-PAGE and pH-stat. Heat-set gels were formed by SPI, hydrolysates, and dialyzed hydrolysates at different protein concentrations (6%, 10%, 14%). Rheological properties of these gels in the linear regime were determined by small amplitude oscillatory shear (SAOS) tests, while their non-linear rheology was studied by large amplitude oscillatory shear (LAOS) tests. Scaling theory, CLSM, and SEM images were used to link the rheological behavior to differences in the gel microstructure.Our results showed that hydrolysates had a shorter gelling time and lower gelling temperature, but the non-network peptides made hydrolysate gels less elastic and resulted in a lower critical linear strain than SPI gels. At low concentration and DH, DβH not only had a shorter gelling time but also formed stiffer gels than SPI, which could be due to increased hydrophobic interactions and disulphide bonds, as well as the more homogeneous gel network. On the other hand, DGH formed weaker gels than SPI at every concentration, and their gel structure was coarse, consisting of small and branched flocs. With increased strain amplitude, all gels showed a transition from predominantly elastic to plastic behavior. DβH displayed intercycle strain softening, while DGH and SPI displayed a weak and strong strain overshoot, respectively. In the medium strain range, SPI showed stronger intracycle shear stiffening and DβH gels showed stronger intracycle shear thinning. At the large strain regime, SPI and DβH gels showed abrupt yielding behavior while DGH gels yielded more gradually. Overall, the “rheological fingerprint” of soy protein heat-set gels was established, and these rheological properties can be tailored by selectively hydrolyzing different components of soy protein, and varying the DH.