Structural characteristics and acid-induced emulsion gel properties of heated soy protein isolate–soy oligosaccharide glycation conjugates

Glycation conjugates were prepared by heating the mixed dispersions of heat-denatured soy protein isolate (HSPI) and soy oligosaccharide (SOS) at 90 °C for 30, 60, 90 and 120 min. The structural characteristics and acid-induced emulsion gel properties of HSPI–SOS conjugates were investigated. The results of degree of graft, pH, intermediates and browning intensity showed that the degree of glycosylation gradually increased with the extension of reaction time, but it decreased at 120 min of reaction. The changes of the absorption peaks in infrared spectra confirmed that HSPI was covalently linked with SOS to form HSPI–SOS conjugates through Maillard reaction. The glycosylation of HSPI with SOS resulted in the transformation form α-type to β-type secondary structure, the reduction in free sulfhydryl groups, and the enhancement of surface hydrophobicity. These structural changes of HSPI–SOS conjugates could influence their glucono-δ-lactone-induced emulsion gel properties. Although the glycosylation decreased the storage modulus of HSPI emulsion gels, the 60 min glycated conjugates accelerated the gelation rate of the emulsions. The conjugate emulsion gels had higher gel hardness and water holding capacity, which exhibited more compact microstructure with smaller and more uniform emulsified oil droplets. Moreover, the glycosylation of HSPI with SOS for 60 min improved the thermal stability of conjugate emulsion gels. Therefore, the structure of HSPI–SOS conjugates can be modified by controlling the degree of glycosylation to improve the physicochemical properties and stability of the emulsion gels.