Structure and acid-induced gelation properties of soy protein isolate–maltodextrin glycation conjugates with ultrasonic pretreatment

Glycated conjugates were prepared by heating aqueous dispersions of soy protein isolate (SPI) and maltodextrin (MD) after ultrasonic treatment (138 W/cm2, 20 kHz) for 5, 15, and 25 min. The effect of the ultrasonic pretreatment on the structure, physicochemical characteristics, and acid-induced gelation properties of the resulting SPI–MD conjugates was investigated. The analyses of free amino groups, degree of graft, intermediates, and browning intensity confirmed the formation of SPI–MD conjugates. The ultrasonic treatment promoted the glycation reaction, which led to the increase in the β-type ordered secondary structure and less compact tertiary conformation of conjugates. The ultrasonic treatment for 5 min increased the isoelectric point, surface hydrophobicity (H0), free sulfhydryl (-SH) groups, and particle size of the conjugates. These changes in the structural and physicochemical properties of the conjugates affected their acid-induced gelation. Glucono-δ-lactone was used to form acid gels at 25 °C. Although the glycation with MD deteriorated the acid-induced gelation properties of SPI, the 5 min ultrasonic conjugates formed gels faster than the untreated conjugate, and their gels exhibited higher storage modulus (G′), gel strength, and water holding capacity (WHC), as well as denser microstructure, than the untreated conjugate gels. However, prolonged ultrasonic treatment (15 or 25 min) weakened the gelation properties of the conjugates by promoting the glycation reaction and reducing the H0. Thus, an optimum ultrasonic pretreatment can be used to modify the structure of soy protein glycation conjugates to control their gelation properties.