Mechanism of microwave-accelerated soy protein isolate–saccharide graft reactions

To investigate the mechanism of microwave-accelerated soy protein isolate (SPI)–saccharide graft reactions, we examined effects of microwave heating (MH) and water-bath heating (WH) on protein structures and reaction properties. Under high-power microwave irradiation, protein disulfide bonds were broken to cause an increase first and a decrease later in free sulfhydryl contents over time, thus inducing subunit disaggregation. And the solvent (water) exposure of hydrophobic core residues was enhanced in MH-treated SPI, which led to protein disaggregation and unfolding and much smaller aggregates in the protein solution. Calculations showed that graft reactions by MH had much lower activation energies than those by WH. Moreover, microwave can reduce the occurrence of caramelization and thus improve the graft reaction selectivity. It is seen that the microwave radiation power strongly influenced the reaction kinetics. Consequently, the results suggest that improvement of microwave-assisted SPI–saccharide graft reactions is mainly due to a thermal microwave effect and therefore the reaction mechanism could be reasonably inferred.

Research highlights
► Microwave can break disulfide bonds, thus inducing subunit disaggregation.
► Microwave can cause protein unfolding and much smaller aggregates in the solution.
► Microwave-assisted graft reactions have much lower activation energies.
► Microwave can improve the graft reaction selectivity.
► Improvement of reactions is mainly due to a thermal microwave effect.