Thermal versus Microwave Inactivation Kinetics of Lipase and Lipoxygenase from Wheat Germ

The inactivation kinetics of enzymes is an important way to reveal the mechanism of enzyme inactivation by microwaves. In this paper, lipase (LA) and lipoxygenase (LOX) were treated by both microwave irradiation and conventional heating. A temperature-controlled water bath was used for conventional heating, and a microwave monomode reactor with a simultaneous cooling and precise temperature control system was used for microwave irradiation. The results showed that the effects of both microwave irradiation and conventional heating on enzyme inactivation were enhanced as system temperature increased; LOX was more sensitive to heat than LA. Therefore, compared with conventional heating, microwave irradiation shows no significant differences in inactivating LA or LOX in aqueous solution, and the thermal effect of microwave irradiation is the main cause of enzyme inactivation. Practical ApplicationsWheat germ has a short shelf life because of the high activity of endogenous enzymes. Two relevant endogenous enzymes are involved in the degradation of nutrients, lipase (LA) and lipoxygenase (LOX); therefore, the inactivation of LA and LOX is an essential prerequisite for long-term storage of wheat germ. Microwave irradiation treatment (MIT) has long been applied to the inactivation of enzymes and has several advantages such as having a minimal effect on food product quality and providing rapid and uniform heating. However, the mechanism of enzyme inactivation by microwaves is still controversial. In this study, wheat germ endogenous enzymes were treated by MIT and conventional heat treatment. Based on residual enzyme activities and first-order rate of inactivation kinetics, the mechanism of microwave inactivation was demonstrated, offering a theoretical basis for the future application of this technique in grain stabilization.