Microwave versus hot air drying of germinated red adzuki beans: from the perspective of drying kinetics, bioactive compounds and starch structural changes

This study investigated the effects of microwave drying (MD) at 6, 8, and 10 W/g and hot air drying (HAD) at 50, 60, and 70 °C on drying kinetics, bioactive components (γ-aminobutyric acid (GABA), phenolics, and total flavonoids), and starch structure of germinated red adzuki beans (GRAB). MD reduced drying time with higher drying rates than HAD. MD increased the retention of individual phenolic compounds (such as gallic acid and quercetin), as well as total phenolic and flavonoid content, whereas HAD induced their degradation. Both drying methods lead to a reduction in GABA. MD and HAD increased the degree of starch gelatinization ( DSG ) by reducing the gelatinization enthalpy (Δ H ), and SEM and CLSM analysis collectively confirmed that MD induced a more pronounced DSG compared to HAD. FT-IR analysis showed that both MD and HAD had preserved the starch functional group, with no new chemical bonds forming. Analysis by 1 H NMR revealed that the rapid heating associated with microwave processing specifically cleaves α-1,6-glycosidic bonds in amylopectin, reducing branched-chain structures. This study may deepen the understanding of the interaction between microwaves and starch and provides a theoretical basis for developing GRAB-based food products.