Combined microwave-electron beam irradiation preserves lipid stability in germ-remained milled rice: dual mechanisms of enzyme inactivation and metabolite-mediated inhibition

Germ-remained milled rice (GR) is susceptible to lipid oxidation. This study examined the effects of microwave (MW), electron beam irradiation (EBI), and their combined pretreatment (ME) on the lipid stability and volatile profiles of GR during storage. ME treatment demonstrated superior efficacy in inhibiting lipid oxidation, reducing lipase (LPS) and lipoxygenase (LOX) activities by 69.10% and 14.23%, respectively, compared to 46.44% and 8.18% for MW, and 57.90% and 10.79% for EBI alone. After 28 days, ME-treated GR retained superior lipid integrity, exhibiting only 12.82% crude fat reduction, 16.89% less saturated fatty acid accumulation, and the highest unsaturated/saturated fatty acid ratio (3.66). Kinetic modeling and correlation analysis identified (Z)-3-methyl-2-undecene and 4-methyldodecane as signature metabolites strongly negatively correlated with LOX activity. Molecular docking revealed moderate binding of these hydrocarbons to the LOX catalytic domain (ΔG: -5.3 to -4.1 kcal/mol). Crucially, in vitro enzymatic assays confirmed that 4-methyldodecane inhibits LOX in a dose-dependent manner (IC 50 = 70.81 μmol/L), providing direct validation of metabolite-mediated feedback suppression. ME inactivates lipolytic enzymes through thermal denaturation by MW and radical-mediated disruption by EBI. Concurrently, 4-methyldodecane could act as an endogenous LOX inhibitor via a feedback mechanism. This dual inhibitory strategy improves storage stability of lipid-rich grains.