Lipid Deterioration Mitigation in Brown Rice Milled from Long-Term Stored Paddy by Microwave: A Lipidomic Perspective

The utilization of brown rice from long-term stored paddy is severely limited by lipid deterioration, which is primarily characterized by a high free fatty acid value (FAV). Although microwave treatment shows promise in mitigating lipid deterioration, its underlying mechanism in degraded grains remains poorly understood. This study systematically investigated the efficacy and mechanism of microwave treatment using a multi-analytical approach. Brown rice from long-term stored paddy (Longjing-46, stored for 6 years) was treated using a laboratory microwave oven (420 W or 560 W, 1-5 min). The reduction in FAV was quantified, lipid structural changes were analyzed by FT-IR spectroscopy, and lipid metabolic alterations were profiled using untargeted lipidomics. Results showed that microwave treatment significantly reduced FAV in a time- and power-dependent manner, with a maximum reduction of 76.3%. Treatment at 420 W for 4 min was identified as the optimal condition. FT-IR analysis confirmed that the treatment inhibited lipid oxidation and hydrolysis at the molecular level. Importantly, lipidomics revealed that the mechanism extended beyond simple enzyme inactivation. Microwave treatment induced a reprogramming of the lipid metabolic network, characterized by the synergistic downregulation of key lipid species and the activation of the autophagy pathway. This study provides a comprehensive, lipid-centric explanation of microwave-mediated quality improvement in long-term stored brown rice, integrating enzyme inactivation with metabolic network reprogramming. The findings offer a novel scientific basis for applying this technology to valorize degraded grain stocks, contributing to the reduction in postharvest loss.