Abstract:
Objective: This study investigates the impact of microwave processing on lipid stability in brown rice derived from paddy stored under ambient conditions (approximately 20?). By comparing brown rice from one-year stored (control) and five-year stored (over-stored) paddy, the research focuses on elucidating the molecular mechanisms underlying microwave-induced reduction of fatty acid value (FAV) and related lipidomic alterations.
Materials & Methods: Brown rice samples were obtained from paddy stored for one year (control) and five years (over-stored) under ambient conditions (~20?). The brown rice was treated in a microwave oven at 420 W and 560 W for 1–5 minutes. FAV was determined via alkali titration. Untargeted lipidomic analysis was performed using liquid chromatography–mass spectrometry (LC–MS). Oxidized lipids were quantitatively profiled using ultra-performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS). Data were processed using multivariate statistical approaches, including principal component analysis (PCA), orthogonal partial least squares-discriminant analysis (OPLS-DA), and variable importance in projection (VIP) analysis. KEGG pathway enrichment was employed to identify relevant metabolic pathways.
Results & Discussion: Microwave treatment significantly reduced FAV in brown rice from both storage periods in a time- and power-dependent manner (P < 0.05). The most pronounced reduction was observed in the five-year stored sample, with FAV decreasing by 75.5% after 4 minutes of treatment at 420 W. Lipidomic profiling revealed distinct separation between treated and untreated groups, indicating substantial lipid compositional changes. A total of 33 differentially abundant lipids were identified, including 14 upregulated and 19 downregulated species. Among these, oxidized lipid profiling via UPLC-MS/MS detected 14 significantly altered oxidized lipids, of which 5 were upregulated and 9 were downregulated post-treatment. The differentially regulated oxidized lipids primarily belonged to arachidonic acid, dibomo-gamma-linolenic acid, eicosapentaenoic acid, docosahexaenoic acid, and linoleic acid classes. Key downregulated lipids included phospholipids such as PMeOH (16:0_18:2) and PMeOH (18:2_18:2), as well as sphingolipids such as Cer (t18:0/26:1(2OH)), all strongly correlated with FAV reduction (VIP > 1). Pathway analysis suggested the involvement of autophagy and glycerophospholipid metabolism in microwave-mediated lipid degradation.
Conclusion: Microwave processing effectively mitigates lipid rancidity in brown rice from over-stored paddy through targeted degradation of rancidity-associated lipids and modulation of oxidized lipid profiles, particularly phospholipids and sphingolipids. The observed lipidomic shifts provide a mechanistic explanation for the FAV reduction and support the potential of microwave technology for quality improvement of aged grains stored under ambient conditions.
Keywords: brown rice, over-stored paddy, microwave treatment, fatty acid value, lipidomics, oxidized lipids, lipid degradation.
