Lipidomics reveals the molecular mechanisms underlying the changes in lipid profiles and lipid oxidation in rape bee pollen dried by different methods

Drying is essential for preserving fresh bee pollen. However, the effects of different drying techniques on lipid quality are unknown. This study aimed to compare the effects of four drying methods (freeze-drying (FD), infrared drying (IRD), hot-air drying (HAD), and pulsed vacuum drying (PVD)) on the drying kinetics, lipid oxidation, lipid profiles, and lipid metabolic pathways of bee pollen. IRD and HAD had the fastest drying rates but the highest degree of lipid oxidation. Lipidomics analysis of the bee pollen identified 1541 lipid metabolites from 20 subclasses. Glycerophospholipids were the most abundant, followed by glycerides, glycolipids, and sphingolipids. Drying not only reduced the lipid content, but also altered the structure of the triglyceride (TG) and fatty acid (FA), which might be caused by degradation and oxidation. Principal components analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) showed that IRD and HAD had the greatest effects on lipid metabolites, whereas FD had the smallest influence. Lipid oxidation during drying was correlated with differential lipids and three main metabolic pathways, including glycerophospholipid, linoleic acid, and glycerolipid metabolic pathways, in which phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidic acid (PA), and lyso-phosphatidylcholine (LPC) were the key lipids. Our results provide comprehensive lipid profiles and potential mechanisms of lipid oxidation during bee pollen drying.