Lipid Analysis of Walnut Kernel Using Ultra-high Performance Liquid Chromatography Coupled to Orbitrap High Resolution Mass Spectrometry

An ultra-high performance liquid chromatography coupled to orbitrap high resolution mass spectrometry (UPLC-Orbitrap HRMS) method was developed to characterize the lipid composition of walnut kernel. According to the MS₁ and MS₂ data acquired in the positive and negative ion modes by electrospray ionization (ESI), a total of 525 lipids belonging to four classes were detected in walnut kernel, including 250 glycerolipids (GL), 221 glycerophospholipids (GP), 36 sphingolipids (SP) and 18 saccharolipids (SL), accounting for 87%, 12.97%, 0.02% and 0.01% of the total lipids, respectively. Diacylglycerols (DG) and triacylglycerols (TG) were the dominant glyceride components, accounting for 45.23% and 41.77% of the total lipids, respectively. TG (18:2/18:2/18:3) was the major TG and DG (18:2/18:2) was the major DG. Essential fatty acids such as linoleic acid and linolenic acid were the major fatty acids in DG and TG. There were a variety of phospholipids in walnut kernel, including phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidyl serine (PS), phosphatidyl choline (PC), phosphatidyl ethanolamine (PE), phosphatidyl inositol (PI), phosphatidylinositol di phosphate (PIP), lyso-posphatidyl choline (LPC), lyso-posphatidyl ethanolamine (LPE), lyso-phosphatidylglycerol (LPG), lyso-phosphatidyl inositol (LPI) and cardiolipin (CL). Among them, PA was the most predominant, accounting for 43.88%. The lipid profile constructed in this study can provide a theoretical basis for the functional development of walnut kernel and further research on its nutritional quality.