Determining the lipid components of wheat germ by nuclear magnetic resonance

Wheat germ is rich in lipids with a very complex composition. The lipids in wheat germ are prone to be hydrolyzed and oxidized during processing and storage. In this study, phospholipids and neutral lipids were extracted from wheat germ with different solvents. Then the phospholipids, mono-, di-, and triacylglycerols, as well as fatty acyl group contents were analyzed by nuclear magnetic resonance (NMR) spectroscopy. The results showed that six kinds of phospholipids were detected by ³¹P NMR in wheat germ, including phosphatidylcholine (PC), phosphatidyl ethanolamine (PE), phosphatidyl inositol (PI), phosphatidylglycerol (PG), phosphatidic acid (PA), and phosphatidylserine (PS). Furthermore. There were also five kinds of lysophospholipids (lysophosphatidylcholine (LPC), lysophosphatidylglycerol (LPG), and lysophosphatidylethanolamine (LPE)) and two non-lipid phosphorus compounds (inorganic phosphate (pi) and phosphatidylcholine glycerol (GPC)). GPC was the hydrolytic product from the removal of fatty acyl groups by two-step hydrolysis of PC. PC had the highest content of phospholipid in the wheat germ. Specifically, the molar concentration of PC was 0.42 μmol/g wheat germ, the molar fraction of PC was 28.50 %, the mass concentration of PC was 0.31 mg/g wheat germ, and the mass fraction of PC was 30.20 %. The signals of PE, PC, LPC, GPC, and choline (Cho) in ¹H NMR spectra were also used to determine the phospholipids in wheat germ. Compared with ¹H NMR, ³¹P NMR spectra were better resolved and more suitable for qualitative and quantitative analysis of phospholipid components, as ³¹P NMR only determined the characteristic signal of phosphorus-containing compounds. The composition and content of free fatty acids, mono-, di-, and triacylglycerols were determined in the wheat germ by ¹H NMR. The proportion of free fatty acids was 12.35 % in the wheat germ. The highest content was triacylglyceride (TG), accounting for 77.25 %. The contents of 1,3-diacylglycerol (1,3-DG) and 1, 2-diacylglycerol (1,2-DG) were 5.80 % and 4.41 %, respectively, and the contents of 1-monoacylglycerol (1-MG) and 2-monoacylglycerol (2-MG) were 0.17% and 0.03%, respectively. Moreover, TG was gradually hydrolyzed to produce diglycerol (DG), monoglycerol (MG), free fatty acid (FA) and glycerol in the wheat germ during storage. FA was always produced in the wheat germ throughout the hydrolysis reaction, leading to a higher content, compared with DG and MG. Six kinds of fatty acyl groups were detected by ¹H NMR in triglycerides and phospholipids of wheat germ, including the acyl groups of docosahexaenoic acid (DHA), eicosapentaenoic acid and arachidonic acid (EPA+ARA), linolenic acid (Ln), linoleic acid (L) and oleic acid (O). Linoleic acid was the most abundant unsaturated fatty acyl group in triglycerides and phospholipids. Saturated and modified acyl groups (S+M) were also detected by ¹H NMR. There was significant difference in fatty acid composition between triglyceride and phospholipid in wheat germ. Lysophospholipids and GPC were the hydrolyzed products of phospholipids, while DG and MG were the hydrolyzed products of triglycerides. As such, their contents shared the degree of lipids hydrolysis in the wheat germ. Therefore, NMR can be used to determine the lipid compositions and hydrolysis in the wheat germ. In short, this NMR is a powerful tool for lipids analysis of wheat germ products.