关键词: 树坚果, 油体, 油质蛋白, 膜磷脂, 稳定性

Abstract:

【Objective】 In oilseeds, lipids are mainly stored in the organelles called oil bodies. Tree nuts, such as walnut and hazelnut, were important oilseed crops. Among the 5 selected tree nuts, the differences in morphology, structure and stability of their oil bodies were evaluated and discussed, which could provide the new ideas for the utilization of the nut oils.【Method】 5 tree nuts were used as materials, including walnut, almond, hazelnut, pecan, and pine nut. The morphology and size of the oil bodies inside the cells of different nut seeds were observed and compared by employing transmission electron microscope. The oil body membrane proteins were separated by SDS-PAGE, followed by the LC-MS/MS identification of the protein bands. The oil body membrane phospholipids, as well as the intrinsic triacylglycerols, were also analyzed through lipidomics. The stability of oil bodies was evaluated by analyzing their changes in morphology peroxide values and thiobarbituric acid values during storage under room temperature.【Result】 The cells of the nut seeds were occupied by the oil bodies and proteosomes. The asymmetric spheroid-shaped oil bodies were covered by intact membranes. The pecan oil body had the largest average diameter (2.34±0.48) μm, while the almond oil body had the minimum (1.21±0.19) μm. For all nut samples, oleosin was the most important oil body membrane protein, accounting for 72.76%-84.15% of the total amount of membrane proteins. Oleosin had a narrow molecular weight distribution of 14.7-18.8 kDa, yet the numbers of oleosin isomers were quite different. Caleosin accounted for 14.34%-4.96% of the total amount of membrane proteins, and its molecular weight in different nuts was approximately 26.5-27.1 kDa. No caleosin isomer was found in each nut sample. Steroleosin, accounted for 8.95%-12.89% of the total amount of membrane proteins, was identified only in walnut, pecan and pine nut oil bodies. The oil bodies of the 5 tree nuts had different triacylglycerol compositions but similar membrane phospholipid compositions. Phospholipidine (PS) and phosphatidylcholine (PC) were the most important oil body membrane phospholipids, accounting for 67.91%-75.06% of the total amount of membrane phospholipids. The proportion of unsaturated fatty acyl chains in membrane phospholipids was 69.64%-74.52%, while 38.64%-45.15% of the fatty acyl groups were oleoyls. The hazelnut oil body showed the highest stability at room temperature. During storage, the oil bodies aggregated and fused gradually, accompanied by an increasing degree of lipid oxidation. Eventually the phase separation phenomenon in oil body emulsions was observed.【Conclusion】 Among the oil bodies from 5 tree nuts, the pecan oil body was the largest in size, while the hazelnut oil body was the most stable. The different tree nut oil bodies had similar constituents of membrane proteins and membrane phospholipids, but there were significant differences in the number of oleosin isomers, triacylglycerol compositions and constituent proportions. The ratio of lipid content to oleosin, as well as the ratio of oil body membrane phospholipids to oil body membrane proteins, might have important influences on the size and stability of the oil bodies.

Key words: tree nuts, oil body, oleosin, membrane phospholipid, stability