Hydrolysis of Acylglycerols and Phospholipids of Milled Rice Surface Lipids During Storage1

The relative contribution of acylglycerols and phospholipids to the lipid hydrolysis on milled rice was investigated during storage at 37°C and 70% relative humidity for 50 days. The monoglycerol, diglycerol, and lysophospholipid contents of surface lipid were determined by reversed-phase HPLC. Monoand diglycerol content of milled rice increased during storage from 0.06% to 0.18% (w/w milled rice) with monoglycerols content increasing more than diglycerols. Lysophosphatidylcholine increased throughout the duration of the study from 0.013% to 0.034% (w/w) while lysophosphatidylinositol and lysophosphatidylethanolamine contents initially increased from 0.002 to 0.003% and from 0.005 to 0.009% (w/w) respectively, and then stabilized until day 50. The relative percentage of triacylglycerols and phospholipids hydrolyzed in milled rice increased rapidly during the first 3 days of storage from 12.3 to 37.6% and 25.0 to 62.5% (w/w) respectively, and steadily increased to 53.1% and 73.8% (w/w) on day 50. A higher percentage (62.5%) of phospholipids were hydrolyzed relative to triglycerols (37.6%) after 3 days and probably contributed significantly to milled rice lipid hydrolysis during early storage. However, triacylglycerols concentration (0.57%, w/w) was much higher relative to phospholipids (0.08%, w/w) in surface lipid, and therefore triacylglycerols hydrolysis was the major contributor to FFA development and quality of stored milled rice.