Hydrolysis of acylglycerols and phospholipids of milled rice surface lipids during storage

Abstract

The relative contribution of acylglycerols and phospholipids to the lipid hydrolysis in milled rice was investigated during storage at 37°C and 70% RH for 50 d. The MAG, DAG, and lysophospholipid contents of surface lipid were determined by reversed-phase HPLC. The MAG and DAG content of milled rice increased during storage from 0.06 to 0.18% (w/w milled rice), with the MAG content increasing more than that of the DAG. Lysophosphatidylcholine increased throughout the study from 0.013 to 0.034% (w/w), whereas 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 TAG and phospholipids hydrolyzed in milled rice increased rapidly during the first 3 d 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 was hydrolyzed relative to TAG (37.6%) after 3 d and probably contributed significantly to milled rice lipid hydrolysis during early storage. However, TAG concentration (0.57%, w/w) was much higher relative to phospholipids (0.08%, w/w) in surface lipids, and therefore TAG hydrolysis was the major contributor to FFA development and the quality of stored milled rice.Rice bran lipid is prone to hydrolysis due to the presence of endogenous lipases (1). Bran streaks on commercially milled rice contain rice bran lipid, 85% of which is TAG that readily hydrolyze to FFA. The TAG FA are mainly unsaturated and rapidly oxidize, compromising the milled rice flavor quality. Phospholipids constitute about 2% of the total rice bran lipid (2) and form the membranes of the spherosomes, which contain rice bran TAG (3). They decompose immediately after milling (4). Lipid hydrolysis and oxidation in rice bran streaks on milled rice plays an important role in milled rice quality. Brewers are a major rice user in the United States, and they are particularly concerned about lipid hydrolysis and oxidation that can cause flavor problems in beer.We have determined that linoleic and oleic acids were the main FA released during milled rice surface lipid hydrolysis (5). However, it was not clear whether the FA originated primarily from acylglycerol or phospholipid hydrolysis. Takano (4) indicated that phospholipids decomposed rapidly at the beginning of rice bran storage, but no study has been conducted on milled rice. Other studies of triglycerol decomposition in rice bran (6-9) investigated FA composition, oxidative changes, and prevention of lipid hydrolysis. A study into the nature of triglycerol and phospholipid hydrolysis in milled rice would provide insight into their relative contributions to FFA formation.The objective of this study was to determine the origin of FFA and acylglycerol products of TAG and phospholipid hydrolysis on milled rice, and the relative contributions of TAG and phospholipids to the total milled rice lipid hydrolysis. EXPERIMENTAL PROCEDURES...

Quality changes in perilla seed powder related to storage duration and temperature

“…1b). This would be in contrast to what has been observed in milled rice stored at 37 °C for 50 days with 70% RH (Lam and Proctor 2004). As was observed with the LPEs, the relative abundances of maltose, tryptophan, 12-hydroxyjasmonic acid, tocopherol, and sitosterol decreased with increasing storage duration and temperature.…”

Effects of Artificial Aging on Rice Lysophospholipids

“…Rice LPLs would leak from damaged spherosomes, and some reactive lipases in aleurone and germ may also be deposited on the surface of polished rice kernel due to the disruption of individual cells and spherosomes in paddy rice bran and germ during dehusking and milling, thus reducing the storage stability of white rice. 31 Furthermore, the aging process could also lead to more strengthened and lignified cell walls and organized structures in rice grains, subsequently reducing starch granule disruption and molecule leaching. 32 Here, the starch and nonstarch LPLs' composition and concentration of stored paddy and milled rice were significantly changed (Tables 1 and 3) and strongly related to storage temperature and humidity (Tables 2 and 4).…”