Phospholipid changes in muscles of plathead grey mullet ( Mugil cephalus) during frozen storage

Abstract

Small and large size whole and eviscerated plathead grey mullet fish ( Mugil cephalus) were stored at − 14°C in polyethylene bags for 6 months. Samples were taken at intervals for 6 weeks, lipids extracted and analyzed for total lipid content, free fatty acids (FFA), peroxide value, total phospholipid phosphorus and individual phospholipids. The apparent total lipid content increased during frozen storage. The amount of FFA increased four times in the small fish and six times in the large fish. Viscera stimulated the FFA development. Peroxide values showed marked increases in all fish treatments, indicating that fatty acids were considerably oxidized. Eviscerated small fish showed much higher increases in peroxide values. The phospholipid content decreased by 94·3% and 96·8% of the original content for whole and eviscerated small fish, respectively, and by 80·3% and 84·3% for whole and eviscerated large fish, respectively. The relationship between FFA content and phospholipid phosphorus of total lipid indicated a suitable storage period to be 6 weeks. Eight phospholipid fractions were identified in both sizes of fish. In small mullet, phosphatidyl choline (PC) was the major fraction followed by phosphatidyl serine (PS), lysophosphatidyl choline (LPC), phosphatidyl inositol (PI), sphingolipid (SL), phosphatidyl ethanolamine (PE) and phosphatidyl acid (PA). In large mullet, PS, PC and SL were the major phospholipids followed by PI, LPC, PE and PA. After the storage period, the major fractions declined while PA and LPC showed very high increases, indicating that phospholipases were present and able to degrade the phospholipids. The degradation of phospholipid fractions was much higher in eviscerated fish.