Effect of intermittent frying on the stability of vitamins A and D in commercially fortified oils

Abstract

Fat-soluble vitamins and fatty acids are very sensitive to high temperature and oxidation. In this study, stability of vitamins A and D, and physiochemical changes were investigated in two types of oils, palm olein (PO) and a blend of PO and sunflower oil (OB). The intermittent frying was performed at two different temperatures 160 and 190 °C. Batches of French fries (500 g) were fried in a deep-fat fryer for 5 cycles, a cycle per day. In each cycle, food was fried every 15 min with a total frying time of 5 h per day. Oil samples were taken at the end of each heating cycle to measure vitamins A and D concentration, free fatty acid (FFA), peroxide value (PV) and color. In PO, vitamin A was reduced by 96.7% after 25 h of frying at 160 °C and it was completely depleted after 15 h at 190 °C. In OB, the vitamin was reduced by 72.2% after 25 h of frying at 160 °C; and it was completely depleted at 190 °C at the end of frying cycles. Vitamin D was less stable, its content in PO decreased by 67.0% and 80.0% at 160 °C and 190 °C, respectively, after 5 h of frying. In OB, it was reduced by 50.0% and 67.0% at 160 °C and 190 °C, respectively, after 5 h of frying. It was found that both types of oil and frying temperatures showed significant effect on vitamin A concentration, while they did not show any effect on vitamin D. Red color increased linearly with frying time in both oils, while fried at two frying temperatures. The change in color was significantly associated with the vitamin A depletion. A low, but significant, association was found between color changes and depletion of vitamin D. In conclusion, the stability of these vitamins depended on the types of oil and temperature of frying. These were more stable in OB compared to PO, and similar stability was observed in the cases of FFA and PV.