Effect of repeated deep frying on oxidative stability and quality of unrefined bulk coconut oil

Although repeated use of edible oils for deep frying can adversely alter their physicochemical properties, detailed studies examining these effects on coconut oil remain limited. In this study, the effect of repeated deep frying of unrefined copra coconut oil sold in bulk was evaluated for its selected quality parameters. For this purpose, coconut oil was heated at 180±5°C for 20 min with food (vadai) as a frying treatment, or without food as a frying control, for ten repeated frying cycles. After each frying cycle, the oil samples were analyzed for oxidative stability, physicochemical properties, and structural alterations using standard methods. The levels of free fatty acids and thiobarbituric acid reactive substances (TBARS) in fried coconut oil samples increased significantly (p<0.05), while their saponification and iodine values decreased significantly (p<0.05), with increasing number of frying cycles. Moreover, the peroxide value of coconut oil heated without food showed a significant (p<0.05) increment over all successive frying cycles tested. However, it increased significantly (p<0.05) only up to the sixth frying cycle and a significant (p<0.05) reduction was observed thereafter. The mean FFA content of coconut oil samples heated repeatedly with food was significantly (p<0.05) higher than that of samples heated without food. This indicates that repeated heating of unrefined coconut oil has a significant (p<0.05) adverse effect on its oxidative stability and physicochemical properties, compared to the accepted limits. However, unrefined coconut oil can be used for a maximum of 2-4 repeated deep frying cycles, while maintaining its physicochemical parameters within the limits specified by the Codex and Sri Lankan food standards.