Curcumin, the active principle of turmeric, is known to act as an anti-oxidant, anti-mutagen and anti-carcinogen. This study aimed to find out the thermal and oxidative stability of soybean oil when potatoes marinated with turmeric were deep fried in the oil. Two sets of experiment were carried out. In one set, 1 L of oil was heated for 24 h (8 h daily for 3 consecutive days) and 200 g of potato chips without any marination were fried each time twice daily. Foods were fried in batches to replicate the commercial practice of the food industries. The temperature maintained during the whole experiment was at 180-190 °C i.e. at the frying temperature. About 50 ml of the oil sample was collected after every 4 h. In the second set, another 1 L of soybean oil was heated for 24 h in the similar manner and potato chips marinated with turmeric was fried twice daily. Oil samples were collected as before and comparative studies were done. The chemical parameters like acid value, peroxide value, content of 4-hydroxy-2-trans-nonenal (HNE) and fatty acid composition for all the oil samples of each set were determined. The comparative studies on peroxide value and content of HNE revealed that the antioxidant property of curcumin in turmeric helped in reducing the oxidation of the oil initially, but with increase in duration of time, the antioxidant potency got gradually reduced. The loss of unsaturated fatty acids were calculated from the fatty acid composition and it was found that loss of unsaturation in soybean oil where turmeric marinated potatoes were fried was 6.37 % while the controlled one showed 7.76 % loss after 24 h of heating. These results indicated higher thermal and oxidative stability of the soybean oil in presence of turmeric. However, the antioxidant effect gradually decreased with increase in duration of heating.
Keywords: Antioxidant; Curcumin; Deep frying; Soybean oil; Turmeric.