The effects of antioxidants on changes in oils during heating and deep frying

The effect of antioxidants on the stability of rapeseed oil during heating at 80°C and during deep‐fat frying of potato chips (French fries) was assessed by measurement of the formation of polymers and increase in peroxide value, supported by determination of the increase in mass during heating and changes in tocopherols during frying. The Rancimat was also used to assess the stability of the oil containing antioxidants at 100°C The order of antioxidant activity during heating was: TBHQ > lecithin > ascorbyl palmitate > rosemary extract > BHT, BHA and D ‐δ‐tocopherol. Minor deviations from this order of activity were observed depending on the method of assessment used, with the main difference being the low stability of the oil samples containing lecithin or ascorbyl palmitate in the Rancimat test. Ascorbyl palmitate gave a lower rate of increase of mass and peroxide value, but gave a similar rate of dimer formation compared to the rosemary extract. The rosemary extract and ascorbyl palmitate both retarded formation of dimers during deep‐fat frying and retarded losses of natural tocopherols in the oil. The stability of oil used for deep‐fat frying fell as the tocopherols were consumed and this was reflected in an increase in hydroperoxides generated in oil samples after frying. Oil extracted from the chips was similar to the bulk oil in polymer content.