Prooxidant capacity of thermoxidised plant oils

The prooxidant capacity of rapeseed, sunflower, soybean, and olive oil was determined before and after heating at a temperature of 180°C for 2, 4, and 6 hours. It was quantified as losses of α-tocopherol caused by the studied oils during 24-h incubation of their acetone-methanol solutions with addition of α-tocopherol at 30°C, whereas the decrease in α-tocopherol concentration was studied as a decrease in antioxidant capacity determined by the spectrophotometric DPPH (2,2-diphenyl-1-picrylhydrazyl) method. During heating of all the studied plant oils, the prooxidant capacity grew due to the formation of reactive lipid oxidation products, but, except the sunflower oil, it did not depend on the time of heating - after the initiatory increase, the prooxidant capacity typically remained approximately constant or decreased. The prooxidant capacity of the heated oils ranged from 58 mg to 360 mg α-tocopherol/kg and decreased in the order soybean oil > rapeseed oil > olive oil ≈ sunflower oil. It did not correlate with the content of polymerised triacylglycerols (except the sunflower oil) and was generally higher than the residual content of α-tocopherol.