PV andp-AV values increased in the order deepfrying > air–light exposure > air exposure (Fig. 1,
P< 0.01,Table 2) and also the highest mean difference compared toRpwas calculated on comparison of deep frying with control by Duncans multiple range test (Table 4). Autoxidation occurs via self-propagating free radical mechanism. Since direct reaction of unsaturated fatty acids with O2 is thermodynamically difficult, production of first few radicals necessary to start propagation (initiation) must occur by some catalytic means. It has been proposed that the initiation step may take place by decomposition of preformed hydroperoxides due to heat or exposure to light or by mechanisms where singlet O2is the active species involved. Upon formation of sufficient free radicals the chain reaction is propagated by the abstraction of hydrogen atoms adjacent
to double bonds of fatty acids, followed by O2 attack at these locations and resulting in the production of peroxy radicals. This explains why PV andp-AV values in the oils exposed to air only were significantly less than that in oils exposed to air–light and the oil used for deep-frying. Olive oil gave the lowest values while soybean the highest. The lower values for olive oil compared with soybean and corn oils are based on the fatty acid composition of the oil and are consistent with
the findings ofGertz et al. (2000)