evaluating the performance of peroxide and conjugated diene values in monitoring quality of used frying oils

frying oils showed an increase in peroxide value (pv) at the initial stages of the frying process followed by a decrease, and there was no correlation between pv and total polar compounds (tpc) in the same set of frying oils. all the frying oils had pvs lower than 4.42 meq o2 kg-1 oil during the frying process and, consequently, none of them was above the limit of 10 meq o2 kg-1 oil for edible oils. the variations of conjugated diene value (cdv) presented a pattern different from that of pv. cdv had a good linear relationship with tpc during initial stages of the frying process and then reached a plateau (r2= 0.9838). the plateau occurred beyond the tpc of 24-27% for the frying oils, and there was an obvious increasing trend before the range mentioned. the contents found for con-jugated dienes of the frying oils during the frying process ranged from 5 to 42 mmol l-1. assuming that the limit of acceptance for tpc is 24%, this roughly corresponded to 29 mmol l-1 for cdv.