Carbonyl value in monitoring of the quality of used frying oils

In this study, a set of frying oil samples of different compositional properties but passed qualitative and quantitative standards, which were of various vegetable oil sources (individually or as blends), were obtained from seven of big oil factories in Iran. Before starting the frying process, all the frying oils had carbonyl values (CV) higher than 2 micromol g(-1). The CV of most frying oils linearly increased until the end of the frying process, whereas for some of them, the CV increased and reached a maximum and then decreased to some extent. However, in a set of frying oil samples on average, the CV linearly increased as the frying time increased. There was a linear relationship between the CV and total polar compounds (TPC) throughout the frying process with a high determination coefficient (R(2)=0.9747). The values found for carbonyl compounds of the frying oils during frying process ranged from 7.76+/-0.00 to 123.45+/-3.70 micromol g(-1). Assuming that the limit of acceptance for TPC is 24%, this was roughly corresponded to 43.50 micromol g(-1) for CV.