In this study, changes in the physical parameters of color index (CI), refractive index (RI) and viscosity in a range of commonly used frying oils during frying process were investigated, and their performance compared to the carbonyl value (CV) as a reference method in monitoring of the quality of used frying oils was evaluated. It was observed that the CI of the frying oils linearly increased with frying time. The frying oils initial CI was 0.36±0.15 on average, and it reached 1.73±0.24 after 48 h of the frying process. The CI had a linear relationship with the CV with a high determination coefficient, R2 = 0.97, during the frying process. Assuming that the limit of acceptance for the CV is 43.50 µmol/g, this roughly corresponded to 1.12 for the CI. As the oxidation accelerated by heat proceeded, the value of RI linearly increased. On average, the RI of the frying oils changed from 1.468313±0.00051 (0 h) to 1.469671±0.00032 (48 h). The RI of the frying oils showed a high correlation with the CV (R2 = 0.94). Assuming that the maximum level permitted for the CV is 43.50 µmol/g, the RI of a used frying oil should be ≤ 1.469075. Viscosity of the frying oils at 30 °C increased with time and changed from 45.11±13.84 mPa s (0 h) to 75.03±12.47 mPa s (48 h) on average. There was a strong correlation (R2 = 0.96) between the viscosity and CV of the frying oils. If 43.50 µmol/g of CV is the maximum level permitted in frying oils, viscosity of a used frying oil should be ≤ 74.78 mPa s at 30 °C.

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