The thermal oxidation rate of frying oils were determined based on in specific gravity (SG), which was found to increase linearly with heating time. The heat oxidation rate constant kSG and heat deterioration rate constant kRI of various frying oils were determined by simultaneous measurements of specific gravity (ΔSG) and refractive index (ΔRI). Even when heat oxidation rates of the oils were the same, the deterioration rates would differ in some cases and thought the latter were the same thermal oxidation rates would differ. Thermal oxidation and deterioration are thus differ and must be considered separately. Heat oxidation rate constants kSG ware calculated based on following heat oxidation law of frying oils. dSG/dθ=kSG (S/W) ·P/1013, ΔSG=kSG (S/W) ·θ·P/1013, where kSG (g, cm-2, h-1), SG : specific gravity (25/25°C or 40/25°C), S/W : specific surface (g/cm2), θ : heating time (h), P : atmospheric pressure (hPa), kSG of the frying oils examined were in the range of 3×10-4 to 24×10-4.
If the point at which a frying oil is no longer suitable for frying use is 2527% polar material content, then reacted oxygen are calculated to bring increase of specific gravity 45×10-450×10-4 at ΔV/ΔW=0.