Oxidation degree of soybean oil at induction time point under Rancimat test condition: Theoretical derivation and experimental observation.

Indices including conductivity, short-chain carboxylic acids, total polar material, acid value, peroxide value and fatty acid composition in water or oil were monitored during Rancimat test of soybean oil from 90 °C to 120 °C. Formic, acetic, propionic and butyric acids in water were detected by ion chromatography. The results showed that all indices had two-stage oxidation rates of initiation (k) and propagation (k) at each temperature. Induction period (IP) computed from conductivity and other indices were well correlated (R = 0.9980-0.9999). According to the Arrhenius model and empirical equation, k increased exponentially with temperature (T) while IP decreased exponentially with T. Oxidation degree of oil at induction time point IP*k was found to be independent of heating time and temperature. The transition between two models was theoretically derived and proved by H NMR. Evaluation of IP*k may provide a new route to correlate Rancimat data with shelf life of edible oil.