Oxidation of frying oils during intermittent usage.

We reported previously that in oils used for frying by commercial establishments, a high correlation was observed among their Gardner colors, polar compound contents (PC), carbonyl values (CV) and acid values (AV). However, this was not true for frying oils used in hospitals. In the present study, oils that had been used for deep-frying in hospital kitchens were collected and assayed for PC, CV, AV, and Gardner color value to determine the reason for the differences from oil used in commercial establishments. Hospitals were selected so that variation in the number of inpatients, frying oil fatty acid composition, and frying frequency was obtained. As previously observed, we did not find good correlations between the color of the frying oil and the PC, CV or AV, respectively. The extent of oxidation in batches of oil repeatedly used for deep-frying was in the following order: soybean oil > blended oil > canola oil. After use in deep-frying, where the oxygen content goes effectively to zero, allowing the oil to stand at room temperature resulted in the quick and steady absorption of oxygen until it returned to its initial content. In addition to the effect of thermal treatment of oil, standing time between usages is a significantt cause of oxidation.