The thermally oxidative deterioration of soybean oil was assessed during cooking typical of that in a Japanese home. A large quantity of fresh soybean oil was heated in a steel pan at 180±2°C for 30h, and an aliquot of the oil was withdrawn periodically and then heated as a thin layer in a frying pan at 180±2°C for 5 or 10min. Polymers and polar material content was determined by gel permeation high performance liquid chromatography using a differential refractometer as a detector, and thin-layer chromatography with a hydrogen-flame ionization detector. Chemical characteristics (acid, peroxide, carbonyl, and iodine values), color tone, and fatty acid composition were also measured.
Polar polymerized material content was found to increase linearly with time and constituted the major thermally oxidation products. Thus, both polymerized products and polar material possibly prove quite useful for assessing deterioration of heated oils. However, parameters such as carbonyl and acid values also increased with heating.
The main components in polymerization products from heat oils were found to be dimers. Most of the polymers were polar and there was thus a fairly good correlation between polymer and polar material content with a correlation coefficient larger than 0.95. Oil deterioration progressed much more rapidly during pan-frying than deepfat frying.