Abstract
Soybean oils were heated at 180°C 5 hr/day up to 2030 hr by frying the following food items with an approximately equal in surface area ; potato, chicken, soybean curd, mackerel, whale meat, and green pepper. Amounts of water depleted from each food item was kept at a level of 25ml/hr/500g oil. Polar (P) and apolar (Ap) fractions were separated by TLC by applying either oil itself or its fatty acid methyl esters (the former is called the TG and the latter the FAMe method) and both fractions were monitored directly on the plate by an UV dual wavelength thin layer chromatoscanner at 230nm. Absorbances (expressed in terms of grams of oil or FAMe used for the chromatography) of P fractions increased with increase in heating time, while those of Ap remained fairly constant at low levels. Ratios of P/Ap for fresh oils remained less than 1 by both methods. At 20 hr of heating (which is considered to be a maximum useful life of the oil under the experimental conditions) the ratios by the TG and FAMe method were 47 and 24, respectively, depending on the freshness of oils used. Of the two methods, the TG method was found to be more convenient and an amount of sample necessary for a single analysis was only 0.2mg and the time required for examination of 24 samples/plate was 12 hr.
