Fatty acid methodology for heated oils

Abstract

AbstractCriticisms have been voiced concerning the methods employed for determining the fatty acid composition of vegetable oils used in processing of fried foods. In the present study seveveral different vegetable oils were heated under standardized conditions, at frying temperatures and under air, for various periods of time, and then subjected to analyses for fatty acid composition. The methods employed were: UV spectrophotometry; gas‐liquid chromatography (GLC) employing either normalization of the peak areas, direct standards or internal standards; and the enzymic, lipoxidase procedure. The present findings confirm that the drop in iodine value is a good approximation of the reduction of polyunsaturated fatty acids in heated oils. In evaluating liquid nonhydrogenated vegetable oils that had been subjected to heat abuse under standard conditions, all of the analytical methods for the estimation of fatty acid composition, with the exception of normalization of GLC peak areas, give substantially the same results. With hydrogenated vegetable oils, the analytical methods provide values which differ significantly among themselves. Evidence was also presented to show that the quantity of material retained on the gas chromatographic column is directly related to the loss of polyunsaturates for oils heated under controlled conditions. For evaluation of oils from frying operations, the preferred method is the GLC method utilizing internal standards if the purpose is the determination of total polyunsaturated fatty acids. However, if the study being made involves a question of nutritional value, then the measurement of the essential fatty acids by the lipoxidase enzymic procedure is the more valid method for evaluation, of heated oils.