Criticisms 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 several 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.

AND SUMMARYAn objective procedure which is relatively simple and rapid is under study for the determination of the flavor quality of vegetable oils. This procedure utilizes the direct injection of an oil sample to which has been added an internal standard, into a packed precolumn of a gas chromatograph. The volatiles are swept from the precolumn through a 10% SE-30 column under operating parameters which permit complete elution of all volatiles and internal standard within 20 min. Some 15 to 20 samples can be evaluated in one day before it is necessary to replace any part of the foot-long precolumn. Evaluations have been made by the gas liquid chromatographic (GLC) procedure and by a flavor panel of oil samples subjected to a variety of storage conditions. Generally, differences in the GLC pattern are reflected in the flavor panel results.

Two methods for measuring the cis, cis methylene interrupted polyunsaturated fatty acids (cis, cis-PUFA) content of fats and oils were compared collaboratively. The methods evaluated were one proposed by the Food and Drug Administration (Method I) and one used by industry (Method II). Analyses of variance between methods and samples and between laboratories and methods showed similar select differences significant at the 95% level (P < 0.05) for both methods. The reproducibility standard deviations were lower with Method II for 3 of the 5 samples, but equally higher than with Method I for 2 other samples. The repeatability standard deviation for one of the oils was significantly higher for Method II but were significantly higher for Method I for 2 other oils. Over all of the samples, the 2 methods gave similar results and there was no overall statistical difference in the performance.

In partially hydrogenated fats containing residual linoleic acid, linoleie acid isomers are found which have no essential fatty acid activity but contribute to the C-18 dicne acid values; i.e., to the so-called ]inoleie acid values obtained by the usual physicochemical methods. Such hydrogenated fats and oil blends, particularly margarine oils, have now been analyzed by a biochemical procedure in the attempt to measure specifically the essential fatty acid content by a direct method. The latter procedure makes use of a lipoxidase enzyme specific for the cis, cis, methylene interrupted diene structure in polyunsaturated fatty acids having two or more double bonds. It is concluded that the biochemical method is equally as reliable as the combined use of the speetrophotometrie and thioeyanometrie procedures for estimating with precision the essential fatty acid content of hydrogenated fats containing residual dienes; the simplicity and speed of the biochemical method make it the procedure of choice.