Essential fatty acid contents of various fats: Interpretations of values by physico-chemical tests

Biological assays of oil and fat products, free from isomers of the naturally-occurringcis-9,cis-12 linoleic acid, have been shown to provide estimates of essential fatty acid content which agree well with values obtained by spectrophoto-metric analysis. However, when partially hydrogenated fats, such as those used in margarines, are bio-assayed the estimates obtained are only about 60% of those derived by spectro-photometric tests. In a blended corn oil margarine, good agreement was obtained for linoleic acid content by using biological assay or spectrophotometry, thio-cyanometric procedure, column chromatography for saturates plus iodine value, and gas liquid partition (GLP) chromatography. This margarine fat contained about 29% of the essential form of linoleic acid, and had a ratio to saturated fatty acids of 1.6:1. The hydrogenated corn oil margarine is unlike conventional margarines in providing high amounts of the isomeric forms of linoleic acid which lack essential fatty acid activity. For this reason, poor agreement was obtained between biological assay results and those by physico-chemical measurements of linoleic acid content. Such fat contains only about 6% of the essential form of linoleic acid, with a ratio to saturated fatty acids of ca. 0.3.1. From this study it is now possible to characterize, even without bio-assay data, the fatty acid composition of a highly isomerized fat, such as is found in hydrogenated corn oil margarine. The characterization groups the fatty acids into saturates and total linoleic acids, with the latter including estimates of the positional isomers of linoleic acid with widely spaced double bonds,trans forms of linoleic acid with methylene-in-terrupted double bonds, linoleic acids with the double bonds in conjugated position, andcis-9,cis-12 linoleic acid. The combined use of the spectrophotometric and thiocyanometric procedures makes it possible to estimate the essential fatty acid content of hydrogenated fats containing residual dienes.