Determination of<i>cis</i> and<i>trans</i>‐Octadecenoic acids in margarines by gas liquid chromatography‐infrared spectrophotometry

Abstract

A combined capillary gas liquid chromatography (GLC) and infrared spectrophotometry (IR) method is described for the determination of cis and trans ‐octadecenoic acids in margarines made from partially hydrogenated vegetable oils. The total trans ‐unsaturation of margarine fatty acid methyl esters determined by IR, with methyl elaidate as the external standard, was correlated to the capillary GLC weight percentages of the component trans fatty acid methyl esters by the mathematical formula: IR trans =%18∶1 t +0.84×%18.2 t +1.74×%18∶2 tt + 0.84×%18∶3 t where 0.84, 1.74 and 0.84 are the correction factors which relate the GLC weight percentages to the IR trans ‐equivalents for mono‐ trans ‐octadecadienoic (18∶2 t ), trans, trans ‐octadecadienoic (18∶2 tt ) and mono‐ trans ‐octadecatrienoic (18∶3 t ) acids, respectively. This formula forms the basis for the determination of total trans ‐and cis ‐octadecenoic acids in partially hydrogenated vegetable oils. From the weight percentages of 18∶2 t , 18∶2 tt and 18∶3 t determined by capillary GLC on a cyanosilicone liquid phase and the total trans ‐unsaturation by IR, the percentage of the total trans ‐octadecenoic acids (18∶1 t ) is calculated using the formula. The difference between the total octadecenoic acids (18∶1), determined by capillary GLC, and the 18∶1 t gives the total cis ‐octadecenoic acids.