GC Analysis of Fatty Acid Composition of the Oil Containing Short Chain Fatty Acids

Abstract

GC analysis of the fatty acid composition of the oils containing short chain fatty acids has not been performed satisfactorily because of the loss of short chain methyl esters during the ester preparation procedure. The loss was supposed to be due to volatility and water solubility of these esters. The authors have worked on the simple and quantitative procedure for the preparation of esters from the oils containing short chain acids. It was confirmed that the loss of short chain esters could be avoided by preparing propyl esters with BF3-1-propanol reagent and washing the ester solution with 10.5% aqueous sodium chloride. Then the propyl esters prepared from the standard mixture containing from butyric to octadecanoic acids were analyzed by GC. The propyl esters were prepared by following procedure. Five tenths grams of the oil containing short chain fatty acids and 6 ml of 0.5 N sodium hydroxide in 1-propanol were introduced into an 100 ml flask. Then the mixture was refluxed about 10 min. Seven ml of 14% BF3 1-propanol reagent was added through the condenser and refluxed for 10 min. Then 5 ml of heptane was added and boiled for 1 min. After cooling to room temperature 100 ml of 10.5% aqueous sodium chloride was added to the flask, and the flask was shaken for 1 min. The heptane layer was transfered into a test tube and the aqueous solution was reextracted by 5 ml of heptane. The heptane solutions were combined and used as the sample for GC. It was found that the response of propyl ester in FID increased as the carbon number of fatty acid increased, although the rate of increase was not so large as that of methyl ester. So GC analytical value (area percent) must be corrected using the correction factor obtained through the analysis of standard mixture. Moreover, the corrected analytical value should be converted to the fatty acid composition, since the ratio of propyl moiety in the propyl ester does not respresent the fatty acid composition. The propyl and methyl esters of fatty acids were prepared from milk fat and fatty acid compositions were analyzed by GC.