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United States Department of Agriculture

Agricultural Research Service

Title: Trimethylsilyl Derivatization/gas Chromatography As a Method to Determine the Free Fatty Acid Content of Vegetable Oils

Authors
item Wan, Peter
item Dowd, Michael
item Thomas-Gahring, Audrey
item Butler, Boyce - THIONVILLE LABS, INC.

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 4, 2007
Publication Date: July 27, 2007
Citation: Wan, P.J., Dowd, M.K., Thomas, A.E., Butler, B.H. 2007. Trimethylsilyl derivatization/gas chromatography as a method to determine the free fatty acid content of vegetable oils. Journal of the American Oil Chemists' Society. 84:701-708.

Interpretive Summary: The concentration of free fatty acids (FFA) in crude vegetable oil and oil in seed is an important quality factor to the industry. Traditionally, FFA concentration is measured by dispersing the oil in a solvent and titrating the mixture against a solution of sodium hydroxide. The titration method is quick and reproducible, but it is non-specific in that all acid components are measured. It also uses the dominant fatty acid of the oil as the average molecular weight to estimate the weight percentage FFA from the measured acidity, and titration provides no information on the profile of FFAs which is often desirable to have by researchers. To solve the above mentioned disadvantages of the titration method, we chose to use gas chromatography (GC) for separating and measuring FFAs in vegetable oils as trimethylsilyl esters. In searching for the most appropriate procedure to accurately determine the quantity and distribution of FFAs, several solvents, two derivatization agents and three chromatographic columns were evaluated. Although none of the stationary phases studied provided complete resolution of all the fatty acids found in commodity oils, a 5% diphenyl/95% dimethyl polysiloxane stationary phase provided separation of most of the acids of interest. Thirty-one vegetable oils were analyzed for FFA concentration by both GC and titration. Results from the two approaches were highly correlated (R2 > 0.998) with the GC method yielding, on average, slightly lower values. For oils with > 0.4% FFA, both methods were repeatable with average coefficients of variance of 2 to 3%.

Technical Abstract: The concentration of free fatty acids (FFA) in seed and crude oil is an important quality factor for industry. Traditionally, FFA concentration is measured by dispersing the oil in a solvent and titrating the mixture against a solution of sodium hydroxide. While the method is quick and reproducible, it is non-specific in that all acid components are measured. It also uses an average molecular weight to estimate the weight percent FFA from the measured acidity and it provides no information on the FFA distribution. One alternative to titration is gas chromatography (GC) of the FFAs as trimethylsilyl esters. In this work, we studied the derivativation chemistry necessary to form the esters and evaluated the separation of fatty acids on three chromatographic columns. Although none of the stationary phases provided complete resolution of all commodity oil fatty acids, a 5% diphenyl/95% dimethyl polysiloxane stationary phase provided the best separation of these fatty acids. Thirty-one vegetable oils were analyzed for FFA concentration by both GC and titration. Results from the two approaches were highly correlated (R2>0.998) with GC yielding, on average, slightly lower values. For oils with >0.4% FFA, both methods were repeatable with average coefficients of variance of 2 to 3%.

Last Modified: 12/21/2014
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