Technical note: Common analytical errors yielding inaccurate results during analysis of fatty acids in feed and digesta samples1

The basic rules governing the proper fatty acid analysis of feed and digesta samples are sometimes overlooked, leading to potential errors in reporting the fatty acid content or composition of feed and digesta samples. The direct transesterification procedure of Sukhija and Palmquist (1988, J. Agric. Food Chem. 36:1202-1206) has become popular in analyzing fatty acids in feed and digesta samples obtained from animal feeding trials. One shortcoming of the Sukhija and Palmquist transesterification procedure is inaccurate analysis of fatty acids with conjugated double bonds. Digesta and milk samples from ruminant species typically contain a multitude of conjugated linoleic acid (CLA) isomers that easily undergo isomerization and epimerization following prolonged exposure to methanolic HCl. Modifications to the Sukhija and Palmquist procedure are given in this paper that allow successful determination of CLA isomers. Errors in fatty acid analysis also occur from misuse of internal standards; use of an internal standard is recommended in the Sukhija and Palmquist procedure as the preferred method to quantify total fatty acid content. The choice of internal standard may sometimes be important for obtaining accurate results. As an example, applying the direct transesterification procedure to a fat supplement high in saturated fatty acids yielded 613 mg/g of total fatty acids when C17 was used as the internal standard compared with 930 mg/g total fatty acids when C19 was used as the internal standard. Fatty acid content further increased to 952 mg/g when a unique unsaturated fatty acid (C13:1) was used as the internal standard.