Remarks on official methods employing boron trifluoride in the preparation of methyl esters of the fatty acids of fish oils

Some "official methods" for preparing methyl esters of the fatty acids from oils or fats may be referred to by users as the boron trifluoride (BF3) method and invariably have two stages. The first stage, brief treatment with alkali [commonly NaOH in methanol (MeOH), sometimes NaOCH3] and heat has been popularly described as a saponification step for over 30 yr. in fact, the disappearance of visible fat or oil is mostly transesterification, which can be accomplished in a few minutes under mild conditions. Free fatty acids (FFA) originally present, or produced by saponification, are not converted to methyl esters at this stage. The second stage, heating in BF3-MeOH, has in practice been as short as 2 min. It can convert all FFA to methyl esters, but this step requires at least 30 min. Examples from the recent literature illustrate the necessity of extending the time for BF3-MeOH transesterification of lipids or oils and methylation of FFA. No alkali transesterification is needed.