Comparison of acidic and alkaline catalysts for preparation of fatty acid methyl esters from ovine muscle with emphasis on conjugated linoleic acid.

Methanolic reagents containing acidic catalysts, HCl (0.5 M, 1 h, 80° C) or BF(3) (14%, 1 h, 80° C), or alkaline catalysts, KOH (0.2 M, 15-60 min, 50° C) or NaOCH(3) (0.5 M, 15-60 min, 50° C), were compared for use in preparation of fatty acid methyl esters for GC analysis of total lipids from freeze-dried semitendinosus muscle of lambs fed a 3.6% linoleate diet. Lipid preparations were in duplicate and included a total lipid extract, as well as direct transesterification and direct saponification of freeze-dried muscle. For the total lipid extracts, the weight% of 18:2 cis-9, trans-11 (CLA) with BF(3) (1.15) was 14.0% lower (P=0.001) than with either KOH (1.32) or NaOCH(3) (1.36); however, with HCl (1.25) CLA was intermediate (P=0.02). Concentrations of CLA (mg/g tissue) were similar (P ⩾0.44) within acidic or alkaline catalysts, but were 18.1% higher (P ⩽0.01) with KOH (2.56) and NaOCH(3) (2.52) than with HCl (2.01) or BF(3) (2.12). For direct transesterification, weight% of CLA was similar (P=0.55) with KOH (1.34) and NaOCH(3) (1.33), but each was 11.9% greater (P=0.003) than with HCl (1.18) and 19.1% greater (P=0.005) than with BF(3) (1.08). Concentrations of CLA after direct transesterification were greatest (P ⩽0.04) with KOH (3.31), followed by HCl (2.89, P=0.04), BF(3) (2.42, P ⩽0.004), and lowest (P ⩽0.002) with NaOCH(3) (2.21), indicating differences in efficiency of direct transesterification. Weight% of CLA in semitendinosus muscle, ranked highest to lowest, was lambs fed 3.6% linoleate (P ⩽0.003) > lambs fed 3.8% oleate (P ⩽0.01) > lambs fed a non-fat supplemented control diet (P ⩽0.01) when either BF(3) (saponified lipids) or KOH (direct transesterification) was used. Thus, dietary treatment effects on muscle CLA were not affected by catalyst. For the muscle of high-linoleate, high-oleate, and control lambs, CLA was 20.2, 13.9 and 0.0% higher, respectively, with KOH than BF(3), indicating that degradation of CLA by acidic catalysts decreased with lower starting amounts of CLA.