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

Abstract

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.