A Rapid GC-FID Method for the Determination of Fatty Acids in Walnut Oils and Their Use as Markers in Authenticity Studies

The composition of walnut oils is affected by multiple factors including their variety, their geographical location, and the environmental factors. In this study, the fatty acid composition of walnut oils from Greece and Bulgaria was assessed by gas chromatography coupled with flame ionization detection (GC-FID). The derivatization process involved the use of 14% BF3 after alkaline hydrolysis, and was optimized after investigating the effect of temperature (50–100℃) and methylation time (5–30 min). Twenty walnut oils originating from Greece and Bulgaria were analyzed, and the methyl esters of palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid were tentatively identified on the basis of their commercially available standards, and quantitatively determined. The method demonstrated good linearity (r2 > 0.999) and adequate recoveries (RE = 86.2–92.5%, n = 6). Precision was expressed as repeatability (intra-day precision, n = 3), and as reproducibility (inter-day precision (n = 9) and the results ranged between 2.7 and 4.6%RSD for intra-day experiments, and 4.1–5.8%RSD for inter-day experiments. The limits of detection (LODs) and limits of quantification (LOQs) ranged between 0.21–0.32 mg/kg and 0.63–0.97 mg/kg, respectively. The determined fatty acid methyl esters were quantified and the results were analyzed by Student’s t-test, principal component analysis (PCA), and hierarchical clustering to investigate the differences of the lipid fraction of the walnut oils according to their geographical origin.