Solid-phase extraction-thin-layer chromatography-gas chromatography method for the detection of hazelnut oil in olive oils by determination of esterified sterols.

Abstract

The sterol composition of extra virgin olive oil is very characteristic and, thus, has become a helpful tool to detect adulterations with other vegetable oils. Special attention has been addressed to the separate determination of the free and esterified sterol fractions, since both have different compositions and can thus provide more precise information about the actual origin of the olive oil. In the case of admixtures with small amounts of hazelnut oil, this approach can be extremely useful, because the similarity between the fatty acid compositions of both oils hampers the detection of the fraud. A hyphenated chromatographic method was developed for a sensitive and precise determination of esterified sterols in olive oils. The oil was subjected to silica solid-phase extraction (SPE) fractionation, cold saponification of the collected fraction and purification on silica TLC. The sterol band was then injected into an SPB-5 (30 m x 0.25 mm I.D., 0.25 microM film thickness) and the ratio [% campesterol x (% 7-stigmastenol)2]/(% 7-avenasterol) was calculated. The method was tested on extra virgin olive oil; good sterol recoveries and repeatability were obtained. The results were compared with another method. which has a different sample preparation sequence (silica column chromatography, hot saponification and silica TLC). Similar results were achieved with both methods; however, the SPE-cold saponification-TLC-capillary GC was faster, required less solvent and prevented sterol decomposition. The SPE-method was applied to an admixture with 10% of hazelnut oil and to a screening of 11 oils (husk oil, virgin and refined olive oils) from different Mediterranean countries.