Characterization of γ‐linolenic acid geometrical isomers in borage oil subjected to heat treatments (deodorization)

Heating of borage oil, either under vacuum as a model or during steam‐vacuum deodorization, produces artifacts that are geometrical isomers of γ‐linolenic acid ( cis ‐6, cis ‐9, cis ‐12 18∶3 acid). In a first approach, we have studied the behavior of these fatty acids in the form of either methyl or isopropyl esters on two capillary columns (CP‐Sil 88 and DB‐Wax). From this study, it appears that the DB‐Wax capillary column is the best suited analytical tool to study in some detail γ‐linolenic acid geometrical isomers. In a second approach, the structure of these isomers was formally established by combining several analytical techniques: Argentation thin‐layer chromatography, comparison of the equivalent chainlengths with those of isomers present in NO 2 ‐isomerized borage oil on two different capillary columns, partial hydrazine reduction, oxidative ozonolysis, gas chromatography coupled with mass spectrometry and gas chromatography coupled with Fourier transform infrared spectroscopy. The two main isomers that accumulate upon heat treatments are the trans ‐6, cis ‐9, cis ‐12 and cis ‐6, cis ‐9, trans ‐12 18∶3 acids with minor amounts of cis ‐6, trans ‐9, cis ‐12 18∶3 acid. One di‐ trans isomer, supposed to be the trans ‐6, cis ‐9, trans ‐12 18∶3 acid, is present in low although noticeable amounts in some of the heated oils. The content of these artificial fatty acids increases with increasing temperatures and duration of heating. The degree of isomerization (DI) of γ‐linolenic acid is less than 1% when the oil is deodorized at 200°C for 2 h. Heating at 260°C for 5 h increases the DI up to 74%. Isomerization of γ‐linolenic acid resembles that of α‐linolenic ( cis ‐9, cis ‐12, cis ‐15 18∶3) acid in several aspects: The same kinds and numbers of isomers are formed, and similar degrees of isomerization are reached when the octadecatrienoic acids are heated under identical conditions. It seems that the reactivity of a double‐bond vis‐à‐vis cis‐trans isomerization is linked to its relative position, central or external, and not to its absolute position (Δ6, 9, 12 or 15).