Further studies on artificial geometrical isomers of α-linolenic acid in edible linolenic acid-containing oils

Abstract

Fifteen samples of commercial edible soybean and rapeseed oils (and mixtures of these) from Belgium, Great Britain and Germany have been analyzed for theirtrans-polyunsaturated fatty acid content. Only one sample out of the 13 refined samples, and the two cold-pressed samples, contained trace amounts oftrans isomers. Others contained between 1 and 3.3% of their total fatty acids as geometrical isomers of linoleic and linolenic acids. The degree of isomerization (DI) of linolenic acid varied between 10.5 and 26.9%. Combining results obtained in this study together with corresponding data for French oils (totalling 21 samples) indicates that the relative percentages of individual linolenic acid geometrical isomers depend on linolenic acid DI. Relationships linking these parameters could be approximated by straight lines, at least for DIs lying between 9 and 30%. Extrapolation to DI=0 suggests that the relative probabilities of isomerization of double bonds in positions 9, 12, and 15 are 41.7, 6.1 and 52.1%, respectively, at the very beginning of the isomerization reaction. At that time, the probability of a simultaneous isomerization of double bonds in positions 9 and 15 is close to zero. Thet,c,t isomer is apparently formedvia thec,c,t and thet,c,c isomers, the former being somewhat more prone to a second geometrical isomerization than the latter. The relative proportion of thec,t,c isomer is practically independent from the DI, at least between 9 and 30%, which would suggest that this isomer is an “end-product” of thecis-trans isomerization reaction.