CIS-TRANS ISOMERIZATION OF OCTADECATRIENOIC ACIDS DURING HEATING - STUDY OF PINOLENIC (CIS-5,CIS-9,CIS-12-18/3) ACID GEOMETRICAL-ISOMERS IN HEATED PINE SEED OIL

To understand the heat-induced cis-trans isomerization of ethylenic bonds in octadecatrienoic acids, pine seed oil, which contains the unusual nonmethylene-interrupted pinolenic (cis 5,cis-9,cis-12 18:3) acid as a major component, was heated under vacuum at 240 degrees C for 6 h together with linseed and borage oils. As a result, a small percentage of pinolenic acid undergoes cis-trans isomerization. The main isomer that accumulates is the trans-5,cis-9,trans-12 18:3 acid. Minor amounts of the three mono trans isomers are also present. Identification of isomers was realized by combining gas-liquid chromatography on a CP Sil 88 capillary column, argentation thin layer chromatography and comparing the equivalent chainlengths of artifacts to those of isomers present in NO2-isomerized pine seed oil. Hydrazine reduction was used to demonstrate that there was no positional shift of double bonds. Heat-induced geometrical isomerization of pinolenic acid differs from that of alpha- and gamma-linolenic acids in at least two aspects. The reaction rate is slower (about one-fourth), and mono-trans isomers are formed in low amounts.