Geometrical isomerization of linolenic acid during heat treatment of vegetable oils

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Abstract
Heat treatment of rapeseed (primor) and soybean oils resulted in the geometrical isomerization of linolenic acid. The geometrical isomers were isolated from a rapeseed oil heated at 240 C for 10 hr by a combination of thin layer chromatography (TLC) of the methoxy bromomercuric adducts of the total methyl esters and AgNO3‐TLC. Three major isomers were identified after hydrazine reduction followed by ozonolysis in BF3‐MeOH as 18:3Δ9c, 12c, 15t, 18:3Δ9t, 12c, 15c and 18:3Δ9t, 12c, 15t. These were accompanied by minor amounts of 18:3Δ9c, 12t, 15c, 18:3Δ9c, 12t, 15t and 18:3Δ9t, 12t, 15c. The 18:3 isomers were detected in both soybean and rapeseed oil heated at 240 C for 10 hr. At this temperature, the increase in time of the heat treatment from 10 to 40 hr resulted in a relative increase of the di‐trans and in a decrease of the mono‐trans isomers. Only minor quantities of these isomers were detected in the oils heated at 200 C for 10 hr. At this temperature, the increase in time of the heat treatment resulted in an increase of both the mono‐ and di‐trans isomers.