GEOMETRICAL-ISOMERS OF LINOLENIC ACID IN LOW-CALORIE SPREADS MARKETED IN FRANCE

The fatty acid compositions of 20 samples of low-calorie spreads marketed in France have been examined by gas-liquid chromatography (GLC) of their isopropyl esters on a fused silica capillary column coated with 100% cyanopropyl polysiloxane. Spreads containing linolenic acid at a level of 2.3% or higher (5 out of the 20 samples under study) also contain trans-18:3 isomers. These were identified, after fractionation of their isopropyl esters by thin-layer chromatography (TLC) on silica-gel plates impregnated with AgNO3, by GLC on two capillary columns of different polarities and comparison of their equivalent chain lengths with those of authentic standards. Identifications were supported by GLC/mass spectrometry of the dimethyl esters resulting from ozonolysis in BF3/methanol of the monoenes isolated by AgNO3-TLC after hydrazine reduction of 18:3 isomers. 9c,12c,15t-18:3 and 9t,12c,15c-18:3 were found to be the most abundant 18:3 isomers in the spreads, with small amounts of 9c,12t,15c-18:3. These isomers occurred in the relative proportions 52-55, 41-42 and 4-6%, respectively. These proportions are independent of the origin of the sample. The tentatively identified 9t,12c,15t-18:3 also occurred in some instances. In 2 of the spreads, total geometrical isomers of linolenic acid accounted for 0.9-1% of the total fatty acids (up to 28% of the total 18:3n-3 fraction). The presence of 18:3n-3 geometrical isomers in the spreads is likely due to rapeseed or soybean oils that were deodorized under rather harsh conditions before these were blended with other fats or oils. Partial hydrogenation of these oils may also contribute to accumulation of the same linolenic acid isomers in the spreads.