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Geometrical isomers of linolenic acid in low‐calorie spreads marketed in France
Author(s) -
Wolff Robert L.,
Sebedio JeanLouis
Publication year - 1991
Publication title -
journal of the american oil chemists' society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.512
H-Index - 117
eISSN - 1558-9331
pISSN - 0003-021X
DOI - 10.1007/bf02662159
Subject(s) - chemistry , isopropyl , chromatography , structural isomer , silica gel , fractionation , fatty acid , linolenic acid , cis–trans isomerism , ozonolysis , linoleic acid , thin layer chromatography , organic chemistry
Abstract The fatty acid compositions of 20 samples of low‐calorie spreads marketed in France have been examined by gasliquid 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 AgNO 3 , 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 BF 3 /methanol of the monoenes isolated by AgNO 3 ‐TLC after hydrazine reduction of 18:3 isomers. 9 c ,12 c ,15 t ‐18:3 and 9 t ,12 c ,15 c ‐18:3 were found to be the most abundant 18:3 isomers in the spreads, with small amounts of 9 c ,12 t ,15 c ‐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 9 t ,12 c ,15 t ‐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.