Occurrence of Conjugated Linolenic Acids in Purified Soybean Oil

A high-performance liquid chromatographic
(HPLC) method is described for the determination
of conjugated linoleic acids (CLA) and
conjugated linolenic acids (CLN). Methyl esters prepared
from purified lipid fractions of soybean oil were
analyzed using an HPLC system equipped with photodiode-
array detector to detect peaks having maximum
absorption around 233 and 275 nm. These peaks
were concentrated by AgNO3-silicic acid column
chromatography and reversed-phase HPLC. The
structural analysis, of dimethyloxazoline (DMOX)
derivatized methyl esters, using gas chromatographymass
spectrometry (GC–MS) showed the occurrence of
9,11- and 10,12-CLA and 8,10,13-, 8,10,12-, and 9,11,13-
CLN. The comparison of these conjugated fatty acids
with authentic isomers by HPLC revealed the presence
of isomeric mixtures of CLA [cis (c),trans(t) or t,c and
t,t] and CLN (c,t,t or t,t,c and t,t,t). Traces of 9,11- and
10,12-CLA (c,t or t,c) were found in crude oil. CLN
isomers (8,10,12-18:3 and 9,11,13-18:3) were found to
be forming during the bleaching phase of soybean oil
processing. 8,10,13-CLN and 9,11- and 10,12-CLA (t,t)
were only found in soybean oil after the deodorization
step. CLN contents in commercial soybean oil varied
from 387 to 1,316 mg/kg oil. A decreased level of
bleaching earth and temperature resulted in a reduced
CLN content. It is possible that CLN would be derived
from the linoleate hydroperoxides formed during the
processing and storage of soybean oil.