Volatile compounds from the triacylglycerol ofcis,cis 9,15‐linoleic acid

Abstract

cis,cis 9,15‐Linoleic acid, one of the isomers produced by partial hydrogenation of linolenic acid, may be a precursor of the hydrogenation odor observed during heating of partially hydrogenated soybean oil in air. Odors of hydrogenation, for example the melon odor, are associated with foods cooked in partially hydrogenated soybean oil that has undergone oxidative deterioration during heating. To better understand this oxidation mechanism, the volatiles from decomposition of the oxidation products of a synthesized model compound, tricis,cis 9,15‐linoleoylglycerol, heated in air at 192°C were collected, separated, quantitated by gas chromatography with flame ionization detection and identified by mass spectrometry. Major volatiles observed indicated that each of the isolated double bonds at carbons 9,10 and 15,16 isomerized to form an allylic radical. The allylic radical derived from the isolated double bond at carbons 9,10 reacted with oxygen to form the 8,9,10 and 11 monohydroperoxides. The allylic radical derived from the isolated double bond at carbons 15,16 produced the 14,15,16 and 17 monohydroperoxides. The 8,9,10 and 11 monohydroperoxides are the expected precursors of the following volatiles identified by mass spectrometry: 2,8‐undecadienal (5.9%), 2,7‐decadienal (7.8%), 6‐nonenal (8.4%) and 5‐octenal (1.5% of total volatiles), respectively. These four aldehydes are analogous to major volatiles obtained from thermal decomposition of autoxidized trioleoylglycerol. The 14,15,16 and 17 monohydroperoxides are the expected precursors of 2‐pentenal (9.0%), 2‐butenal (8.0%), propanal (12.1%) and ethanal (2.0% of total volatiles), respectively. Among the above volatiles, 6‐nonenal (cucumber‐melon) is reported to have the most intense odor associated with deteriorated partially hydrogenated soybean oil.