Evolution of short-chain glycerol-bound compounds during thermoxidation of FAME and monoacid TAG

Methyl heptanoate and octanoate, methyl 8-oxooctanoate and 9-oxononanoate, and dimethyl suberate and azelate were quantitated in model systems of FAME, i.e., methyl oleate and linoleate, and monoacid TAG, i.e., triolein and trilin-olein, heated at 180degreesC for 5, 10, and 15 h. Polar compounds, ranging from 20.2 to 53.2% in TAG systems, were also quantitated as a reference for the level of degradation of the samples analyzed. Base-catalyzed techniques were used for methylation as they did not modify the structure of the compounds of interest. The maximum amounts of short-chain FAME and of aldehydic FAME obtained were of the same order (3-4 mg/g sample in methyl linoleate and trilinolein heated for 15 h). After a second methylation step with diazomethane, amounts of dimethyl suberate and azelate were also quantitated. The total amounts of dimethyl esters in TAG model systems ranged from 1.07 to 3.35 mg/g of heated sample and were similar to those found for total aldehydic FAME (from 1.03 to 3.15 mg/g of heated sample), suggesting that oxidation of the aldehydic acyl group to an acid acyl group was a major reaction.