Studies on Colour Development of Frying Oils during Heating. II.

Mechanism of Colour Development of Methyl Linoleate

Abstract

The aim of this study was to elucidate the mechanism of colour development in frying oils. Methyl linoleate was selected as the typical model component of frying oil. The sample was heated for 8 hr at 200°C and the time dependence of chromaticity and characteristic changes were determined. The severer changes were observed in methyl linoleate than in soybean oil. The highly coloured sample was divided into five fractions by the urea adduct method, then above mentioned measurements were made for each fraction. In addition to the above mentioned analyses, the composition and mean molecular weight of the compounds which did not give the urea adduct were determined and the presence of cyclic structure was identified. As a result it was found that isolated cis double bonds isomerizecl to trans double bonds by heating, and simultaneously the cis-trans and trans-trans conjugation of the isolated double bonds followed. In the highly coloured fractions, isolated trans and Irans-trans conjugated double bonds were predominantly observed. Dimerization and timerization occured accompanying the formation of carbonyl, hydroxyl and epoxy groups. The coloured materials which were composed of partly cyclic dimers and trimers were concentrated in the fraction which slid not give the urea adduct. It was surely established that the polar dimer, which contained oxvgens, exhibited deeper red colour.