Effects of thermal stressing on saturated vegetable oils and isolated triacylglycerols - product analysis by MALDI-TOF mass spectrometry, NMR and IR spectroscopy

Abstract

It is well‐known that triacylglycerols in vegetable oils undergo slow oxidative modifications upon storage particularly at elevated temperatures. This has been shown primarily for oils with unsaturated fatty acid residues that are most sensitive towards oxidation. Saturated oils, however, were by far less investigated. In the present study saturated oils (coconut oil) as well as isolated triacylglycerols were exposed to defined thermal stressing and the resulting products were investigated in dependence on temperature and the heating period. Matrix‐assisted laser desorption and ionization spectrometry, 13C and 31P nuclear magnetic resonance spectroscopy and infrared spectroscopy were used for the characterization of the native as well as the thermally stressed oil samples. These methods were used since they provide both, fast and reliable information on oil composition and can be performed faster than other more established methods. We found that the degradation mechanism of saturated fatty acids is completely different from unsaturated fatty acids. Whereas unsaturated oils are primarily depleted under the cleavage of the double bonds, saturated oils undergo a conversion of one methylene group into a carbonyl group. This was independently demonstrated by all applied methods for the triacylglycerols as well as for the free fatty acids derived after saponification.

Rapid screening of mixed edible oils and gutter oils by matrix-assisted laser desorption/ionization mass spectrometry

“…Therefore, the matrix on the plate can be prepared beforehand, and when oils samples are submitted for analysis, the only procedure for sample preparation is to load the oil samples directly to the plate. Compared to previous sample preparation methods that required sample extraction and mixing of sample and matrix [4,17,[28][29][30][31][32][33], the present protocol is much simpler, easier for sampling automation, and allows high throughput analysis.…”

“…Prolonged heating of oils could cause degradation of TAGs and formation of compounds such as diacylglycerols, monoacylglycerols, free fatty acids, oxidized TAGs and TAGs polymers [29,31,[43][44][45]. Gutter oils are illegally used as edible oils directly or after mixing with edible oils [6].…”

“…As a commonly used analytical tool, MS has been widely applied for analysis of edible oils. In addition to coupling with chromatography, direct infusion electrospray ionization [26], direct analysis in real time (DART) [27] and matrix-assisted laser desorption/ionization (MALDI) [4,17,[28][29][30][31][32][33][34] have also been employed. Among these techniques, MALDI-MS has desirable advantages for characterization of edible oils, such as short analysis time, high sensitivity, high tolerance to impurities and capability of analyzing complex samples without the need of chromatographic separation [35].…”

Application of MALDI‐TOF mass spectrometry in lipidomics

“…It is often assumed that only unsaturated oils undergo oxidation in the presence of air. However, it was shown that even completely saturated TAG as present for instance in coconut oil, a typical frying fat, undergo significant alterations upon heating [92]. Although details of the underlying mechanism are so far unknown, it seems that methylene groups are converted upon heating into carbonyl groups.…”

A simple methodology for the determination of fatty acid composition in edible oils through 1H NMR spectroscopy

“…Additionally, these methods deserve attention from an environmental point of view due to the reduction of waste production. Mass spectrometry,14–16 infrared,17–20 and Raman21 spectroscopy have emerged as potential tools in recent years, but without universal acceptance.…”