Reliability of ^1^H NMR analysis for assessment of lipid oxidation at frying temperatures

Authors: Hwang, Hong-Sik; Moser, Jill; Liu, Sean

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2016
Publication Date: 1/4/2017
Citation: Hwang, H.-S., Winkler-Moser, J.K., Liu, S.X. 2017. Reliability of ^1^H NMR analysis for assessment of lipid oxidation at frying temperatures. Journal of the American Oil Chemists' Society. 94:257-270.

Interpretive Summary: Although there are many analytical methods developed for assessment of lipid oxidation, it is often a serious problem that there is lack of consistency in results obtained from different analytical methods. This is because that most methods are designed to detect one kind of oxidation product even though lipid oxidation is a very complicated process producing numerous products. Therefore, the development of more reliable methods that can concomitantly detect many oxidation products is necessary. The other way to accurately assess the lipid oxidation is to measure the changes in oil molecules instead of detecting many oxidation products. The NMR method can monitor changes in oil molecules during the oil oxidation. Some studies were done at lower temperatures than frying temperatures, but no systematic study was conducted to examine the reliability of NMR method for oil oxidation at a frying temperature. In this study we have examined the reliability of NMR method for oils at a frying temperature. We found that the NMR method was as accurate as three conventional methods that are known to be very reliable.

Technical Abstract: The reliability of a method using ^1^H NMR analysis for assessment of oil oxidation at a frying temperature was examined. During heating and frying at 180 °C, changes of soybean oil signals in the ^1^H NMR spectrum including olefinic (5.16-5.30 ppm), bisallylic (2.70-2.88 ppm), and allylic (1.94-2.15 ppm) proton signals relative to glyceride backbone CH2 (5.30-5.46 ppm) and aliphatic CH2 (1.05-1.71 ppm) signals showed strong correlations with conventional analytical methods including total polar compounds (TPC), polymerized triacylglycerols (PTAG), and changes of linoleic acid and linolenic acid peaks in gas chromatography. For oils rich in oleic acid, mid-oleic sunflower oil (NuSun) and high oleic soybean oil (HOSBO), only the olefinic and allylic proton signals are recommended for analysis due to the relatively low intensity of the bisallylic proton signal. Under these heating and frying conditions, signals indicating intermediate oxidation products, hydroperoxides, were not detected while very small signals corresponding to a variety of aldehydes including alkanal, branched alkenal, 2-alkenal, and aldehydes of conjugated dienes and epoxides were observed. In this study, it was found that the ^1^H NMR method is a fast, convenient, and reliable analytical method to determine the oxidation state of frying oil.