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Frying Oil Studied by High Resolution NMR 1H and 13C spectra

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  036096,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.14195) @2003
Authors:   Ming Li, Kevin M. Keener, Brian E. Farkas
Keywords:   NMR 1H spectrum, NMR 13C spectrum, canola oil, oxidation, hydrolysis

In the present investigation, high resolution nuclear magnetic resonance (NMR) proton and carbon spectra were used to non-invasively measure the degradation of frying oil caused by heating. Fresh and aged canola oil samples were analyzed by NMR proton and carbon spectra. Canola oil was heated to 180 C while exposing to (1)air and (2) air and water steam and stirred constantly. The NMR proton spectrum showed that no aldehydes (n-alkanals and branched alkenals) were presented in fresh canola oil, but they appeared after 4 hours of heating. They reached a maximum (1.35%) after 24 hours in air degraded oil and reached a maximum (1.45%) after 36 hours in air and water steam degraded oil. Aldehydes are of concern because they exert a range of negative toxicological effects. The carbon spectrum of fresh canola oil is mainly triacylglycerides. However, in canola oil that was heated for 4 hours two noticeable changes occurred. First, oxidation increased oil saturation level with selectivity towards n-3 double bonds and these reduced fatty acids were then converted into aldehydes. Second, triacylglycerides were broken down into free fatty acids and mono- and di-acylglycerides via hydrolysis. Canola oil heated while exposed to air degraded faster than exposed to air and water steam. These results illustrate the capability of NMR to noninvasively quantify oil degradation and the by-products formed during the frying process.

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