Measuring Oxidation of Cooking Oil Using EPR Spin Trapping

The staling of vegetable oil is a major problem in several food related industries. Rancidity is caused by a free radical process that is both oxygen and temperature dependent. The result is the degradation of long chain free fatty acids to the smaller aldehydes, ketones and alcohols that give rancid foods their characteristic foul odor and flavor. Efforts to decrease the rate of rancidity formation and methods to measure rancidity are of great importance to the food industry.

EPR (electron paramagnetic resonance, also called ESR, electron spin resonance) is a spectroscopic technique that measures samples with unpaired electrons and thus, is very useful for measuring free radicals. At the same time the technique is completely blind to molecules without unpaired electrons, which makes it very specific. Free radicals are short-lived, so to increase our ability to detect them, we add a compound known as a spin trap. The spin trap reacts with the free radical to form a “spin adduct”. Spin adducts are also free radicals, but they are more stable than the original radical which allows us to measure them using an EPR spectrometer.

In this note we describe how to use the EPR technique to measure rancidity in cooking oils. We also used the technique to demonstrate the protective effect that the commercially available antioxidant HerboloxTM (Kalsec, Inc.) has on preventing rancidity.