Comparison of three methods based on electron spin resonance spectrometry for evaluation of oxidative stability of processed cheese

Electron spin resonance (ESR) spectrometry has been adapted to accelerated (light and temperature) storage experiments with processed cheese comparing the spin trapping technique with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin trap added to the cheese and the spin labeling technique with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as spin label added to the cheese. Both methods showed that light was the more important factor compared to temperature for early stages (up to 11 days) in the formation of radicals in processed cheese. For the spin labeling technique other, unidentified reactions interfered during the early stages of the accelerated storage, indicating that longer reaction times are required for the evaluation, and the spin trapping technique is recommended. As a third method, direct measurement of free radicals formed in processed cheese stored for 15 months and subsequently freeze-dried showed that for longer storage, temperature is more important for the steady-state concentration of radicals than light exposure. In agreement with this result, formation of secondary lipid oxidation products determined as thiobarbituric acid reactive substances was found to correlate with the direct ESR measurement.