Light-induced formation of free radicals in cream cheese

Radicals were found, by electron spin resonance (ESR) spectroscopy, to accumulate in cream cheese (26% fat, 7% protein) and more significantly in low fat cream cheese (17% fat, 11% protein) upon light exposure. The decay of radicals following illumination (875 lux, with a strong UV-component for up to 80 min) followed first-order kinetics with a half-life at room temperature of around 0.5 h both for cream cheese and low fat cream cheese. The surprisingly long-lived radicals had a broad structureless ESR-spectrum (g-value of 2.006) which, for partly desiccated cream cheese, changed towards a nitrogen-centred ESR powder spectrum (g-value of 2.0014) typical for immobilised protein-based radicals. The protein oxidation product, dimethyl disulphide, and the lipid oxidation products, hexanal and 2-butanone, were detected by GC-analysis in higher concentrations in the outer 1 mm layer than in the second layer (of 1 mm thickness) of the product, in agreement with absorption of 99% of the UV-light intensity in the outer 1 mm layer. The low fat cream cheese had higher levels of both lipid and protein oxidation products, in agreement with the higher steady state concentration of radicals and confirming the role of proteins in oxidative changes also for lipids in cheese. The low fat cream cheese was initially more oxidised than was the cream cheese, as seen from the peroxide values, and oxidation products in lipid droplets could be visualised in three dimensions by confocal laser scanning microscopy, using the fluorescent probe C11-Bodipy (581/591).