Chapter 2 - ESR Investigation of the Free Radicals in Irradiated Foods

Abstract

The use of gamma ray or high energy electrons for food microbial decontamination at doses up to 10–20kGy determines the generation of a high diversity of free radicals whose presence, at ambient temperature, could be very well confirmed by electron spin resonance (ESR), but only in the hard/crystalline parts of food items (skeletons of animals, birds, and fish, and exoskeletons like shells, armature of crustaceans, crystalline sugars, and cellulose). In the case of soft or water rich parts of food, the irradiation free radicals decay very fast (lifetime in microseconds or less) and cannot be observed by conventional, continuous wave electron paramagnetic resonance (EPR) spectroscopy. Once created, the free radicals undergo a process of recombination, or capture by the other components of the media, leading to compounds that do not possess unbound electrons. The generation, as well as the disappearance (fading) of free radicals could be considered as first order reactions so that the free radical content follows exponential dependencies on the adsorbed dose (saturating exponential) or on the storage time (exponential decay). In the latter case, the higher the storage temperature, the faster the fading. Therefore, pertinent information regarding the number of different free radical species could be obtained by both isothermal and isochronous annealing. In the first case, the analyses of the decay curve could be helpful in individualizing them, while the Arrhenius plots are very useful in the case of isochronous annealing as they allow determination of the activation energy of each species of free radicals. Notwithstanding these considerations, the presence of characteristic free radical ESR signals can demonstrate a previous microbial decontamination by irradiation, but the absence of such signals does not unambiguously signify the absence of an irradiation treatment.