Free radicals in irradiated pepper: An electron spin resonance study

With electron spin resonance (ESR) spectroscopy, we revealed various free radicals in commercially available pepper in Japan before and after γ-irradiation. The representative ESR spectrum of the pepper is composed of a sextet centered atg=2.0, a singlet at the sameg-value and a singlet atg=4.0. The first signal is attributable to a signal with hyperfine interactions of the Mn²⁺ ion (hyperfine constant, 7.4 mT). The second signal is due to an organic free radical apparently induced by a sterilization process. The third signal may originate from the Fe³⁺ ion in the nonheme proteins. The progressive saturation behavior at various microwave power levels indicated quite different relaxation behaviors of those radicals. Namely, the peak intensity of the organic free radical component decreases in a monotonic fashion, whereas the Mn²⁺ and Fe³⁺ ESR signals substantially remain constant. This evidences the presence of three independent radicals in the pepper before irradiation. Upon γ-irradiation, a new pair of signals appeared in the pepper. The progressive saturation behavior of the pair peaks after the irradiation showed a quite different behavior as compared with the free radical centered atg=2.0. For the measure of irradiation effects, we propose a universal index for the ESR analysis of irradiated dry foods.