Radical concentrations in rice and wheat grains and their components after irradiation with Co-60 γ-rays were determined in time course by using ESR spectrometry. These samples were irradiated in N₂ and in air at different moisture contents and then stored at different temperatures.
Trapped radical concentration increased linearly with increasing dose up to about 0.5 Mrad and was known to be greatly influenced by moisture content, oxygen and stored temperature, though no marked difference between rice and wheat and their varieties or classes. A main part of the free radicals observed in the grains of low water content is 23G in peak width and disap- peared fairly quickly in air, and which is known to be due to the radicals trapped in the starch of endosperm. On the other hand, the radicals observed in bran (11G) and gluten (12G) were considerably stable both in air and in N₂.
Consequently, it was demonstrated that no appreciable amount of free radicals remained in rice and wheat grains which were irradiated with the dose levels necessary for the insect killing and stored at room temperature in air for several days, and accordingly the ESR measurement of residual free radicals would not be available as a tool to detect the irradiated grains.