Comparison of gamma and electron beam irradiation in reducing populations of E. coli artificially inoculated on mung bean, clover and fenugreek seeds, and affecting germination and growth of seeds

Sprouts have frequently been implicated in outbreaks of foodborne illnesses, mostly due to contaminated seeds. Intervention technologies to decontaminate seeds without affecting sprout yield are needed. In the present study, we compared gamma rays with electron beam in inactivating E. coli artificially inoculated on three seeds (fenugreek, clover and mung bean) that differed in size and surface morphology. Furthermore, the germination and growth of irradiated seeds were evaluated. Results showed that the D10 values (dose required to achieve 1 log reduction) for E. coli K12 on mung bean, clover, and fenugreek were 1.11, 1.21 and 1.40 kGy, respectively. To achieve a minimum 5-log reduction of E. coli, higher doses were needed on fenugreek than on mung bean or clover. Electron beam treatment at doses up to 12 kGy could not completely inactivate E. coli inoculated on all seeds even though most of the seeds were E. coli-free after 4-12 kGy irradiation. Gamma irradiation at doses up to 6 kGy did not significantly affect the germination rate of clover and fenugreek seeds but reduced the germination rate of mung bean seeds. Doses of 2 kGy gamma irradiation did not influence the growth of seeds while higher doses of gamma irradiation reduced the growth rate. Electron beam treatment at doses up to 12 kGy did not have any significant effect on germination or growth of the seeds. SEM imaging indicated there were differences in surface morphology among the three seeds, and E. coli resided in cracks and openings of seeds, making surface decontamination of seeds with low energy electron beam a challenge due to the low penetration ability. Overall, our results suggested that gamma rays and electron beam had different effects on E. coli inactivation and germination or growth of seeds. Future efforts should focus on optimization of electron bean parameters to increase penetration to inactivate E. coli without causing damage to the seeds.