Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


School of Nutrition and Food Sciences

First Advisor

Robert M. Grodner


Among food-borne pathogens, Listeria monocytogenes is one of the more resistant to food processing methods, including irradiation. USFDA approval for irradiation of processed meat and seafood products likely to contain Listeria spp. is $<$3 kGy gamma radiation. The incidence of L. monocytogenes in food products varies between 5-30% and at levels $<10\sp4$ CFU/g. With the capacity of this bacterium to multiply at refrigeration temperatures and the zero tolerance level mandated by the USFDA it becomes imperative to maximize the efficiency of the irradiation process. The first purpose of this study was to determine if initial cell concentration and/or temperature at time of irradiation influences the radiosensitivity of L. monocytogenes. Secondly, to determine whether split dose application would effect the radiation response of this bacterium. Concentrations of 10$\sp3,$ 10$\sp6$ and 10$\sp9$ cfu/ml of L. monocytogenes Scott A, were suspended in tryptic soy broth and exposed to 0-5 kGy gamma (1.25MeV) radiation at 20, 4, and -80$\sp\circ{\rm C}.$ Split doses were applied in equal portions to initial cell concentrations of 10$\sp6$ and 10$\sp9$ at the same temperatures. Surviving cells under the various conditions were enumerated and irradiation D-values were calculated from linear regression curves. At all temperatures, cell concentrations of 10$\sp3{\rm CFU/ml}$ were reduced to non recoverable levels with 2 kGy gamma radiation. The irradiation D-value of 0.43 kGy for frozen (-80$\sp\circ{\rm C})$ cultures of 10$\sp6$ CFU/ml was significantly lower (p $<$ 0.05) than the D-values (0.58 and 0.62 kGy) for the same cell concentration at the other two temperatures (20$\sp\circ$ and 4$\sp\circ{\rm C},$ respectively). For higher cell concentration of 10$\sp9$ cfu/ml, a D-value of 0.42 kGy was obtained for both 4$\sp\circ{\rm C}$ and -80$\sp\circ{\rm C}$ these were significantly lower (p $<$ 0.05) than 0.50 kGy for 20$\sp\circ{\rm C}$ suspensions. With both cell concentrations, the lowest D-values were obtained when cells were irradiated in the frozen state indicating a that primary cell damage was due to direct effects of irradiation. Split doses of 1 and 2 hours time between fractions approached the generation times for this bacterium and were significantly reduced at 20$\sp\circ{\rm C}.$ However, there was no consistent trend in reductions at 4$\sp\circ$ or -80$\sp\circ{\rm C}.$ Split dose application of irradiation to food pathogens should be further explored.