Effect of electron beam irradiation and sugar content on kinetics of microbial survival

Abstract

The killing effectiveness of electron beam irradiation has not been completely characterized. The type of microorganisms and the composition of food have a direct effect on the efficiency of this technology. The objectives of this study were to select a surrogate suitable for use in electron beam irradiation studies of fruits and to evaluate the effect of sugar content on the kinetics of microbial damage and recovery. A 2.0 MeV Van de Graaff linear accelerator was used to apply irradiation (up to 5.0 kGy), using different configurations, on gelatin-based systems with the addition of sugars. The systems were inoculated with pathogenic and non-pathogenic bacteria strains (surrogates). Initial studies showed that Escherichia coli K-12 MG1655 is a suitable surrogate that represents the damage induced to common fruit pathogens by irradiation. The reduction in bacteria population can be maintained by storing samples at 4°C. An increase in temperature up to 20°C was enough for the damaged population to recover in 48 hours. Gelatin-based systems proved to be a simple and inexpensive medium to evaluate the effects of irradiation (up to 5.0 kGy) on selected bacteria. Reduction of the system dimensions and their positioning related to the beam source were key factors in increasing the killing effectiveness of irradiation. The sugar levels (up to 8 %) used to mimic the maturity of cantaloupes had no effect on the radiation D10 values and the recovery of the surrogate population quantified as Generation Times. The resistance of the surrogate to irradiation was validated in an optimum configuration and in cantaloupes. Temperature and sugar content caused significantly higher changes to the physical structure of the gel-based systems than irradiation (1.0 kGy). Plate counts and light microscopy techniques demonstrated that the structure of the gelatin-based systems allow for motility of the bacteria in a 3-D array (length, width and depth). When little information was available about the effectiveness of using a low energy linear accelerator, the inoculation of gelatin-based systems proved to be a reliable method to select a suitable surrogate and to predict the effects of irradiation on bacteria as a function of sugar content.