Inactivation kinetics of Escherichia coli in cranberry juice during multistage treatment by electric fields.

The inactivation of Escherichia coli inoculated in cranberry juice by processing with radio frequency electric fields was studied. E. coli ATCC 35218 was chosen among three non-pathogenic strains based on its ability to survive in low pH cranberry juice. Studies were conducted by measuring the survival population when changing the electric field strength between 2.2 and 13.2 kV cm-1, number of treatment stages from 1 to 6 and flow rates between 13 and 25 L h-1 at moderate temperatures of 20, 30 and 40 °C. A minimum inactivation of 5-log reduction, as requested by the Food and Drug Administration (FDA), can be achieved by increasing the number of treatment stages, temperature or both. At 40 °C and 6 treatment stages, 6.57 ± 0.02 log CFU ml-1 reduction in the initial population of E. coli (ATCC 35218) was obtained. At a constant electric field, increasing the temperature produced higher microbial inactivation, consuming lower radio frequency energy input, than increasing the number of treatment stages. Furthermore, a primary model that accounts for the combined effect of time and electric field is proposed. The model represented the sigmoidal curve composed of shoulder, log-linear and tailing sections as observed when changing electric fields. A secondary model that accounts for the effect of temperature and flow rate on the primary model constants is also proposed. The combined primary and secondary models were found to fit the data well with a high coefficient of determination (R2 = 0.965). The proposed model can be extended to kinetic models for pulsed electric fields.