Survival of Escherichia coli O157:H7 in Cucumber Fermentation Brines

Abstract

Bacterial pathogens have been reported on fresh cucumbers and other vegetables used for commercial fermentation. The Food and Drug Administration currently has a 5-log reduction standard for E. coli O157:H7 and other vegetative pathogens in acidified pickle products. For fermented vegetables, which are acid foods, there is little data documenting the conditions needed to kill acid resistant pathogens. To address this knowledge gap, we obtained 10 different cucumber fermentation brines at different stages of fermentation from 5 domestic commercial plants. Cucumber brines were used to represent vegetable fermentations because cabbage and other vegetables may have inhibitory compounds that may affect survival. The 5-log reduction times for E. coli O157:H7 strains in the commercial brines were found to be positively correlated with brine pH, and ranged from 3 to 24 d for pH values of 3.2 to 4.6, respectively. In a laboratory cucumber juice medium that had been previously fermented with Lactobacillus plantarum or Leuconostoc mesenteroides (pH 3.9), a 5-log reduction was achieved within 1 to 16 d depending on pH, acid concentration, and temperature. During competitive growth at 30 °C in the presence of L. plantarum or L. mesenteroides in cucumber juice, E. coli O157:H7 cell numbers were reduced to below the level of detection within 2 to 3 d. These data may be used to aid manufacturers of fermented vegetable products determine safe production practices based on fermentation pH and temperature. Disease causing strains of the bacterium E. coli may be present on fresh vegetables. Our investigation determined the time needed to kill E. coli in cucumber fermentation brines and how E. coli strains are killed in competition with naturally present lactic acid bacteria. Our results showed how brine pH and other brine conditions affected the killing of E. coli strains. These data can be used by producers of fermented vegetable products to help assure consumer safety.