Purpose: This investigation was undertaken to determine how salt type and concentration influence the growth and death of cocktails of pathogenic Escherichia coli strains (STEC), pathogens of concern for acid foods, or Lactobacillus species (LAB, including Lactobacillus plantarum, Lactobacillus brevis, and Lactobacillus paracasei) found in vegetable fermentations.
Methods: Using a cucumber juice medium (CJ), growth rates were determined with a microtiter plate reader for selected NaCl (0 to 1 M) and CaCl2 (0 to 0.6 M) concentrations. Five-log reduction times were determined by incubating cultures in CJ at pH 3.2 with 50 mM lactic acid and plating on Lauria agar (STEC) or 350 mM lactic acid and plating on MRS agar (LAB). Growth and death rates were calculated with custom Matlab software.
Results: For NaCl, growth rates of STEC strains were not significantly different from the no-salt control. However, growth rates decreased for CaCl2 between 0 and 0.2 M and then increased to the level of the no-salt control, up to 0.6 M. The growth rates for LAB strains increased between two- and three-fold as salt concentration increased for both NaCl and CaCl2. Escherichia coli strains had five-log reduction times of 20.4 h, 14.1 h, and 16.6 h for 2% NaCl, 6% NaCl, and 1.1% CaCl2, respectively. For LAB, the five-log reduction times with the same salt concentrations were 33.3 h, 31.2 h, and 70.3 h.
Significance: The data indicate that salt type and concentration have relatively little effect on the growth or death of STEC strains. Increasing NaCl or CaCl2 concentration enhanced growth of LAB species; and CaCl2 may enhance LAB survival and fermentation safety.