Effects of sodium chloride or calcium chloride concentration on the growth and survival Escherichia coli O157:H7 in model vegetable fermentations

Authors: DUPREE, DOROTHY - University Of Georgia; Price, Robert; BURGESS, BREANNE - North Carolina State University; ANDRESS, ELIZABETH - University Of Georgia; Breidt, Frederick

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2018
Publication Date: 4/1/2019
Citation: Dupree, D., Price, R.E., Burgess, B., Andress, E., Breidt, F. 2019. Effects of sodium chloride or calcium chloride concentration on the growth and survival Escherichia coli O157:H7 in model vegetable fermentations. Journal of Food Protection. 82(4):570-578. https://doi.org/10.4315/0362-028X.JFP-18-468.
DOI: https://doi.org/10.4315/0362-028X.JFP-18-468

Interpretive Summary: Vegetable fermentations are becoming very popular in the US. These fermentations have been done for thousands of years, and are not known to cause disease, although some people in the last few years have gotten sick from lightly fermented vegetables that are not very acidic. There is relatively limited research published describing how quickly disease-causing bacteria are killed during fermentation of vegetables. We studied the effects of brining conditions, such as salt type and concentration on the survival of one of the most acid resistant pathogens in vegetable fermentations, Escherichia coli O157:H7. This bacterium has caused disease in many different food products but its acid resistance makes it especially of concern for fermented foods. We found that salts principally may help promote the growth of the lactic acid bacteria that naturally are present in vegetable fermentation more than the growth of the E. coli. These data are important for novel commercial cucumber fermentation technology that uses low salt (calcium salt) fermentations in place of high sodium salt fermentations. The data will be useful to producers, regulators and scientists to help understand the factors leading to safe fermentations.

Technical Abstract: Salt concentration has long been considered an important factor for the quality of fermented vegetable products, but the role of salts in bacterial growth and death during vegetable fermentation remains unclear. We compared the effects of various sodium chloride (NaCl) concentrations, including 1 M (6%) NaCl used in commercial cucumber fermentations and 0.34 M (2%) NaCl used in cabbage and other ready-to-eat vegetable fermentations, on the growth and death of lactic acid bacteria (LAB) of the genus Lactobacillus and pathogenic Escherichia coli (Shiga toxin–producing E. coli, or STEC) strains. We also investigated calcium chloride salt conditions. calcium chloride is being used at 0.1 M (1.1%) in low-salt commercial cucumber fermentations that lack added NaCl. STEC strains have previously been shown to be among the most acid-resistant pathogens in fermented or acidified vegetables. The data showed that 1.1% calcium chloride, and especially 1% NaCl, had a stimulatory effect on the growth rates of STEC and LAB compared with a no-salt control, but higher NaCl concentrations decreased growth rates for STEC; to a lesser extent, LAB growth rates were also reduced. For most salt concentrations tested, maximum cell densities achieved during growth of STEC were reduced compared with those of the no-salt controls, whereas LAB mostly had cell densities that were similar to or greater than those of the no-salt controls. No consistent pattern was observed when comparing death rates with salt type or concentration for the STEC or LAB cocktails undergoing lactic acid stress (50 or 350 mM, respectively) at pH 3.2 and when comparing STEC survival in competitive culture experiments with LAB. For vegetable fermentation safety concerns, the results suggest that an important effect of salt addition is enhancement of the growth of LAB compared with STEC strains. Further research will be needed to determine factors influencing STEC survival in competition with LAB in vegetable fermentations.