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Title: Antimicrobial effects of weak acids on the survival of Escherichia coli O157:H7 under anaerobic conditions

Author
item LU, HUIYING - North Carolina State University
item Breidt, Frederick
item Perez Diaz, Ilenys
item OSBORNE, JASON - North Carolina State University

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2011
Publication Date: 6/1/2011
Citation: Lu, H.J., Breidt, F., Perez-Diaz, I.M., Osborne, J.A. 2011. Antimicrobial effects of weak acids on the survival of Escherichia coli O157:H7 under anaerobic conditions. Journal of Food Protection. 6:893-898.

Interpretive Summary: Food acids can be used to kill bacteria that may be present on ready to eat (uncooked) foods, including the E. coli and Salmonella strains that have been involved in recent foodborne disease outbreaks. For acidic foods that are not heat processed, including some pickled vegetables, and related products, manufacturers must file a process with FDA that shows sufficient acid is present to kill disease causing bacteria. Depending on conditions, up to six day holding times are required to allow acid killing. Recently we found that the lack of oxygen in sealed jars used for pickled vegetables and juice products unfortunately helps the survival of bacteria in acid solutions. This finding prompted further study of acids under oxygen free conditions. In the current manuscript, we have shown that there is a very large difference (over 100 fold) in the rates at which different food acids kill E. coli in sealed jars with no oxygen present. Some food acids, including commonly used food preservatives were found to be much more efficient at killing E. coli than acetic acid, which is used in most pickled vegetables and other acidic foods. Further work will be needed to understand how these different acids work to kill bacteria. However, application of these results may allow manufactures to significantly reduce the required holding times for acid killing of bacteria during the production of acidified foods.

Technical Abstract: Outbreaks of disease due to vegetative bacterial pathogens associated with acid foods (such as apple cider) have raised concerns about acidified vegetables and related products that have a similar pH (3.2 to 4.0). Escherichia coli O157:H7 and related strains of enterohemorrhagic E. coli (EHEC) have been identified as the most acid resistant vegetative pathogens in these products. Previous research has shown that the lack of dissolved oxygen in many hermetically sealed acid or acidified food products can enhance survival of EHEC compared with their survival under aerobic conditions. We compared the antimicrobial effects of several food acids (acetic, malic, lactic, fumaric, benzoic, and sorbic acids and sulfite) on a cocktail of EHEC strains under conditions representative of non–heat-processed acidified vegetables in hermetically sealed jars, holding the pH (3.2) and ionic strength (0.342) constant under anaerobic conditions. The overall antimicrobial effectiveness of weak acids used in this study was ranked, from most effective to least effective: sulfite > benzoic acid > sorbic acid > fumaric acid > L- and D-lactic acid > acetic acid > malic acid. These rankings were based on the estimated protonated concentrations required to achieve a 5-log reduction in EHEC after 24 h of incubation at 30°C. This study provides information that can be used to formulate safer acidand acidified food products and provides insights about the mode of action of weak acids against EHEC.