Evaluation of NaCl, pH, and lactic acid on the growth of Shiga toxin-producing Escherichia coli in a liquid Cheddar cheese extract

Abstract

A Cheddar cheese model system, Cheddar cheese extract, was used to examine how different levels of known microbial hurdles (NaCl, pH, and lactic acid) in Cheddar cheese contribute to inhibition of bacterial pathogens. This knowledge is critical to evaluate the safety of Cheddar varieties with altered compositions. The range of levels used covered the lowest and highest level of these factors present in low-sodium, low-fat, and traditional Cheddar cheeses. Four pathogens were examined in this model system at 11°C for 6 wk, with the lowest levels of these inhibitory factors that would be encountered in these products. The 4 pathogens examined were Salmonella enterica, Staphylococcus aureus, Listeria monocytogenes, and Shiga toxin-producing Escherichia coli (STEC). None of these organisms were capable of growth under these conditions. The STEC exhibited the highest survival and hence was used to examine which of these inhibitory factors (NaCl, pH, and lactic acid) was primarily responsible for the observed inhibition. The STEC survival was examined in Cheddar cheese extract varying in NaCl (1.2 vs. 4.8%), lactic acid (2.7 vs. 4.3%), and pH (4.8 vs. 5.3) at 11°C for 6 wk. The microbial hurdle found to have the greatest effect on STEC survival was pH. The interactions between pH and levels of protonated lactic acid and anionic lactic acid with STEC survival was also evaluated; only the concentration of protonated lactic acid was determined to have a significant effect on STEC survival. These results indicate that, of the pathogens examined, STEC is of the greatest concern in Cheddar varieties with altered compositions and that pH is the microbial hurdle primarily responsible for controlling STEC in these products.