Determining the effect of sodium reduction on the survival of Listeria monocytogenes and Bacillus anthracis in Cheddar cheese

Abstract

Sodium reduction from foods has been established as a public health priority. Cheddar cheese is a food targeted for this reduction because of its relatively high salt content of 600 mg sodium/100 g cheese. Among the roles of salt in Cheddar cheese, inhibition of spoilage and pathogenic microorganisms is a critical one. Listeria monocytogenes is a foodborne pathogen commonly found in dairy processing facilities that could easily contaminate Cheddar cheese. Additionally, a bioterrorism agent that is able to survive pasteurization, such as Bacillus anthracis could be intentionally added to cheese. The objective of this study was to determine the impact of sodium reduction on the ability of L. monocytogenes and B. anthracis to survive in Cheddar cheese. Stirred curd Cheddar cheese was manufactured at full, reduced, and low sodium concentrations using two separate, single-strain starter cultures. In the reduced and low sodium treatments, curds received an application of either sodium chloride only, or sodium chloride with potassium chloride (KCl) to replace the effect of sodium chloride on water activity. After manufacture, cheese was analyzed for composition and aged. One week after manufacture, cheese samples were separately inoculated with L. monocytogenes and with spores of B. anthracis (at a level of 4 and 3 log CFU/g respectively), and stored at 4° or 12°C for 27 to 63 days. Microbiological testing of the cheese for pathogen survival, and natural microbial counts took place in three phases. Naturally present microorganisms included total aerobic count, lactic acid bacteria (LAB) count, and and natural spore forming bacteria count The first stage of testing was run immediately after manufacturing, and simulates results of contamination during the cheesemake. During the first phase of testing, the L. monocytogenes population declined by 4 log CFU/g over 60 days of storage at 4°C. No difference based on starter culture or salt treatment was observed. The count of B. anthracis spores declined by 1.25 log CFU/g during this same storage period. The second and third phases were conducted shortly after the completion of phase one and simulate post aging contamination of the cheese—both accidental and intentional. In the second and third phases of testing, the pH of the cheese had risen from an initial measurement of 4.7 to 5.1 and continued to rise throughout testing to 5.3. The survival of L. monocytogenes and B. anthracis was improved during these stages. The population of L. monocytogenes declined by only 1.5 to 2.5 log CFU/g (depending on stage), with few differences in population based on salt treatment. Under these same conditions, the B. anthracis spore counts declined by only 0.5 to 1 log CFU/g (depending on the stage), with few differences based on salt treatment. The survival of total aerobic, LAB and spore-forming bacterial counts was not affected by sodium reduction. These data suggest that the low salt levels used in this study did not affect the survival of L. monocytogenes or B. anthracis at the experimental incubation temperatures, compared to controls. It follows that sodium reduction may not compromise the safety of food products like Cheddar cheese. These results may offer some assurance for the food industry to move forward in meeting sodium reduction initiatives. However, this progression must take place with caution. Although sodium reduction did not enhance the survival of the organisms, L. monocytogenes and B. anthracis were still capable of surviving in Cheddar cheese. This observation stresses the requirement of pH control during cheese manufacture, adherence to good manufacturing practices, careful handling of products to prevent them from becoming contaminated in post processing operations such as slicing or shredding, and protection of product from intentional adulteration.