Elimination of Listeria monocytogenes in a Soft Cheese, Fromage Blanc, Using Processing Methods, Formulation Changes, and Additive Bacteriocin Nisin
Batches of fromage blanc, a soft white cheese were prepared from whole pasteurized cowâs milk. Processing and formulation methods were used in cheese making to reduce Listeria monocytogenes in artificially contaminated cheese. Treatments implemented included use of additional starter culture in formulation (25% more starter culture than original formulation), use of a higher temperature draining process (at 45oC instead of 22oC), addition of the anti-listerial bacteriocin nisin (Danisco Nisaplin) in formulation at different levels (125 ppm, 250 ppm, 400 ppm), and combinations of these treatments. Characteristics including pH, fat content, protein content, and color were evaluated for each treatment cheese. Statistically significant differences (p<0.0001) were found between the population (log CFU/g) values of L. monocytogenes in the different treatment cheeses and control cheese. Treatments using additional starter culture or higher temperature draining alone were not successful in significantly reducing numbers of L. monocytogenes, but when combined, a 1 log reduction resulted. Of the different concentrations of nisin used in cheese formulation, the level of 250 ppm nisin was used in combination treatments. The treatments using 250 ppm nisin were able to reduce numbers of L. monocytogenes by 2 log 24h after addition. Combination treatments with 250 ppm nisin and additional starter culture in formulation reduced the level of L. monocytogenes by only 1 log, while combination treatments coupling 250 ppm nisin with a higher temperature draining and treatments with 250 ppm nisin, additional starter culture, and a higher temperature draining were able to reduce the pathogen by 2 log. There were statistically significant (p<0.0001) differences found between cheese treatments for values of pH, fat content, and protein content. This soft cheese could be standardized for each of these parameters by the processor before packaging and sale of cheese. There were no statistically significant (p>0.05) differences found between colorimetric values for different cheese treatments.