Potential growth of Listeria monocytogenes in Italian mozzarella cheese as affected by microbiological and chemical-physical environment

In the present study, 33 brands of mozzarella cheese (pasteurized cow milk mozzarella obtained by direct acidification through the addition of food-grade citric acid or obtained by natural acidification through the addition of thermophilic starter cultures, mozzarella for pizza mainly obtained by addition of citric acid, and pasteurized buffalo milk mozzarella obtained by adding microbial rennet) were characterized for the factors potentially influencing the growth of Listeria monocytogenes (microbial populations, moisture, pH, and organic acids). Then, the growth potential of L. monocytogenes in mozzarella was investigated by challenge tests performed at different temperatures. The presence of heterogeneous microflora (lactobacilli, streptococci, Pseudomonas spp., and, for buffalo mozzarella, yeasts) was evidenced. Almost all the product typologies were classified as high-moisture mozzarella cheese because moisture was >52%. Moreover, pH varied from 5.32 to 6.43 depending on the manufacturing methodology applied. Organic acid concentrations too showed great variability depending on the mozzarella production method, with values ranging from less than limit of detection (LOD; 16 mg/kg) to 14,709 mg/kg, less than LOD (216 mg/kg) to 29,195 mg/kg, and less than LOD (47 mg/kg) to 1,725 mg/kg in the water phase of lactic, citric, and acetic acids, respectively. Despite this presence, the concentration of undissociated acids was lower compared with the minimum inhibitory concentrations estimated for L. monocytogenes by other authors. This was confirmed by the results of the challenge tests conducted inoculating the pathogen in mozzarella produced with the addition of citric acid, as the microorganism grew fast at each temperature considered (4, 9, 15, and 20°C). Good hygiene practices should be strictly applied, especially with the aim of avoiding postproduction contamination of mozzarella, as the presence of organic acids and microflora is insufficient to prevent L. monocytogenes growth.