BEHAVIOR OF LISTERIA-MONOCYTOGENES IN MOZZARELLA CHEESE IN PRESENCE OF LACTOCOCCUS-LACTIS

The behavior of Listeria monocytogenes (Scott A) on fully processed It alian Mozzarella cheese was examined in presence and in absence of bac teriocins produced by Lactococcus lactis ssp. lactis strains (DIP 15 a nd DIP 16). These strains, isolated from raw milk, produced heat stabl e bacteriocins that were inactivated by pronase, alpha- chymotrypsin a nd proteinase K, but not by pepsin, trypsin and catalase. The addition of crude bacteriocins to the growing culture of Listeria monocytogene s resulted in a significant reduction in cell number at 5 degrees C, b ut not at 30 degrees C. Mozzarella cheese was inoculated with the List eria culture to obtain an initial level of approximately 30 CFU/cm(2) surface of Mozzarella and approximately 10(3)CFU/ml of the surrounding fluid and then packaged in bags containing the heat-treated neutraliz ed-cultures of Lactococcus lactis ssp. lactis in skim milk (in Italy, Mozzarella is sold in small size pieces, individually packaged in bags containing some fluid). Bags were stored at 5 degrees C up to 21 days . The presence of bacteriocins resulted in apparent death of Listeria monocytogenes after 24 h storage. After 7 days of storage, a revival o f Listeria monocytogenes was observed, followed by an increase in numb er. However, for a storage period of 2-3 weeks the number of Listeria monocytogenes remained significantly below the number observed for Moz zarella cheese packaged in absence of the heat-treated cultures of Lac tococcus lactis.