Ml. Stecchini et al., BEHAVIOR OF LISTERIA-MONOCYTOGENES IN MOZZARELLA CHEESE IN PRESENCE OF LACTOCOCCUS-LACTIS, International journal of food microbiology, 25(3), 1995, pp. 301-310
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.