The use of the bacteriocin, nisin, as a preservative in ricotta-type cheeses to control the food-borne pathogen Listeria monocytogenes

The efficacy of nisin to control the food-borne pathogen Listeria monocytogenes in ricotta-type cheeses over long storage (70 d) at 6-8 degrees C was determined. Cheeses were prepared from unpasteurized milk by direct acidification with acetic acid (final pH 5.9) and/or calcium chloride addition during heat treatment. Nisin was added in the commercial form of Nisaplin(R) pre-production to the milk. Each batch of cheese was inoculated with 1.0(2)-10(3) cfu g(-1) of a five-strain cocktail of L. monocytogenes before storage. Shelf-life analysis demonstrated that incorporation of nisin at a level of 2.5 mg l(-1) could effectively inhibit the growth of L. monocytogenes for a period of 8 weeks or more (dependent on cheese type). Cheese made without the addition of nisin contained unsafe levels of the organism within 1-2 weeks of incubation. Measurement of initial and residual nisin indicated a high level of retention over the 10-week incubation period at 6-8 degrees C, with only 10-32% nisin loss.