A study on survival of Listeria monocytogenes during manufacture and ripening of Kashar cheese

Abstract

This study was performed to determine the survival of Listeria monocytogenes during the manufacture and ripening of Kashar, a popular cheese in Turkey. Raw milk was inoculated at 4 different levels with L. monocytogenes serotype 4b (ca. 3, 4, 5, and 6 log cfu ml(-1) for group 1, 2, 3 and 4 cheeses, respectively) and made into Kashar cheese using the traditional technique. Following dry salting for 10 days at 18 +/- 2degreesC, the Kashar cheese was ripened for 20 days at the same temperature and then stored at 5 +/- 1degreesC for 3 months. Samples of milk, coagulum, curd and cheese were tested for Listeria numbers by surface plating on Oxford Agar. In all groups the numbers of pathogens remained nearly constant during the initial stages of cheese production. However, an approximately 1 log cfu g(-1) increase was detected in L. monocytogenes counts, while the pH decreased from 6.37-6.40 to 5.03-5.07, during curd acidification. Heat treatment of ripe curd at 75degreesC for 5 min greatly influenced viability, but was not enough to destroy all the bacteria, especially when inoculated at high levels. The pathogen was not isolated on the 60(th) and 120(th) days of ripening in group 2 and 3 cheeses, respectively. Meanwhile. 1.83 log cfu g(-1) bacteria remained viable in group 4 even after 120 days of ripening. As a consequence, it was determined that in conjunction with the heat treatment of curd, a ripening period of 4 months was necessary to eliminate L. monocytogenes when its levels were lower than 6 log cfu ml(-1). It was concluded that the techniques used in the production stages of Kashar cheese are not sufficient to eliminate high contamination (6 log cfu ml(-1)) by L. monocytogenes and the cheese may pose a risk of food-borne listeriosis.