Evaluation of Listeria monocytogenes populations during the manufacture and vacuum-packaged storage of Kashar cheese

Abstract

Behaviour of Listeria monocytogenes was investigated during the manufacture and storage of Kashar cheese. For this purpose, pasteurized milk was inoculated at four different levels varying between 3 to 6 log cfu.ml(-1) with the pathogen and its survival was determined in milk, coagulum, curd and cheese received from the production steps and at the 1(st) and 7(th) days of the storage. The heat treatment process applied after curd acidification step at 75 degrees C for 5 min was the most efficient barrier and greatly influenced the viability of the bacteria. Nevertheless this step was not enough to destroy all the bacteria especially when inoculated > 3 log cfu.ml(-1) levels. For all the groups, this stage resulted in approximately 3 log decrease. Storage at 6 +/- 1 degrees C had no influence on the viability and the count of pathogen remained nearly constant throughout storage. Results of this study indicate that neither production steps nor storage at 6 +/- 1 degrees C for 7 days in vacuum packages was enough to eliminate L. monocytogenes in Kashar cheese particularly if the initial level was high. Thus, in addition to the pasteurization process, hygienic measures should be taken into account to avoid possible secondary contaminations.