Absence of growth ofListeria monocytogenesin naturally contaminated Cheddar cheese

Abstract

Each cheese producer is responsible by the legislation for the number ofListeria monocytogenesin cheese and is required to prove that numbers will not exceed 100 cfu/g throughout the shelf-life of the cheese. Even in the case of hard-cheese such as Cheddar cheese, the absence of growth ofList. monocytogenesduring ripening has to be demonstrated to comply with EU legislation. Studies dedicated to assessingList. monocytogenesgrowth throughout cheese shelf-life are generally based on artificially contaminated cheeses. Contrary to the majority of works, the current study focused on the growth ofList. monocytogenesin naturally contaminated raw milk farmhouse Cheddar cheeses during a five-month ripening period.List. monocytogenesgrowth was assessed by direct count and its presence was detected by enrichment in two naturally contaminated cheese batches. In order to track routes of contamination, 199 processing environment samples from inside and outside the processing facility were taken, and their analysis for the presence ofList. monocytogeneswas performed on four occasions over a 9-month period.List. monocytogenesisolates were differentiated using PFGE and serotyping.List. monocytogenesnever exceeded 20 cfu/g in the cheeses and could not be detected after five months of ripening. Eleven pulsotypes were identified. One pulsotype was found in the yard outside the processing facility, in a vat, on the processing area floor and in a cheese. This indicated that the outside environment constitutes a potential source of contamination of the processing environment and of the cheese. These results demonstrate that this farmhouse Cheddar cheese does not supportList. monocytogenesgrowth and suggests that the efforts to reduce processing environment contamination are worthwhile.