ABSENCE OF GROWTH OF LISTERIA MONOCYTOGENES IN NATURALLY CONTAMINATED CHEDDAR CHEESE.

In the EU, each cheese producer is responsible by legislation for the number of Listeria monocytogenes in cheese and is required to prove that numbers will not exceed 100 colony forming units (cfu) per gram throughout the shelf-life of the cheese. Even in the case of hard-cheese such as Cheddar cheese, the absence of growth of List. monocytogenes during ripening has to be demonstrated to comply with EU legislation. Studies dedicated to assessing List. monocytogenes growth throughout cheese shelf-life are generally based on artificially contaminated cheeses. Contrary to the majority of monitoring research, a study by Marion Dalmasso and Kieran Jordan focused on the growth of List. monocytogenes in naturally contaminated raw milk farmhouse Cheddar cheeses during a five-month ripening period. The results were published in the Journal of Dairy Research, Volume 81 of 2014, pages 46 to 53, under the title: Absence of growth of Listeria monocytogenes in naturally contaminated Cheddar cheese.                                                                                                                                 

Listeria monocytogenes growth 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 of List. monocytogenes was performed on four occasions over a 9-month period.                                                                      

 Listeria monocytogenes counts never exceeded 20 cfu per gram in the cheeses and could not be detected after five months of ripening. Eleven specific types were identified. One specific type each 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 support List. monocytogenes growth and suggests that the efforts to reduce processing environment contamination in the EU are worthwhile.

One wonders what the situation would be in South Africa, since Listeria monocytogenes is such a dangerous organism. Do we monitor often enough?