Physico-chemical characteristics of Belgian artisanal cheeses and prevalence of Listeria monocytogenes in these products

Introduction Regulation (EC) N°2073/2005 demands the absence of L. monocytogenes in 25g of cheeses theoretically allowing the growth of the bacterium. It is assumed that the bacterium cannot develop if the pH is under 4.4 or if the water activity (aw) is under 0.92. Furthermore, a combination of pH<5 and aw<0.94 could also prevent its growth (1). A study was performed among Belgian artisanal cheeses in order to investigate their physico-chemical properties, as well as the prevalence of L. monocytogenes. Materials and Methods Sixty-five Belgian artisanal cheeses were collected, respecting geographical dispersion and taking into account several parameters, including type of milk, milk heat treatment, ripening and texture. Salt, fat and dry matter content, as well as aw in the core and pH in the core and on the surface were measured for all samples. The presence of L. monocytogenes in 25g was also tested, with an enumeration in case of detection. For one highly contaminated cheese, the whole batch (10 pieces) was stored until the end of shelf-life. Enumeration was then done with 30 repetitions. Discussion Regarding physico-chemical characteristics, fresh, soft and hard cheeses, pH in the core ranged respectively from 4.5 to 4.7, from 4.7 to 6.9 and from 4.7 to 6.0. On the surface, pH of some cheeses was higher than 7, probably due to the proteolytic activity of the surface microflora. Regarding aw, values were always >0.95. Only based on these pH and aw values, all cheeses from the panel have to be considered as risky regarding L. monocytogenes. L. monocytogenes was isolated from 2 of the 65 cheeses (3.07%).The first one was a semi-hard soft cheese from raw cow milk, for which enumeration was under the LOQ. The second one was a mold-ripened soft cheese from raw goat milk. Enumeration was higher than 4 log cfu/g. This contamination is really high but not really surprising, given that this type of cheese is known for supporting the growth of the pathogen (2). At the end of shelf-life, 14 of 30 samples from this batch were still positive, with levels of contamination ranging from 2 to 6 log cfu/g. A great heterogeneity in the distribution of L. monocytogenes was observed between pieces from this batch, but also inside some pieces. For instance, 3 repetitions in one cheese gave levels of the pathogen ranging from 10 to 17 000 cfu/g.