The use of challenge studies to explore the fate of Listeria monocytogenes in artisanal cheese varieties from Belgium

Most cheese varieties are considered as allowing the growth of L. monocytogenes, and producers must guarantee its non-detection in 25 g before commercialization. Besides safety concerns, detection of L. monocytogenes can lead to harmful moral and economic consequences for producers. Defining more accurately the growth of L. monocytogenes in different types of cheese is a priority. Currently, challenge studies are the most appropriate method for this purpose. Here, the growth potential of L. monocytogenes was assessed in 32 cheese varieties using challenge studies: unripened acid-curd cheeses, mold-ripened soft cheeses, smear-ripened soft cheeses, and Gouda-type and Saint-Paulin-type semi-hard cheeses. The number of batches to include in challenge studies was determined using Sym’Previus, a predictive microbiology software. Following guidelines provided by European Union Reference Laboratory for L. monocytogenes, a cocktail of three strains was inoculated in cheese, targeting a level of 100 cfu/g. Samples were stored at 8±1°C during respective shelf-life. Growth potential was considered as the difference between contaminations at the use-by-date and at day-0. Twenty-three varieties did not allow the growth (growth potential ≤0.5 log cfu/g). Nevertheless, the calculation method recommended by guidelines led to erroneous conclusions when inter-sample variability in L. monocytogenes levels was > 0.5 log cfu/g. Alternative calculation methods could increase accuracy.