Development of a quantitative risk assessment for cheese made from raw goat milk contaminated by Listeria monocytogenes

A retrospective study was performed to assess the potential risk of human listeriosis following a contamination by L. monocytogenes of cheeses made from goat raw milk reported by the Belgian Federal Agency for the Safety of the Food Chain in 2005. The source of the contamination was related to a shedder goat, excreting 2.6 log cfu (colonies forming units) L. monocytogenes / ml without any clinical symptom. On the basis of the collected data, a quantitative microbial risk assessment model was developed covering the production chain from the milking of goats until the consumed products. Predictive microbiology models were used to simulate the growth of L. monocytogenes during the process of cheeses made from goat raw milk. The modular exposure assessment model showed a significant growth of L. monocytogenes during chilling and storage of the milk collected the day before the cheese production (increase of 1.7 log cfu/ml for the median) and during the step of starter and rennet adjunction to milk (increase of 0.8 log cfu/ml for the median). The median estimated final result (in the fresh cheese) was equal to 3.5 log cfu/g. The model estimates (expressed as median final result issued from the exposure assessment) were realistic compared to the number of L. monocytogenes measured in the fresh cheese (3.6 log cfu/g) reported during the cheese contamination period. The average number of expected cases of human listeriosis was between 0 and 1 for a high-risk sub-population and 0 for a low-risk healthy sub-population. Scenario analysis was finally performed to identify the most significant factors and aid in developing priorities for risk mitigation. Thus, by using quantitative risk assessment and predictive microbiology models, this study provided valuable information to identify and to control critical steps in a local production chain of goat cheese made from raw milk.