Case study of contamination by Listeria monocytogenes in raw goat milk cheese: development of a quantitative risk assessment model of the production chain

Introduction: Quantitative Risk assessment could be applied in industries as a tool to control and manage the safety of food products.
Purpose: A contamination by Listeria monocytogenes of cheeses made from raw milk was reported by the Belgian food agency. This contamination was caused by the presence of an asymptomatic “shedder” goat in the herd. With field and laboratory collected data, a quantitative risk assessment model of the production chain was developed.
Methods: A modular risk model was built to simulate the food production pathway covering the milking of goats until the final product for the customers. A dynamic square root model was used to predict the growth rate in relation with the temperature, the pH and the water activity along the production chain with predictive microbiology modules.
Results: The shedder goat was identified from the herd and milk samples were taken from the two different parts of the mammary gland with 2.6 log cfu (colonies forming units) Listeria monocytogenes/ml for the right part and absence in 25 ml for the left part of the mammary gland. Numbering of Listeria monocytogenes was carried out on the final products with 3.6 log cfu/g in the fresh not ripened cheeses. The modular risk assessment shows a significant growth of Listeria monocytogenes during chilling and storage of the milk collected the day before the cheese production (increase of 0.6 log cfu/ml) and during adjunction of ferment and rennet to milk (increase of 0.8 log cfu/ml). The model confirms the results obtained in the final products.
Significance: The modular risk model gives valuable informations to identify and to control critical steps in the food production chain of goat cheese made from raw milk. Only one shedder goat can cause a high risk to become ill for the consumers of contaminated products.