Survival kinetics of Mycobacterium bovis during manufacture and ripening of raw milk Cheddar and Caerphilly cheese produced on a laboratory-scale

Abstract

Persistence of Mycobacterium bovis was investigated in UK raw milk cheeses. Replicating traditional cheese production methods under stringent CL3 containment conditions, Cheddar and Caerphilly cheeses were produced with Myco.bovis inoculated raw milk. High-inoculum investigations used three Myco.bovis genotypes; later low-inoculum investigations used only Myco.bovis AF2122/97. High-inoculum Cheddar (n=9) and Caerphilly (n=9) were matured for a minimum of 12 and 4months respectively; maturation of low-inoculum Cheddar (n=3) and Caerphilly (n=3) was up to 11weeks. Survival of Myco.bovis was monitored by enumeration at different points throughout cheese manufacture and ripening. D values were calculated as follows: 57 and 59days in high-inoculum Cheddar and Caerphilly, respectively, and 41 and 24days in low-inoculum Cheddar and Caerphilly respectively. Mycobacterium bovis is concentrated in cheese curd and a proportion lost with the whey. Reduction in viability during manufacturing is limited, while significant Myco.bovis inactivation occurs during maturation. Inactivation was improved, during Caerphilly ripening, when acid development was enhanced by increasing the proportion of starter culture. Mycobacterium bovis inactivation data obtained could be used to inform assessment of the risk posed to consumers by raw milk dairy products.