Background

The disputed link between the presence of Mycobacterium avium paratuberculosis (Map) in foods and the occurrence of Crohn's disease in humans is of continuing concern for food manufactures. This is especially the case in light of conflicting reports of the sensitivity of Map to inactivation in the industrial pasteurisation process. Heat sensitivity of various mycobacterial species has been the subject of numerous studies in the past, though many have relied on the holder method of laboratory pasteurisation (62.5°C for 30 min) which has been criticised as not reflecting the industrial process.

Materials and Methods

In this study, a number of mycobacterial species including Map, M. fortuitum and M. smegmatis, inoculated into low thermoduric count milk, were heated in a pilot plant pasteuriser under high temperature short time (HTST) conditions (72.5°C for 27s) and survival of the mycobacterial species determined. Pasteuriser performance was carefully monitored to ensure correct operation. Recovery conditions for the heat stressed bacteria were optimised to ensure detection of any sublethally injured cells. This involved no decontamination step following pasteurisation and a minimum recovery period of 24 hours at 4°C prior to assay. Enterococcus faecium, a recognised thermoduric species, was included in the study and its survival compared with that of the mycobacteria.

Results and Discussion

Under HTST conditions no surviving mycobacteria were detected in milk containing an initial inoculum of 10⁴-10⁵ cfu/ml. In contrast, under similar conditions E. faecium was readily detected and survivors increased two fold following a recovery period of 24h at 4 C. Partial kinetic data on inactivation were obtained for the mycobacterial and enterococcus species at 65°C and 70°C, respectively. The D65 value for the mycobacteria ranged from 10-28s and the D70 value was 11s for E. faecium. These results show a minimum 4 log reduction for mycobacteria at HTST conditions suggesting that current milk pasteurisation procedures are effective, considering numbers of Map in raw milk are typically less than 10² cfu/ml.