Effect of higher pasteurization temperatures, and longer holding times at 72°C, on the inactivation of Mycobacterium paratuberculosis in milk

Raw cow's milk spiked with 10⁶ cfu ml^-1 of Mycobacterium paratuberculosis was subjected to heat treatments of 72, 75, 78, 80, 85 or 90°C for 15 s, and 72°C for 20 and 25 s, using laboratory pasteurizing units. Three bovine strains of Myco. paratuberculosis were studied (NCTC 8578, B2 and DVL 943). Each strain was subjected to all the heat treatments indicated on three separate occasions. Although each of the heat treatments achieved a substantial (5-6 log₁₀) reduction in numbers of viable Myco. paratuberculosis, small numbers of the organism (4-16 cfu 10 ml^-1) survived in a proportion of the milk samples at each of the higher temperatures investigated, right up to 90°C for 15 s. A longer holding time of 25 s at 72°C was found to be more effective at inactivating Myco. paratuberculosis. Only one of the three strains studied, B2, yielded small numbers of survivors after heating at 72°C for 20 s, but it was completely inactivated by extending the holding time at 72°C by a further 5 s to 25 s. It was concluded that a longer holding time is more likely to achieve the complete inactivation of Myco. paratuberculosis in milk than a higher pasteurization temperature.