Effect of Commercial-Scale High-Temperature, Short-Time Pasteurization on the Viability ofMycobacterium paratuberculosisin Naturally Infected Cows' Milk

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Abstract
Raw cows' milk naturally infected withMycobacterium paratuberculosiswas pasteurized with an APV HXP commercial-scale pasteurizer (capacity 2,000 liters/h) on 12 separate occasions. On each processing occasion, milk was subjected to four different pasteurization treatments, viz., 73°C for 15 s or 25 s with and without prior homogenization (2,500 lb/in2in two stages), in an APV Manton Gaulin KF6 homogenizer. Raw and pasteurized milk samples were tested forM. paratuberculosisby immunomagnetic separation (IMS)-PCR (to detect the presence of bacteria) and culture after decontamination with 0.75% (wt/vol) cetylpyridinium chloride for 5 h (to confirm bacterial viability). On 10 of the 12 processing occasions,M. paratuberculosiswas detectable by IMS-PCR, culture, or both in either raw or pasteurized milk. Overall, viableM. paratuberculosiswas cultured from 4 (6.7%) of 60 raw and 10 (6.9%) of 144 pasteurized milk samples. On one processing day, in particular,M. paratuberculosisappeared to have been present in greater abundance in the source raw milk (evidenced by more culture positives and stronger PCR signals), and on this occasion, survivingM. paratuberculosisbacteria were isolated from milk processed by all four heat treatments, i.e., 73°C for 15 and 25 s with and without prior homogenization. On one other occasion, survivingM. paratuberculosisbacteria were isolated from an unhomogenized milk sample that had been heat treated at 73°C for 25 s. Results suggested that homogenization increases the lethality of subsequent heat treatment to some extent with respect toM. paratuberculosis, but the extended 25-s holding time at 73°C was found to be no more effective at killingM. paratuberculosisthan the standard 15-s holding time. This study provides clear evidence thatM. paratuberculosisbacteria in naturally infected milk are capable of surviving commercial high-temperature, short-time pasteurization if they are present in raw milk in sufficient numbers.