Effect of turbulent-flow pasteurization on survival of Mycobacterium avium subsp. paratuberculosis added to raw milk

Abstract

A pilot-scale pasteurizer operating under validated turbulent flow (Reynolds number, 11,050) was used to study the heat sensitivity of Mycobacterium avium subsp. paratuberculosis added to raw milk. The ATCC 19698 type strain, ATCC 43015 (Linda, human isolate), and three bovine isolates were heated in raw whole milk for 15 s at 63, 66, 69, and 72 degrees C in duplicate trials. No strains survived at 72 degrees C for 15 s; and only one strain survived at 69 degrees C. Means of pooled D values (decimal reduction times) at 63 and 66 degrees C were 15.0 +/- 2.8 s (95% confidence interval) and 5.9 +/- 0.7 s (95% confidence interval), respectively. The mean extrapolated D72 degrees C was <2.03 s. This was equivalent to a >7 log10 kill at 72 degrees C for 15 s (95% confidence interval). The mean Z value (degrees required for the decimal reduction time to traverse one log cycle) was 8.6 degrees C. These five strains showed similar survival whether recovery was on Herrold's egg yolk medium containing mycobactin or by a radiometric culture method (BACTEC). Milk was inoculated with fresh fecal material from a high-level fecal shedder with clinical Johne's disease. After heating at 72 degrees C for 15 s, the minimum M. avium subsp. paratuberculosis kill was >4 log10. Properly maintained and operated equipment should ensure the absence of viable M. avium subsp. paratuberculosis in retail milk and other pasteurized dairy products. An additional safeguard is the widespread commercial practice of pasteurizing 1.5 to 2 degrees above 72 degrees C.

FIG. 1

Temperature control during a continuous pasteurizer run of whole milk spiked with M. avium subsp. paratuberculosis. Samples for enumeration were taken after the temperature had equilibrated for at least 1 min at each selected temperature.

FIG. 2

RTD E(t) of whole milk at 74°C after pumping at 120 liters/h through a 15 s holding tube (ID, 7.7 mm; Re, 11,050).

FIG. 3

Heat inactivation of M. avium subsp. paratuberculosis. A log₁₀ k plot of strains ATCC 19698, Linda, A, B, and D at 63, 66, 69, and 72°C is shown. Data are from duplicated runs at 63 and 66°C (10 data points each) and at 69°C (1 data point). The absence of data points at 69 and 72°C indicates no survivors.

FIG. 4

Heat inactivation of M. avium subsp. paratuberculosis. A log₁₀ kill plot of strains ATCC 19698, Linda, A, B, and D, each pasteurized at 63, 66, 69, and 72°C for 15 s, is shown. Data are the same as for Fig. 3. The absence of data points at 69 and 72°C indicates that the kill exceeded the >4 log₁₀ to 5 log₁₀ detection limit. Solid line, mean kill; dashed lines, upper and lower 95% confidence intervals.