Thermal inactivation of Mycobacterium paratuberculosis, a suspected hu
man pathogen, was determined in ultrahigh-temperature whole milk. Thre
e strains of M. paratuberculosis were examined for survival at tempera
tures from 55 to 75 degrees C using a submerged glass capillary tube m
ethod, Clumped and declumped suspensions of the cultures were used to
determine the rate of heat inactivation and survival at pasteurization
temperatures. Methods for declumping M. paratuberculosis included the
use of glass beads, vortexing and passing the cells through a 26-gaug
e needle. The latter procedure was found to be superior over other met
hods and did not affect the viability of cells. Capillary tubes filled
with milk containing 4 x 10(6) to 3 x 10(7) CFU/ml were heated at tem
peratures ranging from 55 to 75 degrees C. At 55 degrees C, minimal th
ermal inactivation was observed for clumped and declumped cells. At 58
degrees C, thermal inactivation ranging from 0.3 to 0.7 log reduction
was observed for both clumped and declumped suspensions. D values at
60 degrees C ranged from 8.6 to 11 min and 8.2 to 14.1 min for clumped
and declumped cells, respectively. At 63 degrees C, the D values rang
ed from 2.7 to 2.4 and 1.6 to 2.5 min for clumped and declumped cells,
respectively. Survival of M. paratuberculosis at initial levels rangi
ng from 44 to 10(5) CFU/ml at pasteurization treatment (63 degrees C f
or 30 min and 72 degrees C for 15 s) was also determined. No survivors
were observed after incubating plates for up to 4 months on Middlebro
ok 7H11 agar and up to 2 months on Herrold's egg yolk medium. The sens
itivity of the plating method was 1 CFU/250 mu l. These results demons
trate that low levels of M. paratuberculosis, as might be found in raw
milk, will not survive pasteurization treatments.