The thermal resistance of Listeria monocytogenes strain Scott A that had been associated with a recent milkborne outbreak of listeriosis was determined in whole and skim milk, heavy cream, and ice cream mix. L. monocytogenes suspended at concentrations of approximately 1 × 10 5 cells/ml was heated at temperatures ranging from 52.2 to 79.4°C at various contact times. The D 71.7°C values computed for milk samples ranged from 0.9 to 2.7 s. The D 7.94°C value in ice cream mix was 0.5 s. The z D value for fluid products ranged from 5.8 to 7.1°C; the z F value for ice cream mix was 7.0°C. The L. monocytogenes suspensions would not survive a proper pasteurization process given to raw dairy products.
The thermal resistance of Salmonella typhimurium cultures that had been associated with a major milkborne oubreak of salmonellosis was determined in raw whole milk. Thirteen patient stool isolates and 24 implicated pasteurized milk isolates at concentrations of 1 × 10 5 /ml were screened for heat resistance at 51.8°C. A representative milk strain was heated in replicate at four temperatures from 51.8 to 68.3°C. The z D value was calculated to be 5.3°C. Mean D-value estimates at 51.8°C were 24.0 and 22.8 min for patient and milk isolates, respectively. Extrapolated D 71.7°C values were 0.24 and 0.22 s, and did not differ significantly (α = 0.05). These isolates would not survive proper pasteurization.
The thermal resistance of Listeria monocytogenes associated with a milkborne outbreak of listeriosis was determined in buffer and whole milk. Thermal resistance was stable over a 2-year period and could not be altered by selecting heat-stressed survivors. The rate of inactivation was linear and did not differ significantly between pH 5.5 and 9.0. When portions of whole milk containing 1 × 10 5 cells of L. monocytogenes / ml were heated at seven temperatures from 52.2 to 74.4°C, the D-values ranged from 1683.7 to 0.7 s, respectively. The z D -value was 6.3°C. The D-value at 71.7°C was 0.9 s. L. monocytogenes would not survive the pasteurization process.