Comparison of Heat Resistance of Listeria monocytogenes in Milk as Determined by Two Methods.

The thermal resistance of 3 strains of Listeria monocytogenes was compared using test tube versus sealed tube methods of thermal inactivation. All L. monocytogenes strains were rapidly inactivated in milk when survival was measured using sealed tube thermal inactivation methods. Calculated D62°C values ranged between 0.1-0.4 min for the three strains tested. In contrast, total inactivation of L. monocytogenes populations using test tube methods of thermal inactivation could not be accomplished within 30 min at 62°C. Extensive tailing of survivor curves was consistently observed. When an initial population of 5 × 106 L. monocytogenes /ml was heated at 72, 82, or 92°C, consistent survival of a population of 102-103 L. monocytogenes /ml after 30 min was observed. The results prove that the test tube method for measuring thermal resistance of L. monocytogenes is inaccurate. Reports of extraordinary heat resistance based upon this method are correspondingly inaccurate. L. monocytogenes cells, dispersed freely in milk, will not survive pasteurization.

Psychrotrophic Growth and Thermal Inactivation of Listeria monocytogenes as a Function of Milk Composition.

Listeria monocytogenes strains 19111, 19113, 19115, F5027 and F5069 were grown in 11% nonfat milk solids, skim milk and whole milk at 4, 10, 22, and 37°C to determine the influence of temperature and milk composition on growth and thermal resistance. Milk composition affected cellular growth. The psychrotrophic growth of L. monocytogenes serotype 4b strains was enhanced in whole milk when compared to skim milk or 11% NFMS. This enhancement of psychrotrophic growth was not observed for serotype 1 or 3 strains. The stimulatory effect of whole milk on serotype 4b L. monocytogenes strains was most dramatic at 10°C where cells increased from 7.9 × 10° to 5.8 × 106 CFU/ml within 48 h. Milk composition did not affect the thermal resistance of L. monocytogenes . All strains used in this study had a D62.7°C value of 1.0 min or less, therefore, pasteurization as defined by current FDA guidelines should eliminate this organism from raw milk with a large margin of safety. Post-pasteurization contamination of dairy products with L. monocytogenes must be eliminated since the psychrotrophic nature of this organism ensures survival and proliferation during refrigerated storage.