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

Abstract

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.