Thermal Inactivation of Salmonella Species in Fluid Milk ¹

The thermal resistance of Salmonella senftenberg 775W, Salmonella muenster previously isolated from raw fluid milk, and two mixtures each consisting of ten Salmonella strains commonly isolated from human or non-human sources was tested. Cells were suspended in whole milk at a final concentration of 10⁵ cells/ml. The inoculated milk was thermally processed at temperatures ranging from 60°C to 74°C using a pilot-scale plate pasteurizer unit. The mean and minimum residence time of milk in the holding tube of the pasteurizer was 17.6 and 16.2 s, respectively. The maximum temperature at which viable salmonellae were detected in the human (61.5°C) and non-human (64.5°C) mixtures was considerably lower than that obtained with S. senftenberg 775 W (67.5°C). S. muenster failed to show any milk-adapted response and could not be recovered at temperatures greater than 63.0°C. Treatment at 63°C produced a 4 log₁₀ or greater reduction in the number of viable Salmonella including the heat resistant S. senftenberg 775 W, and a minimum 2 log₁₀ decrease at 60°C. These findings warrant caution in the use of subpasteurizing temperatures for thermal processing of fluid milk.