Addendum: Survie de Staphylococcus aureus au cours de la préparation et de la conservation de lait écrémé en poudre. Problèmes posés par le dénombrement des survivants

Abstract

Skim milk powders were contaminated with seven strains of Staphylococcus aureus previously isolated from dairy products. These organisms were either added to the concentrated milk before spray-drying or directly to the milk powder. Survival under spray-drying conditions similar to those used in commercial operations was dependent on drying conditions and on the test strain; 1/5 to 1/250 000 of the original populations could be recovered. Survival during storage of milk powder at room temperature depended on the test strain, moisture content, and the method used to contaminate the powder. Our results suggest that powder stored at room temperature for more than 3 months may contain enterotoxin but no detectable surviving S. aureus.Of the different media tested, direct inoculation of Baird–Parker agar and PPSA supplemented with Tweens reliably detected maximal numbers of viable cells. Other selective counting methods gave variable results dependent on the test strain, the age of the powder, and the method by which the powders were contaminated.The nature of cellular damage depended on whether or not the organisms had been exposed to the spray-drying process. Enumeration of populations in spray-dried samples was "superior" when selective media were supplemented with egg yolk or Tweens. [Translated by the journal]