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

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Abstract
Seven S. aureus strains isolated from milk products were used to contaminate skim milk powder. Contamination occurred either before spray drying by inoculating the culture in concentrated milk or by inoculating the culture directly in the milk powder. During spray-drying, survival depended on the strains and drying parameters. In normal industrial conditions, survival was 1/15th to 1/250,000th depending on the strains. During storage at room temperature, the rate of destruction varied as a function of the strain, water content and means of contamination. Results showed powdered milk stored for more than 3 mo. at room temperature retained enterotoxins while containing no detectable surviving S. aureus. Certain media inoculated directly made it possible to count the maximum surviving S. aureus. Other methods of selective counting varied in effectiveness depending on storage temperature, means of contamination and the strain considered. Damage to S. aureus cells in the milk powder was different for cells subjected to drying than for those not dried. S. aureus subjected to spray-drying were more easily counted if the medium contained egg yolk or some Tweens.