To test the performance of the Listeria isolation methods, reference samples consisting of gelatin capsules filled with spray-dried milk powder containing Listeria have been developed. During the spray-drying process the Listeria cells are exposed to heat stress and are susceptible to osmotic stress during the reconstitution procedure. To limit the effect of osmotic shock, the milk powder has to be encapsulated in gelatin in order to guarantee slow dissolution. Furthermore, the capsules have to be preen-riched in a nonselective medium. The practical consequences of these findings are discussed.

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