Full scare commercial pasteurization equipment operated at 72–73°C with a holding time of 15–16 s was used to determine the ability of commercial thermal processing to inactivate Listeria monocytogenes strain Scott A. Three methods of providing L. monocytogenes concentration in raw milk were employed: freely suspended (extra-cellular), inside bovine phagocytes (in vitro procedure), and inside bovine phagocytes in experimentally infected cows (in vivo). Three enrichment methods were used to assay for L. monocytogenes after pasteurization: cold enrichment (4°C, 28 d), selective enrichment of Lovett et al. (FDA procedure) (17), and the USDA-FSIS two stage enrichment procedure. In addition, a 1-L sample taken just before the vacuum breaker was incubated undiluted in the original sample container (4°C, 4 weeks). None of the four assay methods could detect Listeria in the pasteurized milk.

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