Survival of Listeria monocytogenes During the Manufacture and Storage of Nonfat Dry Milk

Abstract

The ability of Listeria monocytogenes to survive in skim milk during spray drying and to persist in nonfat dry milk during storage was examined. Concentrated (30% solids) and unconcentrated skim milks were inoculated with ca. 105 to 106 L. monocytogenes/ml and spray dried (inlet temperature, 165 ± 2°C; outlet temperature 67 ± 2°C) to a moisture content of 3.6 to 6.4%. The nonfat dry milk was packaged in moisture-resistant film and stored at 25°C for up to 16 wk. A reduction of ca. 1 to 1.5 log10 L. monocytogenes/g occurred during the spray drying process, irrespective of whether the milk was concentrated or not before spray drying. The organism progressively died during storage at 25°C, with a >4-log10 CFU/g decrease occurring within 16 wk of storage.