Environmental Survey for Listeria Species in California Milk Processing Plants

This survey was undertaken to determine the prevalence of Listeria monocytogenes and Listeria spp. in environmental samples obtained from milk processing plants located throughout California. Identification of Listeria spp. other than L. monocytogenes was not made. Milk processing plants were categorized as to type of product produced and subdivided into areas based on manufacturing activity occurring in that area. A total of 597 environmental samples from 156 plants were analyzed in the six month period of January through July 1987. Listeria spp. were isolated from 75 (12.6%) of the samples. Thirty-eight of the Listeria spp. (50.7%) were identified as L. monocytogenes . Forty-six plants (29.5%) yielded positive results for Listeria spp., of which, 31 plants (19.9%) were positive for L. monocytogenes . Listeria was recovered more frequently from fluid milk product plants and frozen milk product plants. Likewise, Listeria was isolated most frequently from the packaging-filling room location within a plant and least frequently from the raw milk receiving room. Although Listeria spp. was isolated most frequently from the conveyor site, the drain site within a plant location gave a Listeria isolation rate similar to the total number of Listeria positive sites within that plant location.