Bacterial tracking in a dairy production system using phenotypic and ribotyping methods

A systematic sampling plan was designed to collect raw and pasteurized milk samples throughout a single-raw milk source, dairy-processing operation experiencing reduced product shelf lives due to bacterial contamination. The objectives were to track bacterial contamination sources throughout a complete dairy production system and use this information to reduce bacterial spoilage losses in processed fluid products. Over a 5-week period, 233 bacterial isolates were collected, representative of different colony morphologies on psychrotrophic bacteria count (PBC) plates. Forty-five isolates (19%) were obtained from pasteurized milk and 188 (81%) were isolated from raw product. Thirty isolates were identified as Pseudomonas spp. by Gram stain and biochemical methods. Of these, 27 (90%) were postpasteurization isolates and 3 (10%) were raw milk isolates. Automated ribotyping revealed that raw and pasteurized Pseudomonas fluorescens isolates were indistinguishable (similarity index > 0.93), suggesting the possibility of postpasteurization contamination with bacteria from raw product. In the plant, filler nozzles were identified as the primary reservoirs of postpasteurization contamination. Nozzle replacement produced significantly lower finished-product PBCs at 7 days postprocessing (> 4-log reduction) and extended fluid product shelf life.