Contamination of milk with Gram-negative spoilage bacteria during filling of retail containers

Pasteurised milk can become recontaminated by Gram-negative psychrotrophic bacteria (GNP) during the automatic filling of the retail packages. The immediate sources of contaminating bacteria have been identified and means of reducing the recontamination have been examined. Retail packages with pasteurised milk, and water samples from different water sources associated with filling equipment, were collected throughout the production day in two dairy plants. The water samples were inoculated to sterile milk and were together with the milk packages incubated at 7°C until the aerobic plate count reached 106–107 cfu/ml, or the number of Gram-positive spore-forming bacteria (GPS) exceeded 104 cfu/ml. Colonies were randomly selected and identified as GNP or GPS. Gram-negative isolates were genomically typed by the polymerase chain reaction-based (PCR) method randomly amplified polymorphic DNA (RAPD). The RAPD-typing traced several contaminating GNP to the waste water at the bottom of the filling machine and to condensed water on the filling nozzles. GNP isolates of identical RAPD-type were found in milk and water samples collected at different sampling occasions in one of the dairy plants. Three to 6 months had passed between these sampling occasions. Rinsing with water inside the filling machine or in its immediate surrounding, to remove milk residues associated with start-up, product changes and processing failures, was minimised on three sampling occasions. On another two sampling occasions the production was run under normal conditions. The rinsing with water gave rise to significantly increased recontamination frequency of GNP.