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 imm ediate sources of contaminating bacteria have been identified and means of reducing the recontamination have been examined. Retail packages with paste urised milk, and water samples from different water sources associated with filling equipment, were collected throughout the production day in two dai ry plants. The water samples were inoculated to sterile milk and were toget her with the milk packages incubated at 7 degrees C until the aerobic plate count reached 10(6)-10(7) cfu/ml, or the number of Cram-positive spore-for ming bacteria (GPS) exceeded 10(4) 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 pol ymorphic DNA (RAPD). The RAPD-typing traced several contaminating GNP to th e waste water at the bottom of the filling machine and to condensed water o n the filling nozzles. GNP isolates of identical RAPD-type were found in mi lk and water samples collected at different sampling occasions in one of th e dairy plants. Three to 6 months had passed between these sampling occasio ns. Rinsing with water inside the filling machine or in its immediate surro unding, to remove milk residues associated with start-up, product changes a nd processing failures, was minimised on three sampling occasions. On anoth er two sampling occasions the production was run under normal conditions. T he rinsing with water gave rise to significantly increased recontamination frequency of GNP. (C) 2000 Elsevier Science B.V. All rights reserved.