Bacteriological Hygiene in the Production of Pasteurised Milk

University dissertation from Food Technology, Lund University, P.O.Box 124, SE-221 00 Lund, Sweden

Abstract: Critical contamination sites and sources for the Gram-negative psychrotrophs (GNP) of the taxa Pseudomonas, Enterobacteriaceae and Aeromonas, and the Gram-positive spore-forming (GPS) species Bacillus cereus, have been traced in the production line of pasteurised milk. Samples of raw and pasteurised milk from different sampling sites along the line were collected. After incubation at 7°C until the total aerobic plate count reached 106 to 107 cfu/ml, or the number of GPS exceeded 104 cfu/ml isolates were picked randomly and identified. All GNP present in the raw milk were properly killed during pasteurisation. A recontamination by GNP was observed in 40% of the consumer packages. This recontamination was focused to the filling procedure, and Pseudomonas was the dominating GNP in all packages. Environmental samples, as water from different water sources in the dairy plant, air collected by a RCS-sampler on water-agar strips, and empty, formed and sealed packages taken from the filling machines, were inoculated with sterile milk. GNP isolates were randomly picked and identified after incubation at 7°C. Samples of water and air collected in the immediate surroundings of the filling machine contained large numbers of GNP causing growth in refrigerated milk. GNP isolates from the milk samples and from the environmental samples were genomically typed by the PCR-based method Randomly Amplified Polymorphic DNA (RAPD). The RAPD-typing showed that there are many different RAPD-types involved in the recontamination of pasteurised milk. Several of the milk contaminating RAPD-types were found in condensed water on the filling nozzles, in waste-water at the bottom of the filling machine, and in air surrounding the filling machine. Some contaminating RAPD-types were present inside the filling machine over long periods, especially at the bottom of the filling machine. The effect on post-pasteurisation contamination by reducing the usage of water around the filling machine was studied. A clear link between the usage of water inside the machine and the frequency of consumer packages contaminated by GNP was found. B. cereus were involved in, and sometimes solely responsible for, the spoilage of pasteurised milk. The B. cereus infection can originate from heat-resistant spores present in the raw milk, but B. cereus can also enter the milk at the dairy plant. By using traditional microbiological methods and RAPD-typing, the pasteurised milk was once found to be contaminated by B. cereus while passing through one pasteuriser, and commonly found to be contaminated during the filling procedure.

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