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Critical contamination sites of the gram-negative psychrotrophic bacterial groups, Pseudomonas, Enterobacteriaceae and Aeromonas and the gram-positive spore-forming bacterium, Bacillus, have been traced in the production lines for pasteurised milk in three dairy plants, situated in Sweden or in Norway. Samples of raw and pasteurised milk were collected at six sites along the line. All milk samples were incubated at 7°C until the aerobic plate count had reached 106–107 cfu mL-1 or the number of gram-positive spore-forming bacteria had exceeded 104 cfu mL-1. Colonies were picked randomly and identified. Nearly all recontamination by gram-negatives occurred in the filling step. Gram-negative bacteria grew to high numbers during incubation in about 40% of the sampled milk packages. Pseudomonads were isolated from all of these packages (100%), Enterobacteriaceae from 9% and aeromonads from 3% of the packages, i.e. these gram-negative bacteria constituted a major part of the bacteria that actually spoiled the milk upon prolonged storage. The rest of the packages, 60%, were spoiled by gram-positive spore-forming bacteria like Bacillus. Although spores of Bacillus are present in raw milk and survive pasteurisation, some indications of post pasteurisation recontamination were found as well. |
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