Studies on the Psychrotrophic Bacteria of Dairy Plant Milk

Abstract

The conditions of contamination with bacteria grown at 5°C for 10 days, 20°C for 4 days, and 35°C for 2 days incubations of raw milk obtained at 2 dairy plants, were studied on 563 samples. After that, 393 strains isolated at 5°C incubation from raw milk, 302 strains at 20°C and 362 strains at 35°C were identified and were tested their growth abilities at 0°C for 14 days, 5°C for 10 days, 20°C for 4 days, and 35°C for 2 days incubations respectively.The results obtained are summarized as follows:1) Bacterial counts of most of raw milk samples were ranged from the level of 103/ml to 106/ml at 5°C incubation, and, from the level of 104/ml to 106/ml at both 20 and 35°C incubations. The number of samples with bacterial counts more than 106/ml was many at 20°C incubation, without the distinctions of season and milking method. On comparison of bacterial counts of samples from hand-milking and milker-milking, their bacterial counts differed in area and season. But it showed a tendency that the number of samples with bacterial counts more than 106/ml was many in milker-milking.2) Total strains of gram-negative bacilli such as Pseudomonas, coliform bacteria, Flavobacterium etc. occupied 80.8% of all strains isolated at 5°C incubation of raw milk. In strains isolated at 20°C incubation, total strains of gram-negative bacilli such as Flavobacterium, coliform bacteria, Pseudomonas etc. were 69.0%. On the other hand, in strains isolated at 35°C incubation, total strains of cocci consisted of Streptococcus mostly were 55.2%, and gram-negative bacilli were only 30.7%.3) Out of 393 strains isolated at 5°C incubation, all but one were well grown at 20°C in growth-test. One hundred and ninety six strains (64.9%) of 302 strains isolated at 20°C incubation were well grown at 5°C. Ninety five point nine percent of 362 strains isolated at 35°C incubation was well grown at 20°C.4) The method at 20°C for 4 days incubation is supposed as more superior than the one at 35°C for 2 days incubation to estimate the viable bacterial counts including both psychrotrophs and mesophiles in raw milk, especially in raw milk arrived at milk plant.