Examination of the biochemical characteristics of bacilli isolated from raw milk
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A total of 114 samples of raw milk were examined for the presence of bacilli. It was found that the level of bacillus contamination in raw milk depended on the place where the samples were taken. Raw milk on the farms was contaminated with bacilli at the average level of 0,92 log, while at collection points the milk contained bacilli at the level of 1,56 log and in the dairy it was also 1.56 log/ml. Bacilli were isolated after warming the milk for 10 min. at 80°C, inoculating a nutrient culture medium and incubation for 48 to 72 hours at 30°C. Bacillus ability to grow in psychrotrophic conditions was determined by storing the thermally treated milk samples for 10 days at the temperature of 7°C. Bacillus multiplication in psychrotrophic conditions varied so the milk samples could be divided into five groups. In the first group it was noticed that bacillus number decreased in psychrotrophic conditions from log 1.57 to log 0.72 which represented a highly significant environmentaly influen...ced difference as determined by the t test. In the second group of milk samples there were bacilli that did not grow and survive in low temperature conditions, and after the milk samples were stored for 10 days at 10°C, the number of bacilli fell to less then 1/ml of milk. In the third group of samples bacilli did not show differences in number before and after storing in psychrotrophic conditions Bacilli grew well at 7°C in milk samples placed in the fourth group and adapted to the environmental difference which was shown by the t test to be very significant. The fifth group also exibited the appearance of psychrotrophic strains because these were detected milk stored psychrotrophic under conditions. Examination of the ability of bacillus to grow depending on the type of medium indicated that out of 135 strains isolated, 133 grew in broth at the temperature of 45°C, 121 strains grew at 55°C, 125 bacillus strains grew in the presence of 7% NaCl and 132 strains grew in broth at pH 5.7. More than half of the examined strains (67.40%) were lecithin positive and 46.15% strains were haemolytic, which indicates the possible presence of bacilli pathogenic for people. Determination of enzyme activity of the isolated bacillus strains, which may affect production processes in the dairy industry, showed that 40,0% degrade starch, 5.92 gelatin and 80.74% of the strains hydrolyzed casein.
Keywords:bacilli / raw milk / thermally treated milk
Source:Acta veterinaria - Beograd, 1998, 48, 1, 59-68
- Univerzitet u Beogradu - Fakultet veterinarske medicine, Beograd