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An attempt to show the effect of temperature on the rate of growth of bacteria in milk was made by finding the generation times of Salmonella typhosa, Shigella dysenteriae, Streptococcus haemolyticus, Micrococcus pyogenes aureus, Lactobacillus acidophilus, Escherichia coli, Bacillus subtilis and Alcaligenes faecalis at temperatures ranging from 4 to 60°C. Growth was quantitatively measured by means of plate counts during the logarithmic period which was previously determined for each organism. The following is a summary of the results obtained:
At 4° C. none of the micro-organisms showed evidence of multiplication during the 6hr.incubation.
As temperatures of 5–45° C. were approached the generation time decreased until the optimum temperature for each organism was reached; beyond this point a slowing of growth was observed, until at 60° C. viability was apparently lost by all the organisms.
Streptococcus haemolyticus and Micrococcus pyogenes aureus have a generation time of 37–23 min.
Salmonella typhosa and Shigella dysenteriae have a generation time of 50–26 min.
Escherichia coli and Alcaligenes faecalis have a generation time of 41–16 min.
Bacillus subtilis grows at about the same rate as Alcaligenes faecalis.
The slowest organism of all appears to be Lactobacillus acidophilus with a generation time of 52–125 min.
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