6 Dairy Bacteriology. 



concentrated to permit of bacterial growth, but if diluted 

 considerably, rapidly undergoes fermentation. The keep- 

 ing quality of these products is dependent upon this prop- 

 erty. 



Chemical reaction of medium. As a rule, bacteria prefer 

 a slightly alkaline to an acid medium, but there are so 

 many exceptions to this rule, that the force of it as a gen- 

 eral statement is not very strong. Those organisms that 

 are normal inhabitants of milk are, as a rule, less sus- 

 ceptible to slight variations in the reaction of the food 

 medium than many others. 



11. 2. Temperature. A certain degree of heat is ab- 

 solutely necessary before the spores of bacteria can ger- 

 minate, just as seed grain will not sprout when the ground 

 is too cold. As the temperature of a fluid increases, the 

 rapidity with which the bacteria multiply also increases 

 for a time. Beyond a certain point, however, a heat rigor 

 sets in that destroys the activity of the protoplasm. There 

 is therefore, an optimum or best temperature for growth, 

 and minimum and maximum points as well, below and above 

 which, development is impossible. These three cardinal 

 growth points vary considerably with different germs. 



The temperature limits of growth, i. e., the range be- 

 tween the maximum and minimum points of development, 

 are much wider with bacteria than with almost any other 

 forms of living matter. For this reason, bacteria are 

 more widely distributed than any other class of living 

 organisms. 



Some forms thrive at 32 F., while others are able to 

 grow at temperatures approximating 140 F. With the 

 great majority of bacteria, especially those growing in 

 milk, this range is not so great. Most milk bacteria fail 

 to develop at a point below 40-50 F . , while the maximum 

 growth point does not exceed 105-110 F., the optimum 



