/102 Dairy Bacteriology. 



tablets to an eight ounce bottle of water. In determin- 

 ing the acidity in each patron's milk, a number of com- 

 mon white cups are used, one for each patron. Two 

 measures full of the alkali solution are placed in each cup, 

 and then as the milk is received at the weigh- can, one- 

 half as much milk is added to the alkali solution in the 

 cup, 1 and the whole gently, but thoroughly shaken. If 

 the pink color of the alkali solution persists even faintly, 

 it shows that there is not enough acid in the milk to neu- 

 tralize the same; if it disappears altogether, leaving the 

 milk white in color, it indicates that there is more acid 

 in the milk than can be neutralized by the alkali of the 

 tablet solution. Any standard desired can be chosen, but 

 where the relation of the milk to the alkali solution is 

 maintained in above ratio (two to one), it indicates 

 that 0.2% acidity is present, if the alkali is completely 

 neutralized. Extended experience has shown the neces- 

 sity of selecting milks for pasteurizing that come within 

 this standard. 



103. Temperature and time limits. The time and 

 temperature limits in pasteurizing are subject to consid- 

 erable variation. The minimum temperature, however, 

 must exceed the thermal death-point at which the milk 

 bacteria are destroyed. As the tubercle bacillus is some- 

 times found in milk, and as it is one of the most resist- 

 ant organisms in its vegetative state that is known, the 

 thermal death-point of this germ serves as a minimum 

 standard for efficient pasteurizing. 



The determination of this point depends on the follow- 

 ing conditions: 



1. The temperature of heat used. 



2. The length of exposure to the heat. 



1 Farrington, Wis. Expt. Stat., Bull. 52. 



