190 Reductase, Catalase, Etc. 



The degree of acidity of milk depends on the lactation period. 

 Colostrum, milk of animals with affected udders, and milk from 

 freshly milking cows have an abnormally high acidity, while milk 

 from animals in the last stages of lactation, and sometimes milk 

 from affected udders, may be lower than normal. 



Besides these factors the degree of acidity of milk is also 

 influenced by the growth of bacteria, the species of bacteria, and 

 therefore by all factors which have an influence on the bacterial 

 growth, such as cleanliness in milking, cooling, outside tempera- 

 ture, age of the milk, etc. 



Plaut, for instance, demonstrated that milk which has been kept 

 At a temperature of showed multiplication of and voluntarily coagulated 



bacteria after after 



10 deg. C. 48 to 72 hrs. 100 hrs. 



15 deg. C. 20 to 24 hrs. 63 hrs. 



20 deg. C. 12 to 20 hrs. 48 hrs. 



25 deg. C. 8 hrs. 24 hrs. 



31 deg. C. 7 hrs. 22 hrs. 



37 deg. C. 5 hrs. 12 hrs. 

 Koning has also kept milk at various temperatures and titrated the degree of acidity 

 after varying periods: 



Kept at 7 to 9 deg. C. Kept at 22 deg. C. 



Deg. of aciditv Deg. of acidity 

 with l/jo NaOH 



Milk after delivery 13.6 13.6 



After 15 hours 14.6 14.6 



After 29 hours 14.6 20.6 



After 41 hours 16.0 62.6 



After 53 hours , 16.0 71.0 



After 65 hours 16.8 



After 77 hours 17.6 



After 89 hours 18.8 



Since the growth of various bacteria depends on the method 

 of keeping the milk, therefore the acid formation varies in accord- 

 ance with the same conditions during the same time. Koning's 

 experiments confirm these findings : 



Time of Milk in shallow vessels In tall cylinders 



delivery at 22 deg. at 37 deg. C. at 22 deg. " at 37 deg. C. 



Shortly after milking. 18.6 



After 24 hours 16.4 18.5 18.8 32.4 



The degree of acidity depends, furthermore, upon whether 

 fresh milk is boiled or raw; in boiled milk it is lower than in raw 

 milk, and it also depends on the aeration of the milk. 



Milk drawn carefully into bottles 25 minutes after the milk- 

 ing has 17.4; after being aired by pouring from a height of 

 y 2 meter, 16.4; after repeated aeration 16.1, and after boiling 

 only 16 degrees of acidity (Koning). The escape of the volatile 

 carbonic acid seems to play a part in this. 



Finally, the degree of acidity depends on the method by which 

 it is tested. For instance, if the milk is diluted with water for 

 the purpose of titration (method of Thorner, Pfeifer, etc.), then 

 through this addition of water, a solution of alkaline calcium 

 phosphate takes place and the acidity becomes less. 



