52 MILK HYGIENE 



ulent material, the nature of which is not definitely 

 established. During this disease, several noticeable 

 changes occur in the milk and it is, therefore, very prob- 

 able that pathological processes take place in the cells 

 of the gland and so an excretion of virus is only to be 

 expected. In other acute diseases, as anthrax, hemor- 

 rhages may occur in the tissues of the udder and the 

 admixture of bacteria from the blood follows naturally. 

 The changes in the milk caused by microbes, and par- 

 ticularly by bacteria, which comprise by far the greatest 

 number of micro-organisms of milk, are very numerous. 

 These are most readily studied by working with sterile 

 milk (the best way is to use milk which was taken from 

 the udder in sterile condition, for milk undergoes quite 

 essential changes when heated) and with pure cultures 

 of bacteria. The most important changes are the 

 following : 



a. The milk is unchanged in appearance, reaction, 

 odor and taste in spite of bacterial growth. 



b. The bacteria form alkali (probably ammonia) ; 

 the reaction grows more and more alkaline and when, 

 after some time, a certain alkalinity is reached, the fat 

 is saponified, the neutral casein-lime compound becomes 

 a base and, as a result of this, the milk is changed to a 

 yellowish translucent, whey-like liquid. 



c. The bacteria split up the lactose, forming acid. 

 The main product of this division is lactic acid, some- 

 times d-lactic acid, sometimes 1-lactic acid, sometimes a 

 mixture of both, i-lactic acid. The reaction may be ex- 

 pressed by this formula: C 12 H 22 O 11 +H 2 0=4(C,H 6 3 ). 

 In fact, however, this process is far more complicated 

 for beside the lactic acids, a number of other com- 

 pounds are formed in small quantity as by-products : 

 alcohol, valeric acid, acetic acid, succinic acid, carbonic 

 acid, etc. After a certain amount of free lactic acid has 



