I QO BACTERIA 



Milk-sugar, or Lactose (C, 3 H 34 O 12 ). This is an import- 

 ant and constant constituent of milk. It forms the chief 

 substance in solution in whey or serum. Milk-sugar ap- 

 proximates to dextrose in its action on polarised light. By 

 boiling with sulphuric acid it is converted into dextrose and 

 galactose. 



Fat occurs in milk as suspended globules, and by churning 

 may be made into butter. 



The Proteids include casein, albumen, lactoprotein, and a 

 small quantity of globulin. These are the nitrogenous 

 bodies. 



Mineral Matter. The ash of milk, obtained by careful 

 ignition of the solids, contains calcium, magnesium, potas- 

 sium, sodium, phosphoric acid, sulphuric acid, chlorine, and 

 iron, phosphoric acid and lime being present in the largest 

 amounts. 



(i) Lactic Acid Fermentation. If milk is left undisturbed, 

 it is well known that eventually it becomes sour. The casein 

 is coagulated, and falls to the bottom of the vessel; the 

 whey or serum rises to the top. In fact, a coagulation 

 analogous to the clotting of blood has taken place. In 

 addition to this, the whole has acquired an acid taste. Now, 

 this double change is not due to any one of the constituents 

 we have named above. It is, in short, a fermentation set 

 up by a living ferment introduced from without. The con- 

 stituent most affected by the ferment is the milk-sugar, 

 which is broken down into lactic acid, carbonic acid gas, 

 and other products. 



For many years it has been known that sour milk con- 

 tained bacteria. Pasteur first described the Bacillus acidi 

 lactici, which Lister isolated and obtained in pure culture. 

 Hueppe contributed still further to what was known of this 

 bacillus, and pointed out that there were a large number of 

 varieties, rather than one species, to be included under the 

 term B. acidi lactici. We have already seen that these 



