Fermentations of Milk. 85 



produced in milk by the ordinary acid-forming bac- 

 teria. Milk contains about 4 per cent of milk sugar, 

 all of which is fermentable* If this were all decom- 

 posed by bacteria, the acidity of the milk would actu- 

 ally exceed 4 per cent. It is thus evident that the rea- 

 son why more acid is not formed in milk is not because 

 of any lack of sugar. The bacteria, like all other kinds 

 of living things, are injured by their own by-products, 

 unless these are constantly removed in some way; in 

 milk the bacteria cannot escape the action of the acid 

 which they themselves have formed, consequently 

 growth ceases. The amount of acid formed is depend- 

 ent on the kind of bacteria present and on the compo- 

 sition of the milk. Certain bacteria will not produce 

 enough acid to cause the curdling of the milk; still 

 others will form 2 or even 3 per cent. These types, 

 however, do not play any important part in the sponta- 

 neous souring of milk. 



In milk the acid first formed combines with the ash 

 constituents and the casein to form salts which do not 

 seriously affect the growth of the bacteria. Ultimately, 

 the limit of the ash and casein to take up acid is reached, 

 and free lactic acid which is harmful to bacterial 

 growth appears. If the content of casein and ash con- 

 stituents is high, a higher degree of acidity will be 

 reached than in a milk with a lower content. If a 

 large part of the volume of the milk is made up of a 

 compound that has no role whatever in the acid fer- 

 mentation, such as the butter fat in cream, the amount 

 of acid formed per unit volume of milk will be reduced, 

 since in determining the acidity, a definite volume of 

 milk is taken, and the acidity is expressed, as such a 

 per cent of this amount. 



