143 DAIRY FARMING 



NORMAL FERMENTATIONS. 



We speak of normal fermentations because milk always 

 contains certain classes of bacteria even when drawn and 

 kept under cleanly conditions. These fermentations will 

 be discussed in the following pages. 



i. LACTIC FERMENTATION. 



This is the most common and by far the most important 

 fermentation of milk. Indeed it is indispensable in the 

 manufacture of butter of the highest quality. The germ 

 causing this fermentation is called Lactici Acidi. It is 

 non-spore bearing and has its optimum growth tempera- 

 ture between 90 and 98 F. At 40 its growth ceases. 

 Exposed to a temperature of 140 for fifteen minutes 

 it is killed. 



The souring of milk and cream, as already mentioned, 

 is due to the action of the lactic acid bacteria upon the 

 milk sugar changing it into lactic acid. Acid is therefore 

 always produced at the expense of milk sugar. But the 

 sugar is never all converted into acid because the pro- 

 duction of acid is limited. When the acidity reaches 

 about .9% the lactic acid bacteria are either checked or 

 killed and the production of acid ceases. Owing to the 

 universal presence of these bacteria it is almost impossible 

 to secure milk free from them. 



Under cleanly conditions the lactic acid type of bacteria 

 always predominates in milk. When, however, milk is 

 drawn under uncleanly conditions the lactic organisms 

 may be outnumbered by other species of bacteria which 

 give rise to the numerous .taints often met with in milk. 



Contradictory as it may seem, the lactic acid bacteria 

 are alike friend and foe to the butter maker. Creamery 



