20 MARKET DAIRYING 



be built up and used as plant food. In the art of butter 

 and cheese making bacteria are indispensable. The to- 

 bacco, tanning, and a host of other industries cannot 

 flourish without them. 



Lactic Acid Bacteria; In milk and cream intended 

 for direct consumption, bacteria of any kind are enemies 

 to dairymen. The class of bacteria that is feared more 

 than any other, -perhaps, is that which causes milk to 

 sour. The organisms belonging to this class are known 

 as lactic acid bacteria. They are non-spore bearing and 

 grow best at a temperature of from 90° to 98° F. At 

 40" F. their growth ceases; exposed to a temperature of 

 140° for 15 minutes they are killed. 



The souring of milk and cream 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 production of acid is 

 limited. When the acidity reaches about .9% the lactic 

 acid bacteria are either checked or killed and the pro- 

 duction 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. 



While the acid bacteria are objectionable in milk in- 

 tended for direct consumption, in cream made into butter 

 they are indispensable. The highly desirable aroma in 

 butter is the result of the growth of these organisms in 

 the process of cream ripening. There are a number of 



