86 PHYSIOLOGICAL ACTIVITIES 



The "lactic acid bacteria" form a very large and important 

 group and are indispensable in many commercial processes. 



Fig. 62. — A loaf of "salt rising" bread. The porous structure is due to the 

 gas formed by bacilli and not by yeasts. 



In the making of butter the cream is first "ripened," as is 

 the milk from which many kinds of cheese are made (Fig. 64) . 



Fig. 63.— Ohio Swiss cheese. The "eyes" are due to gas formed by bacteria 

 during the ripening of the cheese. 



The chief feature of this "ripening" is the formation of lac- 

 tic acid from the milk-sugar by the action of bacteria, and a 



