CHAPTER XXII 



The Relations of Micro-Organisms and 

 Enzyms to Cheese-Making 



Milk, on standing under ordinary conditions, under- 

 goes a variety of changes sooner or later, many of 

 which destroy its value for cheese-making purposes. 

 The most common and extensive changes occurring 

 in milk are due to fermentations. One result of some 

 kinds of fermentation is the production of bad flavors, 

 but these may be acquired also by direct absorption 

 from the surrounding air or from the food consumed 

 (p. 6). We shall see that certain kinds of fermenta- 

 tions are useful and necessary in cheese-making, while 

 others make it difficult or impossible to prepare a good 

 product. 



FERMENTATIONS AND FERMENTS 



The souring of milk is one of the most familiar 

 cases of fermentation. The important change taking 

 place is the formation of lactic acid from milk-sugar 

 and the change is caused by certain living organisms. 

 An equally familiar case of fermentation is the 

 coagulation of milk by rennet-extract. In this case 

 the change is produced, not by a living organism, but 

 by a chemical substance. That which causes fermenta- 

 tion is called a ferment. 



Fermentation may be defined as a chemical change 

 of an organic compound through the action of living 



