224 BACTERIA IN CHEESE MAKING. 



CHANGES AFFECTING THE SOLUBILITY OF THE CASEIN. 



The chemical changes that occur in the ripening of cream 

 affect all of its ingredients, but the most significant feature 

 is a modification of the casein. When freshly precipitated, 

 either by acids or by rennet, casein is insoluble in water, and, 

 especially after heating, is a hard, tough mass, difficult of 

 digestion. During the ripening of the cheese this material 

 becomes partly converted into more soluble bodies. The 

 cheese becomes softer, more readily handled by the digestive 

 processes in the stomach, and forms, therefore, a more 

 wholesome food. 



In the production of these changes there appear to be two 

 quite different processes at work. Since the beginning of 

 the study of modern bacteriology it has been thought that 

 this ripening was the result of bacteria growth. It was long 

 ago proved that bacteria multiply in the cheese during the 

 ripening, and it was also known that certain species of bac- 

 teria found in cheese (the peptonizing class) are capable of 

 producing chemical changes in the casein quite similar to 

 those that occur in the cheese during the ripening. Lastly, 

 it was found that if cheese was treated with antiseptics, or by 

 the action of heat in such a way as to prevent the growth of 

 bacteria, the ripening did not occur. These facts led to the 

 conclusion that the ripening .was due to bacteria growth. 

 Within the last few years, however, it has become evident 

 that there are other processes concerned in this cheese ripen- 

 ing besides bacterial action. 



Action of Enzymes. Fresh milk contains a chemical fer- 

 ment or enzyme known as galactase (246). This enzyme is 

 a normal ingredient of milk and, inasmuch as it is present in 

 the milk before bacteria have a chance to grow, it is not pro- 

 duced by the action of bacteria. This galactase has a power 

 of acting upon the casein, producing in it a change similar 

 to, if not identical with, that which occurs in the ripening of 



