170 Dairy Bacteriology. 



and plastic under pressure, but the characteristic flavor 

 is developed in greater or less degree. A very consid- 

 erable part of the cheese thus becomes soluble in water, 

 and it is much more easily digested than in an unripened 

 condition. 



The different factors that are operative in the ripen- 

 ing changes are not yet fully known, but in recent years 

 as a result of scientific study, material progress in the 

 study of the changes has been made. 



Rennet. The commercial rennet extract when in con- 

 dition for use contains very few bacteria. A preserva- 

 tive, boric acid, is added by the manufacturer to restrain 

 the bacteria, otherwise the extract would soon be unfit 

 for use. The bacteria in the commercial rennet extract 

 are too few to be of any importance whatever in the 

 ripening process. 



Rennet extract contains an enzyme, rennin, that 

 causes the milk to curdle; also another enzyme, pepsin, 

 that exerts a digestive action on the curdled casein. 

 Pepsin is always found in the stomach juices of all ani- 

 mals, but no digestive action takes place, unless the re- 

 action is distinctly acid, as is the case under normal con- 

 ditions, since hydrochloric acid is excreted by the walls 

 of the stomach. Outside of the stomach, the same condi- 

 tions must obtain with reference to the presence of acid, 

 if pepsin is to exert a digestive effect. In the cheese 

 curd, the milk sugar is rapidly changed into lactic acid 

 by the action of the bacteria. This gives the proper 

 chemical reaction for peptic action, and the enzyme is 

 then able to act on the paracasein, the nitrogenous part 

 of the cheese. If milk contains no acid-forming bac- 

 teria, conditions will not permit of peptic action, and as 

 a consequence, the ripening processes do not take place. 



