RELATION OF BACTERIA TO CHEESfi 107 



by the gas formed from the lactose. The flavor of the cheese 

 is also injured because of other by-products of the acid 

 fermentation. 



Tests for the quality of the milk to be used in the manu- 

 facture of cheese have for one of their purposes the detection 

 of the second class of organisms, the gas-forming bacteria. If 

 a sample of the milk is placed in a container and incubated 

 at 37 C. for a few hours, until curdling has taken place, the 

 quality of the milk will be revealed by the nature of the curd. 

 This forms the so-called Gerber fermentation test, largely used 

 in Germany and Switzerland. 



A more delicate method is known as the Wisconsin Curd 

 Test, and is carried out as follows : Pint jars are filled with 

 the milk to be tested. The jars should be sterilized before 

 being filled, and care should be taken not to contaminate one 

 sample from others, as by the use of dippers, etc. The jars of 

 milk are warmed to 37 C., 0.5 cc. of rennet extract is added to 

 each jar, and the whole is well mixed. The curd is cut with 

 a case knife. The jars are kept at 37-40C., in order to 

 facilitate the shrinking of the curd, as well as the growth of 

 the gas-generating bacteria. After the curd is firm the whey 

 is poured off. Additional portions of whey will be expressed 

 and should also be removed. The jars are kept at the same 

 temperature for eight to twelve hours, and are then examined 

 for flavor, texture, and presence of gas. The particulate bodies 

 in the milk, fat globules and bacteria, are retained in the curd. 

 As the curd shrinks until it occupies but about one sixth of 

 the volume of the milk, the bacteria are concentrated, and 

 the changes produced by them are more apparent. A few gas 

 holes scattered through the total volume of milk, such as is 

 occupied by the curd when produced by the acid-forming bac- 

 teria, might pass unnoticed, but, if present in a small mass of 



