Bacteria in the Cheese Industry. 173 



180. Fermentation tests. Fermentation tests are 

 based upon the principle that if milk is held at a moder- 

 ately high temperature, approximating the blood heat, 

 the bacteria in the same will develop rapidly. Particu- 

 larly is this true with taint-producing organisms as these 

 seem to develop more rapidly at the higher temperature. 



A number of tests have been devised that are based 

 upon the operation of the same general bacteriological 

 principle. In Walther's lacto-fermentator the milk is 

 allowed to stand in bottles or glass jars until it sours 1 . 

 To hasten this process it is incubated in a warm place. 

 If the curd formed is homogeneous and has a pure acid 

 smell, the milk is regarded as all right. If it floats in a 

 muddy serum, is full of gas or ragged holes, it shows 

 abnormal conditions. As usually carried out, no special 

 attempt is made to free the vessels used for this purpose 

 from various kinds of bacteria. Often, too, the samples 

 are left uncovered, thus permitting infection to occur 

 from without. The utility of this method could be ma- 

 terially increased by using sterile tubes and having same 

 protected by a. plug of cotton from external contamina- 

 tion. 



A few drops of rennet are sometimes added to the 

 milk that is incubated in order to enable a more ready 

 detection of the gas that is evolved. The rents and splits 

 in the curdled casein are rendered more evident in this 

 way. 



181. Casein or rennet tests. The action of rennet 

 on milk can be utilized in determining its value for cheese 

 purposes. If a given quantity of rennet solution is added 

 to a definite quantity of milk at a certain temperature, 

 the time of coagulation will give some indication as to 

 the character of the. milk. Good milks ought to curdle 



1 This same principle is employed in Gerber's test. 



