76 Dairy Bacteriology. 



the milk of the different cows and noting whether any ab- 

 normal condition develops in the respective samples. 



Fermentation tests. The moot satisfactory way to de- 

 tect the presence of the taints more often present is to 

 make a fermentation test of one kind or another. These 

 tests are most frequently used at the factory, to enable the 

 maker to detect the presence of milk that is likely to prove 

 unfit for use, especially in cheese making. They are based 

 upon the principle that if milk is held at a moderately high 

 temperature, the bacteria will develop rapidly. A number 

 of different methods have been devised for this purpose. In 

 Walther^s lacto-fermentator samples of milk are simply 

 allowed to stand in bottles or glass jars until they sour. 

 They are examined at intervals of several hours. If the 

 curdled milk is homogeneous and has a pure acid smell, the 

 milk is regarded as all right. If it floats in a turbid serum, 

 is full of gas or ragged holes, it is abnormal. As generally 

 carried out, no attempt is made to have these vessels ster- 

 ile. Gerber's test is a similar test that has been extensively 

 employed in Switzerland. Sometimes a few drops of rennet 

 are added to the milk so as to curdle the same, and thus 

 permit of the more ready detection of the gas that is evolved. 



Wisconsin curd test. The method of testing milk de- 

 scribed below was devised at the Wisconsin Experiment 

 Station in 1895 by Babcock, Russell and Decker. 1 It was 

 used first in connection with experimental work on the 

 influence of gas-generating bacteria in cheese making, but 

 its applicability to the detection of all taints in milk pro- 

 duced by bacteria makes it a valuable test for abnormal 

 fermentations in general. 



In the curd test a small pat of curd is made in a glass 



12 Kept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid., June, 1898. 



