128 Testing Milk and Its Products. 



short time, more whey can be poured off, and the curd 

 thereby rendered firmer. The water around the jars is 

 kept at a temperature of 98, the vat is covered, and the 

 curds allowed to ferment in the sample jars for six to 

 twelve hours. 



During this time the impurities in any particular sam- 

 ple will cause gases to be developed in the curds so that 

 by examining these, by smelling of them and cutting them 

 with a sharp knife, those having a bad flavor or a spongy 

 or in any way abnormal texture may be easily detected, 

 and the milk from which it was made, thereby picked 

 out. 



151. By proceeding in the same way with the milk 

 from the different cows in a herd, the mixed milk of 

 which produced abnormal curds, the source of contami- 

 nation in the herd may be located. Very often the 

 trouble will be found to come from the cows' drinking 

 foul stagnant water or from fermenting matter in the 

 stable. In the former case the pond or marsh must be 

 fenced off, or the cows kept away from it in other ways; 

 in the latter, a thorough cleaning and disinfection of the 

 premises are required. If the milk of a single cow is 

 the source of contamination, it must be kept by itself, 

 until the milk is again normal; under such conditions the 

 milk from the healthy cows may of course safely be sent 

 to the factory. 



152. The fermentation test. The Gerber fermentation test (see 

 fig. 47) furnishes a convenient method for examining the purity 

 of different lots of milk. The test consists of a tin tank which 

 can be heated by means of a small lamp, and into which a rack 



