Testing the Purity of Milk. 139 



ered, and the curds allowed to ferment in the sample 

 jars for six to twelve hours. 



During this time the impurities in any particular 

 sample will cause gases to be developed in the curds so 

 that by examining these, by smelling of them and cut- 

 ting them with a sharp knife, those having a bad flavor, 

 or a spongy or in any way abnormal texture may be 

 easily detected, and thus traced to the milk causing the 

 trouble. 



Since the curd test was first described, several modi- 

 fications have been made in the apparatus. In one of 

 these the bottles are held in a covered metal frame so 

 that all of them can be drained at once by invei'ting the 

 frame. 



155- By proceeding in the way described with the 

 milk from the difl^'erent cows in a herd, the mixed milk 

 of which produced abnormal curds, the source of con- 

 tnmination in the herd may be located. Very often the 

 trouble will be found to come from the eows 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 it is again normal; under such conditions the milk 

 from the healthy cows may, of course, safely be sent 

 to the factory. 



156. The fermentation test. The Gerber fermentation 

 test (see fig. 47) also 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, 



