METHODS OF EXAMINING MILK 279 



sel in which it was collected to the test tube, the necessary 

 precautions should be observed to prevent contamination. 

 Twelve hours after they are placed in the incubator, 

 the samples are examined. If the milk is normal, fresh 

 and of good quality, there will be no change apparent ex- 

 cept perhaps a clean, sour odor. The beginning of fer- 

 mentation or curdling is indicated by an upward bulging 

 of the cream layer and the presence of a greenish layer 

 beneath it. If there is no change at this time, the samples 

 are to be replaced in the incubator and observed again in 

 twelve hours, and subsequently at twelve-hour periods if 

 necessary. If curdling does not take place after forty- 

 eight hours, the reaction of the milk should be taken and 

 tests made for preservatives. When the milk curdles, the 

 time of curdling and the character of the curd are to be 

 noted. The time of curdling depends upon the number of 

 bacteria present which are not inert, while the character 

 of the curd depends upon the kind of bacteria which pre- 

 dominate. The types of curds, the symbols by which they 

 are recorded, and their indications are as follows: 



1. Jelly-like Curd. Ji, solid, smooth, white, jelly- 

 like curd, with no fluid. J 2 , curd of same type, but show- 

 ing a few furrows or gas bubbles. J 3 , curd same, but 

 showing more furrows and gas bubbles and also cracks, 

 with some fluid. The jelly-like curd is produced when 

 the acid- forming bacteria predominate, and if the number 

 of bacteria is excessive it usually indicates that the milk 

 vessels or utensils are unclean, or that the milk is old or 

 has not been kept under proper conditions. 



2. Peptowzed Curd. The curd may be firm, jelly- 

 like, with ragged surfaces, or it may be soft, flocculent 

 or "mushy," associated in either case with more or less 

 fluid. Pi, the amount of fluid is small in proportion to 



