292 MANUAL OF MILK PRODUCTS 



circumstances, it may be desirable to make additional tests of 

 special lots of milk. One of the quick tests (see p. 301) for 

 determining the percentage of acid may prove most useful in 

 determining the quality of some milk. If there is suspicion of 

 its containing microorganisms which will later cause gas or 

 other undesirable fermentations, it may be necessary to make 

 a curd test, which may be done as follows : 

 The Wisconsin curd test. 



Pint glass jars, thoroughly cleaned and sterilized with live 

 steam, are provided ; they are plainly numbered or tagged, one 

 jar being provided for each lot of milk to be tested. The jars 

 are filled about two-thirds full with milk from the various 

 sources; it is not necessary to take an exact quantity; they 

 are then placed in a water tank, the water of which is heated 

 until the milk in the jars has a temperature of 98° F. In trans- 

 ferring the thermometer used from one jar to another, special 

 care must be taken to clean it each time in order to prevent 

 contamination of pure lots of milk by impure ones. 



When the milk has reached a temperature of 98°, add to 

 each sample ten drops of rennet extract, and mix by giving 

 the jar a rotary motion. The milk is thus curdled, and the 

 curd allowed to stand for about twenty minutes until it is firm. 

 It is then cut fine with a case knife, and stirred at intervals for 

 one-half to three-quarters of an hour sufficiently to keep the 

 curd from matting under the whey. When the cubes are quite 

 firm, the whey is poured off and the curd left to mat at the 

 bottom of the bottles if the old form of apparatus is used. The 

 best tests are made when the separation of the whey is most 

 complete. By allowing the samples to stand for a 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 from six to twelve hours. 



During this time the impurities in any particular sample 



