156 MODERN METHODS OF TESTING MILK 



two-thirds full with the milk to be tested and the ster- 

 ilized cover put on at once. The jars are then placed 

 in the tank which is filled with water at 100 to 102 

 F. up to the upper surface of the milk in the jars. The 

 temperature of the water should be kept at 100 to 

 102 F. during the whole operation. To hasten the 

 warming of the milk, the jars are taken out and shaken 

 occasionally. The temperature of the milk is observed 

 with a sterile thermometer, and when the milk has 

 reached 98 F., one adds 10 drops of rennet-extract 

 to each jar and mixes thoroughly by giving the con- 

 tents of the jar a rotary motion. When the milk has 

 coagulated, it is allowed to stand until it is firm, usu- 

 ally about 20 minutes. To enable the whey to sepa- 

 rate more readily, the curd is then cut fine with a 

 thin knife, which must be carefully rinsed with hot 

 water after finishing each jar and before using it in 

 another, in order to avoid carrying contamination from 

 one milk to another and spoiling the test. The curd 

 is allowed to settle completely. When the whey has 

 been separating half an hour, the samples are exam- 

 ined for flavor by smelling, after which the whey 

 is carefully poured out of the jars and this is re- 

 peated at intervals of 30 to 40 minutes for 8 hours 

 or more. Under the favorable conditions of tempera- 

 ture, similar to those employed in cheese-making, the 

 organisms present develop readily and reveal their 

 presence in different characteristic ways. The jars 

 are finally opened, any whey present is drained off, 

 and the following tests are applied: (i) The curd is 

 cut into two pieces. The curd will be solid and free 

 from holes on the cut surfaces, if the milk is not 



