MILK AND ITS PRODUCTS 197 



giv< it a gentle rotary motion until the curd of the milk is entirely 

 dissolved, and the mixture of acid and milk is of a uniform dark cof- 

 fee ?olor and very hot. This change in appearance and in tempera- 

 tun is the result of the action of the acid upon the constituents of 

 the milk, all of which are dissolved except the fat, which is not 

 affe ?ted. 



Whirling the Bottles. The bottles are now put in the centri- 

 fug . They should be quite hot throughout the whirling. Test- 

 ing should not be done in a cold room. It is generally necessary to 

 pla e some boiling water in the bottom of a hand centrifuge to keep 

 up he heat during the whirling. The test bottles should be placed 

 in 1 lie machine so that they balance each other. The machine is 

 ope -ated for five minutes at the proper speed for the machine, 700 

 to 000 revolutions of the disk per minute, depending upon the size 

 of ihe revolving parts. At the end of five minutes the machine 

 is si opped, and hot water is added to each bottle by means of the 

 pip< tte until the contents come up to the bottom of the neck. 

 Tilt ti whirl the machine two minutes more and add hot water 

 to bring the fat column near the top of the graduations on the 

 neck. Whirl one minute and the tests are ready for reading. 

 The fat should be clear and free from black sediment below, or 

 f oai i on top. 



Reading the Test. The test bottles should be placed for a few 

 minutes in a deep pail or pan of water, the temperature of which is 

 regulated to 130 F. The fat should be in a fluid condition. In 

 reading, the test bottle should be held in a perpendicular position 

 on the level with the eye. 



I 1 he reading on the graduated scale should be noted at the top 

 and bottom of the fat column,, This reading is made from the ex- 

 treme bottom of the fat column to the straight line which is seen 

 across the top, and not to the curved line which appears just below. 

 The difference between the two readings is the percentage of fat in 

 the milk. The neck of the bottle is graduated into large divisions 

 which represent per cent, and these again into smaller divisions, 

 each of which may be 0.1 per cent, or 0.2 per cent, depending upon 

 the style of the bottle. 



Suggestions on Making the Bdbcock Test. 1. Use the index 

 finger, not the thumb, for closing the pipette. Keep the finger dry. 



2. Draw the milk above the mark when measuring, and then hold 

 the pipette on level with the eye as the milk is allowed to flow a 

 drop at a time until the mark is reached. 



