No. 6. DEPARTMENT OF AGRICULTURE. 647 



to the bottom of the test hotlle and not drop through the milk in 

 the center of the bottle; and, moreover, unless this is done, the neck 

 is liable to choke up ai)d cause the acid to overtlovv on one's hands. 

 Failure to observe this precaution will generally cause blackening 

 of the fat. The pouring should be slow and steady. It is well, 

 also, while pouring in the acid, to lurii the test bottle around slowly 

 so that the acid may successively come in contact with the different 

 portions of the inside walls of the neck and wash down any milk 

 adhering. Unless this is done, some milk may remain in the neck, 

 in which case its casein will be precipitated and not redissolved 

 aud thus the fat will contain particles of casein. 



(4.) Mixing Milk in Test KoHle. — As soon as the acid has been 

 measured into the test bottle, the acid and milk should be thoroughly 

 mixed. This is best done by giving the Irottle a rotary motion, 

 with gentle shaking. Much motiof) up and down should be avoided, 

 since milk may be thrown up into the neck beyond the reach of the 

 acid and undissolved casei>a resulting from this will mix with the 

 fat; and ,then, violent motion up and down might throw some of the 

 acid out upon one's hands or clothing. When the acid and milk 

 first mix, the casein is precipitated in a more or less solid mass, 

 which gradually redissolves. The mixing once begun should con- 

 tinue mitil one has made certain that the casein is entirely redis- 

 solved. The chemical action of the acid upon the compounds of the 

 milk produces much heat and, as stated above, the solution, at 

 first yellow, changes gradually through varying shades of yellow 

 and brov»n to a dark-brown color, provided the acid is not too 

 strong. This color is due mainly to the charring or partial burning 

 of the milk-sugar by the acid. Samples of milk that have beea 

 preserved for some time with potassium bichromate or formalin 

 require rather more time and agitation for redissolving the casein 

 than do samples of fresh milk. 



83. Whirling Test Bottles. 



(i.) Whirling the Bottles. — The test bottles containing the mixture 

 of milk and acid should be placed in the machine and whirled 

 directly after the acid is added. An even number of bottles should 

 be whirled at the same time, and they should be placed in the 

 wheel in pairs opposite to each other, so that the equilibrium of 

 the apparatus will not be disturbed. ^A'hen all of the test bottles 

 are placed in the apparatus, the cover is placed upon the jacket, 

 and the machine turned at the proper speed, 600 to 1,200 revolutions 

 per minute, according to diameter of tester, for about five min- 

 utes. The test should never be made without the cover being 

 placed upon the jacket, as this not only prevents the cooling of the 



