300 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



apparatus for making this test. In some the milk is 

 permitted to flow through the filter by gravity, while in 

 the others it is forced through by air pressure from a 

 rubber bulb. According to the quantity of dirt collected 

 on the cotton disc the milk is classed as good, medium, 

 fair, and bad. The discs are sometimes dried and mailed 

 to the producer for his examination. There is no direct 

 relation between the quantity of dirt collected on the cot- 

 ton disc and the bacterial content. Milk which would be 

 classed as good according to this test may have a high 

 bacterial content and vice versa. Furthermore, milk pro- 

 duced under unclean conditions will test "good " by this 

 test if it is well strained beforehand. 



Another method of testing for dirt is to place the 

 milk in a conical glass, or in a vessel with a narrow tube 

 at the bottom, and measure the volume of sediment which 

 settles to the bottom after a certain period; or the milk 

 may be centrifugalized and the sediment measured. 



The visible dirt in milk consists principally of par- 

 ticles of feed and litter, manure, hairs, dandruff and dust. 



TEST FOR LACTOSE 



Glage has devised a simple test for lactose which may 

 be applied to samples of individual cow's milk or to 

 samples of milk from the different quarters of the udder. 

 It is made as follows : Place 3 c.c. of milk and 3 c.c. of 

 a 15 per cent, solution of potassium hydroxide in a test 

 tube, boil thoroughly and let stand for 10 minutes. When 

 the milk contains the normal amount of lactose the mix- 

 ture at first becomes pale yellow during the boiling, 

 changing quickly to dark yellow, to orange, and then 

 to brown ; after standing the color of the mixture becomes 

 coffee-brown or red-brown. If the quantity of lactose is 



