PHYSICAL AND CHEMICAL EXAMINATION OF MILK 159 



prevent choking the opening (Figs. 47, 48). The acid should be 

 poured into the bottle with the same care as the milk. The 

 temperature of milk and the acid before mixing should be about 

 21° C. If much higher the mixture is liable to boil and foam will 

 escape. 



The specific gravity of the acid should be 1.82 to 1.83. Com- 

 mercial sulphuric acid has a specific gravity of 1.84. This is cor- 

 rected by pouring the acid into a small volume of water and meas- 

 uring the specific gravity after the mixture has cooled. 



It is important that the acid should have a definite strength. 

 If too strong, some of the milk solids char and black flakes collect 



Fig. 46. — Types of cream and milk test bottles, showing fat columns, menis- 

 cuses, etc. (Bulletin No. 58, B. A. I.) 



at the bottom of the fat column. If the acid is too weak, the 

 casein may not dissolve completely and the fat is cloudy, or some 

 undissolved casein gathers at the lower end of the fat column. 

 Reading cannot be accurate when the column of fat is not clear and 

 well defined. Charring can be prevented by using a smaller quan- 

 tity of acid, but it is better still to add 2 c.c. of an 80 per cent, gly- 

 cerin solution to the milk and mix before the acid is run in. When 

 the acid is too weak a larger quantity than 17.5 c.c. will usually 

 give satisfactory results. Sulphuric acid must be kept in well- 

 closed bottles, otherwise moisture from the air is absorbed and the 

 acid becomes weaker. The specific gravity of the acid is tested 

 with an acidometer. 



