MILK. 143 



gravity and the addition of water lowers it. The average 

 percentage composition of milk is given by Konig as fol- 

 lows : 



Cows', water, 87.17; proteins, 3.55; fats, 3.69; lac- 

 tose, 4.88; mineral matter, 0.71. 



Human, water, 87.41; proteins, 2.29; fats, 3.78; lac- 

 tose, 6.21; mineral matter, 0.31. 



Casein, which is a nucleoalbumin, is not in true solu- 

 tion in milk, since it can be filtered out by unglazed por- 

 celain, though not by filter-paper. It is precipitated by 

 weak acids, as is seen when the milk becomes sour, but is 

 not coagulated by boiling. Eennin breaks up the casein 

 into two compounds: an albumose and an insoluble cal- 

 cium compound (paracasein calcium, or cheese). Coagu- 

 lated human casein is not as hard as that of cows. The 

 difference is partly due to its chemical composition, but 

 largely to the fact that cows' milk contains more casein 

 and calcium than human milk. It can be made to form 

 a soft and spongy coagulum similar to the human by dilu- 

 tion or by the removal of the calcium compounds. 



The fats of milk are a mixture of stearin, palmitin, 

 and olein, with a small amount of the glycerids of some 

 lower members of the fatty-acid series, butyric, caproic, 

 caprylic, capric, etc. The fat exists as an emulsion, a coat- 

 ing of albumin keeping the globules separate. They may 

 be made to collect by dissolving this coating by a chemical 

 agent, like sulphuric acid. Babcock's method for deter- 

 mining the percentage of fat in milk is based upon this 

 principle. Here the volume of the fat is measured and 

 this gives the relative amount in the milk. If the fat rises 

 for twenty-four hours without such decomposition it should 

 form a layer 10 or 15 per cent, of the depth of the milk, if 

 the latter is normal. 



