THE MILK. 439 



first be freed from fats and albumins. To this end, it is most con- 

 venient to dilute with water and to remove the casein by the cau- 

 tious addition of acetic acid. The resulting precipitate, which 

 contains both the casein and the fat, is filtered off and the filtrate 

 boiled. After the removal of the precipitated coagulablo albumins, 

 the sugar is then estimated in the filtrate by titrating with Fehling's 

 solution, as described in the section on the Urine; 10 c.c. of the 

 reagent correspond to 0.06 gramme of lactose, providing that the 

 solution contains from 0.5 to 1 per cent, of sugar. 



In addition to lactose, the milk contains also small amounts of a 

 reducing substance, which is supposedly identical with Landwehr's 

 animal gum (dextrin). It is possible, however, that, as in the case 

 of the reducing substance of the mucins and mucoids, chondroitin- 

 sulphuric acid or an allied substance may be responsible for the 

 reactions. 



Extractives. Among the extractives of the milk, .which com- 

 prise traces of urea, kreatin, kreatinin, xanthin-bases, lecithins, 

 cholesterin, and citric acid, the latter is of especial interest, as it 

 is apparently also formed in the mammary glands, and is not refer- 

 able to the ingestion of the substance as such. It has been found in 

 human milk as well as in cows' milk, and is notably present in com- 

 bination with calcium. Its amount in cows' milk is given as 0.25 

 per cent. Human milk contains a somewhat smaller amount of 

 citric acid ; it varies between 0.024 and 0.07 per cent. (Sieber) ; the 

 higher values are found between the sixth and the eleventh month 

 of lactation. 



To the presence of citric acid apparently the reaction of Umikoff 

 is due. To make the test, about 5 c.c. of milk are treated with one- 

 half the amount of a 10 per cent, solution of ammonium hydrate and 

 heated on a water-bath at 60 C. for fifteen to twenty minutes. 

 Human milk then assumes a violet-reddish color, which is the more 

 intense the older the milk in reference to the time of lactation. Cows' 

 milk treated in the same manner assumes a yellow, at most a yellowish- 

 brown color, so that it is thus possible to distinguish human milk 

 from cows' milk. According to Sieber, it is also possible thus to 

 distinguish the milk of the earlier months of lactation from that of 

 the later months ; after the eighth month, however, the reaction no 

 longer gives uniform results, but is sometimes intense, and at others 

 quite feeble. Sieber has ascertained that the difference between 

 human and cows' milk in this respect is probably primarily depen- 

 dent upon the differing amount of calcium salt that is present. On 

 heating cows' milk with ammonia all the citric acid is precipitated 

 as calcium citrate, while in the case of human milk, which contains 

 only one-sixth the amount of calcium, a certain proportion of the 

 citric acid remains in solution. 



The formula of the acid is CH 2 .COOH.C(OH).COOH.CH 2 .- 

 COOH, viz., C 6 H 8 O 7 ; it is thus oxy-tricarballylic acid. 



