126 THE CHEMISTRY OF THE TISSUES AND ORGANS. 



enable the ether to attack the fat more easily. Moreover, Hoppe-Seyler 

 states that it is not so difficult to remove the fat simply with ether ; 

 the fluid still remains cloudy, it is true, but solutions of caseinogen 

 are always opalescent, and this is increased by the presence in the 

 milk of particles of proteid or proteid-like substances, as described by 

 Kehrer. 



The reaction of milk. Milk readily turns sour from the fermentation 

 of lactose and formation of lactic acid. In carnivora fresh milk has an 

 acid reaction, but in most animals it gives either an alkaline or, more 

 frequently, an amphoteric reaction; the acid phosphates in the milk 

 turn neutral litmus red, and the alkaline phosphates turn it blue. The 

 proportion between these salts varies very considerably in different 

 animals, in the same animal at different stages of lactation, and even 

 between the first and last portions of the same milking (Thorner, 1 

 Sebelien, 2 Courant 3 ). 



Courant estimated the alkaline constituent by titration with 

 decinormal sulphuric acid, with blue lackmoid as indicator, and the acid 

 constituent with decinormal soda with phenolphthalehi as indicator. 

 He found as a mean for the first and last portions of the milking of 

 twenty cows, that the alkalinity of 100 c.c. of the milk was equal to 

 41 c.c., and the acidity equal to 19'5 c.c. of the respective solutions 

 used. In human milk the proportional alkalinity is higher ; the average 

 of the numbers was 10'8 and 3*6 c.c. respectively. 



Constituents of milk. These are water, three proteids (caseinogen, 

 lactalbumin, lacto-globulin), two carbohydrates (lactose, animal gum?), 

 fats, extractives (traces of urea, creatine, creatinine, hypoxanthine, 

 lecithin, cholesterin, citric acid 4 ), salts and gases. Most of these de- 

 mand separate discussion. 



Effect of boiling milk. When milk is heated to, or near to, the 

 boiling point, a scum forms on the surface ; on the removal of this skin 

 it is rapidly renewed, and this can be repeated over and over again. 

 This is probably in part produced by the coagulation of the lact- 

 albumin ; this carries to the surface some caseinogen and fat. 5 Contact 

 with air appears to be the chief influence in causing the solidification 

 which results in the formation of the scum ; evaporation is rapid from 

 the surface exposed to the atmosphere, and thus partial drying occurs 

 there. 



The boiling of milk before it is used as a food is advantageous in 

 two ways (1) all micro-organisms are destroyed; (2) the gastric juice, in 

 virtue of its rennet, causes a flocculent and not a bulky precipitate. 

 These quite outweigh any slight diminution of digestibility alleged to 

 occur. 6 The reason that boiled milk curdles with rennet with greater 

 difficulty than fresh milk appears to be that, by boiling, a part of the 

 dissolved calcium salt is precipitated as tricalcium phosphate. 



As milk turns sour, it is possible to get a bulky heat coagulum by 

 boiling. 7 



1 Chan. Ztg., Cbthen, Bd. xvi. S. 1469. 2 Ibid., S. 597. 



3 Inaug. Diss., Bonn, 1891 ; and Arch. f. d. ges. Physiol., Bonn, 13d. 1. 



4 Soldner, Landw. Versuchs. Stat., Berlin, Bd. xxxv. 



5 See D. F. Harris, Joiirn. Anal, and Physiol., London, 1894, vol. xxix. p. 188. 



6 Raudnitz, ZtsrJir.f. physiol. Chcm., Strassburg, Bd. xiv. S. 1. 



7 Recent work on this question will be found in a paper by Cazeneuve and Haddon, 

 Compt. rend. Acad. d. sc., Paris, 1895, tome cxx. p. 1272. See also influence of boiling on 

 the proteids of cows' milk, Centralbl.f. d. mcd. Wissensch., Berlin, Bd. xxxiv. S. 145. 



