24 PROTEIDS. 



In addition to these, according to the older views, milk, even 

 when quite fresh, frequently contained traces of a proteid which, 

 since it yielded the biuret reaction, was usually spoken of as a 

 peptone, and was by some observers called ' lactoprotein. 1 It 

 was stated to increase in amount in the milk on standing for some 

 time, and more especially if warmed to 40, and to be consider- 

 ably increased during the clotting induced by rennin. 2 Eecent 

 researches have however shewn that perfectly fresh milk contains 

 no substance whicn yields a biuret reaction, its presence being due 

 to its formation during the processes employed in its separation. 3 

 If the milk undergoes an acid (lactic) fermentation a substance 

 may now be obtained from it which yields a biuret reaction, but 

 is not a true peptone, but a primary albumose. 



When milk is kept for some time at a temperature above 50 

 and below its boiling point, a firm skin is formed over its surface 

 composed largely of casein. 4 Its formation is not to be regarded 

 as being specially characteristic of milk, for pure casein dissolved 

 in dilute alkalis exhibits the same phenomenon, as also do alkali- 

 albumin, chondrin, gelatin, and the filtrate from 1 p.c. starch when 

 it is concentrated on a water-bath. Its formation is probably due 

 to the rate of evaporation from the surface of the milk being 

 more rapid than the fluid diffusion into the upper layer ; 5 and in 

 accordance with this it is found that its appearance is considerably 

 facilitated by blowing a rapid stream of air or any indifferent gas, 

 such as carbonic oxide, over the surface of the warmed milk. 



Our knowledge of the chemical properties of casein as already 

 described is based entirely upon researches carried out upon the 

 milk of cows. There is no reason to suppose that all that has 

 been said does not apply equally well to the milk of other ani- 

 mals. Nevertheless human milk shews, apart from the difference 

 of composition (see 513), certain differences from cow's milk, 

 which are due to a distinct but characteristic difference in the 

 reactions of the casein contained in each. 6 This is shewn by the 

 following facts. (1) Human milk clots less firmly than cow's 

 milk, and sometimes not at all with rennin. (2) The casein in 

 hurrian~milk, 011 the addition of acetic acid, yields a very imper- 

 fect jprecipitate whicn is finely floccuientTaTmost granular as com- 

 pared with the compact and coarsely flocculent precipitate yielded 



1 Hamrnarsten, Maly's Bericht. Bd. vi. (1876). S. 13. Palm (Russian), Ibid. Bd. 

 xvi. (1886), S. 143. For other references see Halliburton, loc. cit. p. 459. 



2 Hoppe-Seyler, Handbuch d. phi/s.-path. chem. Anal. 1883, S. 480. 



3 Neumister, Zt. f. Biol. Bd. xx'iv. (1888), S. 280. 



4 Sembritzky, Pfliiger's Arch. Bd. xxxvu. (1885), S. 460. See also Maly's Ber. 

 Bd. xvu. (1887), S. 157. 



5 Hoppe-Seyler, Virchow's Arch. Bd. xvn. (1859), S. 420. 



6 Simon, Animal Chemistry (Sydeuham Soc.), Vol. n. 1846, p. 53. Also in "Die 

 Frauenmilch u. s. w." Berlin, 1838. Biedert, Virchow's Arch. Bd. i>x. (1874), S. 

 352. Biel, see Abst. in Maly's Ber. Bd. iv. (1874), S. 166. Langgaard, Virchow's 

 Arch. Bd. i.xv. (1875), S. 352. 



