26 INTRODUCTORY — THE CONSTITUENTS OF MILK. 



Tyrosine, CaHnNOa, is an oxyphenyl derivative of amino- 

 propionic acid! It crystallises in fine glistening needles usually 

 grouped in bundles, soluble in about 150 parts of boiling water. 

 It gives a red precipitate witb a solution of mercuric nitrate 

 containing nitrous acid, and is capable of forming metallic salts. 



By tryptic digestion more tyrosine is produced than leucine ; 

 other hydrolytic actions produce greater quantities of leucine. 



A. J. Brown and J. H. Millar have shown that tyrosine is 

 separated in the early stages of tryptic digestion, but the tyrosine 

 nucleus is very resistant to peptic digestion. 



Proximate Detenninatioji of the Conslitvtion of Proteins.-- 

 Hausmann's method consists in boiling 1 gramme of protein 

 with strong hydrochloric acid for some hours and determining 



1. The nitrogen split off as ammonia by distillation with 

 magnesia, preferably at a low temperature. 



2. The nitrogen in the insoluble portion. Melanin nitrogen. 



.3. The nitrogen in the phospho-tungstic acid precipitate; 

 basic (lysine, histidine, or arginine) or diamino-nitrogen. 



4. The soluble nitrogen. Mono-amino-nitrogen. 



The only milk protein which has been thus examined is casein, 

 and the mean of the results of Giimbel, Osborne and Harris, and 

 Kutscher is — 



AmmouKi Melanin Dianiino- Mono-anyno- 



Nitrogen. Nitrogen. Nitrogen. Nitrogen. 



1-63 0-27 3-87 9-98 



The Proteins of Milk. — The number of proteins present in 

 milk (of the cow) has been variously stated at from one to eight 

 by different observers. The most recent work has tended to 

 reduce the number to not more than four, the larger number 

 described having been obtained by faulty methods of separating 

 these bodies or by the action of some reagent used on the protein. 

 Many of the products described are now known to be mixtures 

 of one or more of the proteins with various impurities, or decom- 

 position products obtained during the separation of the proteins 

 one from another. 



The theories of leading observers are briefly given as follows : — 

 Duclaux maintains that there is only one protein in milk, 

 which exists in two forms — the coagulable and non-coagulable ; 

 he gives to this protein the name of casein. The first modi- 

 fication is not in a state of solution, and can be separated by 

 filtration through a porous jar ; it is combined with the phos- 

 phates of the alkaline earths, and this causes it to differ in its 

 properties from the otlier modification, which is in a state of true 

 solution and passes through the porous jar. ^^'ere this view 

 correct, the coagulable modification should gradually lose its 

 distinctive properties ns it is purified from phosphates ; and, on 



