168 



PROTEIN. 



other substances, as tannic acid, creosote, al- 

 cohol, and ether ; and its coagulation may 

 also be effected by a current of voltaic elec- 

 tricity. When taken into the stomach it is 

 coagulated by the free acid usually present. 



The curious change which albumen under- 

 goes from the soluble to the insoluble condition 

 is but very imperfectly understood, and it is 

 not known how far the physical state of that 

 coagulated by heat resembles that rendered 

 insoluble by alcohol and the other precipitants. 

 It is said that if an egg be smeared with oil 

 immediately after it is laid, and afterwards ex- 

 posed to heat, the coagulation is incomplete. 

 Coagulated white of egg readily dissolves in 

 alkaline solutions, and is reprecipitated un- 

 changed if the solution be supersaturated with 

 sulphuric acid. If it be digested at a tempe- 

 rature of about 120, with a tolerably strong 

 alkaline solution, the sulphur and phosphorus 

 are separated from the protein; but if the alka- 

 line solution be boiled, further decomposition 

 takes place ; ammonia is given off, and leucin, 

 protid, and other compounds are formed. If 

 the alkaline solution in which white of egg is 

 boiled be rather weak, it acquires, after some 

 hours' boiling, a smell precisely similar to that 

 of boiled fowl. Though perfectly insoluble 

 after coagulation, both in cold and boiling 

 water, it appears to dissolve when heated under 

 pressure to about 300 with that liquid, and 

 the solution thus formed behaves in every re- 

 spect similar to uncoagulated albumen. When 

 exposed to the air in a moist state albumen is 

 extremely prone to enter into putrefaction; 

 but if dry it may be preserved unchanged for 

 any length of time. If boiled for several hours 

 in water it is converted into tritoxide of pro- 

 tein, without passing through the intermediate 

 stage of binoxide, in which respect it differs 

 from fibrine. 



The ready convertibility of albumen into the 

 other protein compounds, as well as into many 

 other animal tissues, is well illustrated in the 

 phenomena of incubation ; where we find all 

 the various compounds which are contained in 

 the hatched bird, derived more or less directly 

 from this substance, which, together with a 

 yellow oil and some inorganic salts, constitutes 

 the whole of the solid contents of the egg. 



Casein is the form in which protein appears 

 in the milk, where it constitutes the chief 

 source of nourishment to the young animal, 

 for which purpose it is admirably adapted, not 

 only from the protein it contains, which is 

 readily converted into fibrin and albumen, but 

 also on account of the inorganic salts, espe- 

 cially phosphate of lime, with which it is always 

 associated. The proportion of casein contained 

 in the milk of different animals varies consi- 

 derably; and a still more striking variation is 

 caused by the food of the animal, as may be 

 seen in the following table. 



WO parts. Casein. 



Cow's milk 4'48 



fed on hay 3'0 



turnips 30 



clover 4'o 



100 parts. Casein . 



Cow's milk potatoes and hay, 3'3 to 5'1 



Ewe's milk 4'5 



Goat's milk 4'02 



Ass's milk 182 



Woman's milk T52 



Casein is scarcely known in a state of abso- 

 lute purity, as it is extremely difficult, to sepa- 

 rate it entirely from inorganic impurities : these 

 consist chiefly of lime, potash, soda, and iron, 

 combined with phosphoric, sulphuric, and hy- 

 drochloric acids. The purest specimens pre- 

 pared by Rochleder left, when burnt, only 0*3 

 per cent, of incombustible ash ; but as it is 

 generally prepared it contains considerably 

 more, sometimes as much as 10 per cent. It 

 appears to be insoluble in water, and owes its 

 solubility in milk to the small quantity of 

 potash which is always present. The best pro- 

 cess for obtaining casein is the following. A 

 quantity of milk is first evaporated to dryness 

 on a- water-bath, and the dry residue, reduced 

 to powder, is boiled in successive portions of 

 ether until the whole of the fatty matter is re- 

 moved ; the impure curd should then be eva- 

 porated to dryness, and the soluble part sepa- 

 rated by digesting in water. To this solution, 

 after filtration, alcohol is added to throw down 

 the casein, which, however, is often still con- 

 taminated with a little sugar of milk : this may 

 be removed by again dissolving in water, and 

 once more precipitating the casein by alcohol. 

 When dry it resembles albumen very much in 

 appearance, and its behaviour with reagents is 

 in most cases very similar ; it differs from it 

 chiefly in not coagulating when heated, and it 

 is precipitated by all the acids, but redissolves 

 in an excess of most of them. Sulphuric acid 

 throws down a compound which has been 

 called sulphate of casein ; this precipitate al- 

 ways contains a certain quantity of phosphate 

 of lime, and it is only by repeatedly dissolving 

 it in an alkaline solution, reprecipitating with 

 dilute sulphuric acid, and well washing with 

 boiling water, that it can be obtained in a state 

 of purity. When milk or a solution of casein 

 is heated under ordinary circumstances, a thin 

 skin is formed on the surface, which, if re- 

 moved, is quickly replaced by another; this 

 substance has never been properly examined; 

 but as it is not formed unless oxygen is pre- 

 sent, it is probably the result of oxidation. 

 Casein is precipitated from its solutions by 

 ferrocyanide and ferridcyanide of potassium, 

 provided the solution is not alkaline, and still 

 more perfectly if a little acetic acid is present. 

 Lactic acid also readily coagulates casein ; but 

 the coagulation appears to be most completely 

 effected by the lining membrane of the 

 stomach of the young animal, an action due 

 either to lactic acid, or, what is perhaps more 

 probable, to the presence of pepsine. 



Casein has been found in some of the animal 

 fluids besides milk : the saliva, the bile, pan- 

 creatic juice, and perhaps the blood, all con- 

 tain it in more or less notable quantity. It 

 affords another instance of the admirable adap- 

 tability of this interesting class of compounds 



