CHAPTER IV. 



PROTEINS. 



Properties. Our present knowledge of the proteins of 

 milk is far from complete, though much work has been 

 done on the subject. This is due to the fact that it is 

 extremely difficult to obtain these compounds in anything like 

 a state of purity. The method of crystallisation, which is so 

 largely depended on in the case of other bodies, is only available 

 in the case of albumin, and as proteins are altered in their 

 essential properties by very many reagents, the choice of methods 

 of purification is limited. The difficulty is still further increased 

 by the peculiar behaviour of casein in retaining calcium salts, 

 once it has been brought into contact with them, as is the case 

 in milk. The proteins of milk have been prepared in as pure 

 a state as possible by the general method of precipitating them 

 by some reagent, dissolving them, reprecipitating as many times 

 as may be thought necessary, and, finally, by eliminating such 

 impurities as may have been introduced during the process. As 

 there is no means of knowing when all the impurities have been 

 eliminated, it is possible that we are yet unacquainted with the 

 proteins of milk in a state of purity. This should not be for- 

 gotten during their study. 



The proteins are composed of carbon, nitrogen, hydrogen, 

 oxygen, and usually sulphur and phosphorus. The exact mode 

 of combination of the elements in any protein is not known, 

 but recent researches, notably by Hofmeister, Schiff, Curtius, 

 Kiihne, Neumeister, Hammarsten, E. Fischer, Abderhalden, 

 Chittenden, Osborne, and Skraup, have thrown much light on 

 the types on which proteins are formed. 



Of the ultimate products obtained by the continued breaking 

 down of proteins either by enzymes, acids or other hydrolysing 

 agents, the most important, both in character and amount, are 

 amino-acids ; it has been found that nearly, if not quite, all 

 the amino-acids obtained from proteins are a-compounds of the 



