18 INTRODUCTORY — THE CONSTITUENTS OF MILK. 



passes over with water to a slight extent as the solution becomes 

 concentrated. 



Lactic acid is soluble in and miscible in all proportions with 

 water, alcohol, ether, and glycerol. It is insoluble in petroleum 

 ether. Fats also dissolve it. It is probable that the lactic acid 

 present in milk is, partially at all events, dissolved in the fat. 



As milk almost immediately after milking contains organisms 

 which produce lactic acid, it may be considered as a normal 

 constituent of milk ; indeed Bechamp has held that it is produced 

 from milk by organisms (micro-zymes) derived from the udder 

 itself. That this view is erroneous is shown by the fact that 

 Lister, Pohl, Warington, and others have succeeded in preserving 

 milk, drawn direct into sterilised vessels, for a considerable 

 length of time without the development of acidity. 



Lactic acid probably exists in milk, not in the free state, but 

 as a salt, at all events until the acidity is sufficient to curdle the 

 milk on boiling. 



The 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, finallv. 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 

 protems of milk m a state of purity. This should not be 

 forgotten during their study. 



The proteins are composed of carbon, nitrogen, hydrooen 

 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 lio-ht on 

 the types on which proteins are formed. ^ 



Of the ultimate products obtained by the continued breakin.^ 

 down of proteins either by enz)-mes, acids or other hydrolvsinS 



