6 CONSTITUENTS OF MILK 



Proteids. The proteids of milk are: 



Per Cent. 



Caseinogen approximately 2.0-3.0 



Lactalbumin. approximately 0.3-0.8 



Lactoglobulin a trace 



Mucoid proteid a trace 



Caseinogen* is a distinctly acid phospho proteid which does 

 not contain purine or pyrimidine derivatives. Lactalbumin, as 

 its name implies, is one of the albumins and, therefore, soluble 

 in water and coagulated by heat. Lactoglobulin is insoluble 

 in water but soluble in salt solutions. 



According to Richmond the proteids of milk are characterised 

 by the following reactions: Caseinogen is precipitated by adding 

 sodium chloride, magnesium sulphate, or ammonium sulphate 

 to saturation: globuUn is soluble in a saturated solution of 

 sodium chloride but is precipitated by magnesium and ammo- 

 nium sulphates: albumin is soluble in saturated solutions of 

 •sodium chloride and magnesium sulphate but is precipitated 

 by ammonium sulphate. Albumin, however, may be precip- 

 itated by magnesium sulphate in slightly acid solutions but is 

 reJissolved on neutrahsation of the solution. These reactions 

 are relative rather than specific and cannot be relied upon for 

 quantitative separation of the various proteids: they may, 

 however, be used for preparing the pure proteids by redissolving 

 and reprecipitating the various fractions. Other methods may 

 also be used for the separation of the proteids. For example, 

 the caseinogen may be removed by the action of chymase, the 

 lab ferment of rennet, or by filtration through coarse porcelain : 

 filtration through fine porcelain or boiling with a small quan- 

 tity of acid followed by filtration will remove all the proteids. 

 Lactalbumin is slowly coagulated by heating at 70° C, but 

 very little is precipitated when the acidity is normal. Casein- 



* Caseinogen is used in these pages to designate the mother substance 

 and paracasein the rennet transformation product: this nomenclature, 

 though not strictly logical, eliminates the ambiguity that arises from the 

 difference in the prevailing EngUsh and American phraseology. 



