40 INTRODUCTORY — THE CONSTITUENTS OF MILK. 



An ash of this composition is only formed when the milk is 

 homogeneous ; if it is curdled, by natural souring or by the 

 addition of acids, the precipitated lumps do not contain sufficient 

 alkali metals to form these compounds, and much calcium and 

 magnesium phosphates are formed ; on dissolving in water, 

 soluble alkaline phosphates go into solution, and calcium and 

 magnesium phosphates, together with varying proportions of 

 double phosphates, are left insoluble. Curdled milk gives the 

 same total proportion of ash as fresh milk, but the soluble ash is 

 higher and the insoluble ash lower. 



About 8 per cent, of the phosphoric acid present in the ash is 

 derived from the phosphorus of the casein ; the traces of carbonic 

 acid present are not true mineral constituents of the milk. 



Deducting these, we have a considerable excess of bases over 

 acids ; in the milk these bases are combined partly with the 

 proteins to form soluble salts, and -partly with citric acid to form 

 citrates. 



Citric acid is contained in milk to the extent of O'l to 0'15 per 

 cent. ; its most characteristic salt is the calcium citrate, Avhich 

 is fairly soluble in cold water, but insoluble in boiling water. 

 It is a tribasic acid, and forms three classes of salts. 



Soldner deduces the following composition as most probable 

 for the salts existing in milk : — 



Sodium chloride, NaCl, 

 Potassium oliloride, KCI, . 

 Mono-potassium pliosphate, KH2PO4, 

 Di-potassium pliosphate, K2HPO4, . 

 Potassium citrate, K;i(C',;H;,07), 

 Di -magnesium phospliate, ilgHPOj, 

 Magnesium citrate, Mg3(C6H507)2, 

 Di-calcium phosphate, CaHP04, 

 Tri-caleium phosphate, Cao(P04)2, 

 Calcium citrate, Ca3(C6H607)2, . 

 Lime combined with protein, 



Per cent. 



10-62 

 016 



12-77 

 I •■22 

 5-47 

 3 71- 

 4-05 

 7-42 

 8-90 



23-55 

 5 13 



100-00 

 The mineral salts, as stated above, would amount to 0-90 per 



cent., as against 0-75 per cent, of ash obtained. 



According to Soldner .36 to 56 per cent, of the phosphoric 



acid and 53 to 72 per cent, of the lime are not in solution, but 



are in the colloidal form. 



The following shows the distribution of the phosphoric acid 



of the milk according to the author's experiments : — 



J PoU.^ as casein, combined with CaNa, . 0-(KiO5 per cent 

 i PA as Ca3(P0j)„, . 0-0625 , 



PAasRoHPOj, . 0-077 



PAasRHjPO^, . . 0-020 



Total P„0-, . 0-220 



