MILK AND MILK PRODUCTS 19 



tose and proteids may be calculated (approximately), the specific 

 gravity, fat, total solids, and ash being known. Thus : 



P-2.8T+ 2-5A-3.33F-o.7-g-; 



in which P is the proteids, T the total solids, A the ash, F the 

 fat, D specific gravity (water at 15.5 being taken as i), and 

 G 1000 D 1000. 



The difference between the total solids and the fat, proteids, 

 and ash gives the lactose. In this formula it has been assumed 

 that everything that is not fat, proteids, or ash, is milk-sugar, 

 an assumption which is not strictly correct, and which intro- 

 duces a small error. Another slight error is introduced by the 

 fact that the ash in milk is not the same as the salts existing 

 in the milk. 



Total Proteids. For practical purposes the total pro- 

 teids are best estimated by calculation from the total nitrogen 

 obtained by the Kjeldahl- Gunning method. Milk contains, 

 however, a sensible proportion of non-proteid nitrogen. Ac- 

 cording to Munk, this may range, in cow's milk, from 0.022 

 to 0.034 per cent., and from 0.014 to 0.026 per cent, in human 

 milk. By these figures, the average proteid nitrogen in cows' 

 milk would be 94 per cent., and in human milk 91 per cent., 

 of the total nitrogen. 



KJELDAHL-GUNNING METHOD 



Reagents 



Potassium Suljate. A coarsely powdered form free from 

 nitrates and chlorids should be selected. 



Suljuric Acid. This should have a sp. gr. 1.84 and be free 

 from nitrates and ammonium.' 



Standard Acid. ~- Sulfuric or hydrochloric acid, the strength 

 of which has been accurately determined. 



Standard Alkali. - - Ammonium hydroxid, sodium hydro xid, 

 or barium hydroxid, the strength of which in relation to the 

 standard acid must be accurately determined. 



