ISO COMPOSITION OF FEEDING STUFFS 



acidsj etc. The separation of these substances from the fat is 

 not easy, and they are therefore included under the heading of 

 crude fat or ether extract. 



Nitrogenous Constituents. 



143. (d) Crude Protein. 



This estimation is based on the incorrect assumption 

 that the proteins in the food-stuff are the only nitro- 

 genous compounds present, and that they contain an 

 average of i6 per cent, of nitrogen. The nitrogen is 

 determined by the Kjeldahl process (7, p. lo) and 

 multiplied by 6-25 or other special factor (see 10, p. 18), 

 to obtain the weight of protein. 



{e) True Protein or Albuminoids. 



In estimating separately the true protein and the amides, 

 amino acids, etc., which are estimated with it under the heading 

 of "Crude Protein," advantage is taken of the property which 

 proteins possess of being precipitated by certain metallic salts 

 or tannic acid. Cupric hydroxide is generally used for the 

 purpose. 



Cupric Hydroxide Reagent. — Twenty grams of copper 

 sulphate are dissolved in i litre of water to which 

 25 grams of glycerol are added ; dilute caustic soda is 

 added until the mixture is just alkaline, and the blue 

 precipitate of Cu(0H)2 is then filtered off and washed 

 with water containing glycerol (5 grams glycerol per 

 litre) till the washings cease to be alkaline. The 

 precipitate is rubbed up in a mortar with water contain- 

 ing 10 per cent, of glycerol to form a uniform 

 gelatinous mass, which can be measured off in a 

 pipette. The amount of Cu(0H)2 per cent, of this 

 liquid is determined. 



Estimation of True Protein. — One gram of the food- 

 stuff sample is added to 100 c.c of water and the 



