408 ANIMAL BIOCHEMISTRY 



ammonia is liberated and is distilled into an acid of known strength. 

 The amount of ammonia present is calculated after titration of the 

 excess acid. This value is converted to nitrogen and thence to the 

 amount of crude protein present. The term crude is used advisedly, 

 since this method determines the total organic nitrogen, regardless of 

 source. There are, of course, small amounts of nitrogenous com- 

 pounds present in feeds which are not proteins. For this reason the 

 term crude protein is applied. 



Crude Fat or Ether Extract 



When dry feeds are extracted with anhydrous ethyl ether, all ether- 

 soluble materials dissolve. This extract, consisting mainly of glycerides 

 of the fatty acids, is weighed after evaporating the ether and the 

 percentage computed. Since the ether extract contains appreciable 

 amoiuits of sterols, pigments, and other ether-soluble materials, the 

 residue has been given the name crude fat or ether extract. 



Crude Fiber 



When a feed is boiled consecutively with sulfuric acid (1.25 per 

 cent) and sodium hydroxide (1.25 per cent), the proteins, fats, and 

 most carbohydrates are hydrolyzed, leaving, after filtration, the more 

 resistant woody material (fiber). This is dried, weighed, and burned. 

 The loss in weight on burning is assumed to be the indigestible organic 

 portion of the feed and is known as "crude fiber." Although it is an 

 index of the amount of indigestible matter in the feed, this designa- 

 tion is only approximately true for the reason that ruminating ani- 

 mals and horses can utilize appreciable amounts of crude fiber. Never- 

 theless it is the least valuable portion of the feed and for this reason 

 should be kept at a minimum in feeds sold on the market as concen- 

 trates. 



Ash 



A weighed portion of a feed is ignited in a muffle furnace or over 

 a gas flame until the carbonaceous matter is completely oxidized and 

 the mineral salts remain in the crucible or dish as a white or grayish 

 ash. This ash does not contain the mineral salts originally in the feed, 

 since many of the salts are changed during the heating process. Or- 

 ganic substances containing such elements as sulfur and iodine are 

 destroyed, and varying amounts of these elements may be lost by 

 volatilization. If the temperature of heating is sufficiently high, it is 

 possible that other elements such as sodium and potassium may be 

 lost also. Usually, however, the temperature is controlled to prevent 



