NITROGENOUS ORGANIC COMPOUNDS OF PLANTS 225 



it contains various nitrogenous bodies, not proteids. 

 Pure protein is a simple chemical compound, while crude 

 protein consists of a group of compounds of which pure 

 protein is one. The albumin of eggs and milk and the 

 gluten of grains are types of 

 pure proteids. In many foods, 

 as potatoes, roots, and fruits, 

 less than half of the crude pro- 

 tein is pure protein. Crude pro- 

 tein, from different sources, is 

 unlike in character, composition, 

 and, to a certain extent, in food 

 value. Less is known of its 

 composition and food value than 

 of any other class of nutrients 

 in foods. 



In the analysis of plant and 

 animal substances, the chemist 

 first determines the per cent, of 

 total organic nitrogen and then 

 multiplies this by 6.25 to obtain 

 the equivalent amount of crude 



protein. This is because the Fig. 81. Digestion apparatus 



used in the determination of 



proteids contain, on the average, nitrogen. 

 about 1 6 per cent, nitrogen, or there is about one part of 

 nitrogen to every 6.25 of protein (100-^-16 6.25). 

 The nitrogen can be determined with accuracy ; in fact, 

 the method for its determination is one of the most accu- 

 rate in chemistry. In brief, the method consists in first 

 digesting a small weighed amount of material in a flask 

 15 



