THE COMPOSITION OF FEEDING STUFFS 9 



of organic plant substances. It was formerly believed that all pro- 

 teins contain about 16 per cent nitrogen, and, since this element 

 can be readily determined by the chemist, the content of protein in 

 a substance was obtained by multiplying the nitrogen content by 1 , 

 or 6.25. Later investigations showed that the different protein sub- 

 stances vary considerably in chemical composition, and that the per- 

 centage of nitrogen they contain may range from 15 to over 19 per 

 cent. Hence, the factor 6.25 is often not correct, but it is generally 

 applied, since our knowledge of the composition of pure proteins 

 from different sources is still incomplete. We shall, therefore, con- 

 tinue the use of the factor 6.25 in this book until chemists have 

 agreed on specific factors to be used in the case of plant materials 

 and feeding stuffs of different origin. 



Besides nitrogen, proteins contain between 50 and 55 per cent 

 of carbon, 6 to 7 per cent hydrogen, 20 to 24 per cent oxygen, 

 0.3 to 2.3 per cent sulfur; the phosphorus content of the proteins 

 in which this element is present ranges between 0.4 and 0.9 per 

 cent. The average composition of protein substances may be given 

 as follows : 



Carbon, 53 per cent. Nitrogen, 16 per cent. 



Hydrogen, 7 per cent. Sulfur, 2 per cent. 



Oxygen, 22 per cent. 



The proteins form a most important group of nutrients, since 

 they furnish the materials for building up body tissues and fluids; 

 other nutrients cannot take their place for this purpose. We shall 

 see, however, that protein may also serve other purposes than to 

 furnish material for tissue building when necessary, viz. : To supply 

 energy that may be used for maintaining body heat, for performing 

 work, or for storage as body fat. 



Classification of Proteins. Protein substances are generally classified 

 as (1) simple, (2) conjugated, and (3) derived proteins. 



1. Simple Proteins. The most important compounds in this group 

 are given below. 



a. Albumins. These are soluble in pure water and are coagulated and 

 rendered insoluble by heat. They are present in small amounts in the sap 

 and seeds of plants. The main proteins found in the animal body belong 

 to this class, viz.: Those of the muscle, blood, milk, and eggs. Leucosin 

 found in the cereals, legumelin found in leguminous seeds, ricin in castor 

 bean, and tuberin in potatoes, belong to this class. 



&. Globulins are insoluble in water, but soluble in a 10 per cent sodium 

 chloride solution. The globulins are abundant in plant materials and have 

 been identified in many seeds of plants. The following are present in 

 the cereals and other common seeds: Maysin in corn kernels, edestin in 

 corn, wheat, cotton seed, and flaxseed, avenalin in oats, legumin and 

 vicilin in leguminous seeds (peas, lentils, horse beans), glycin in soy- 

 beans, and conglutin in lupines. 



