26 NUTRITION OF FARM ANIMALS 



43. Coagulation. An important property of the proteins 

 is that of coagulation. For instance, if a solution of ordinary 

 egg albumin be heated to 55 C. the albumin begins to separate 

 in an insoluble form and at about 6'o C. the precipitation is 

 complete. This change differs from the change in the case of 

 gelatin solutions from liquid to solid in being irreversible, i.e., 

 coagulated protein cannot be changed back to the soluble form. 

 It should be noted that this change is entirely distinct from the 

 precipitation of proteins by means of ammonium sulphate for 

 example. The exact nature of the change is unknown, but it 

 would seem to be in part chemical in character. 



All forms of protein appear to be subject to coagulation in 

 the chemical sense of the word. Thus the precipitated proteins 

 obtained from solutions are at first in the colloidal form but on 

 standing pass more or less rapidly into the coagulated or " de- 

 natured " form. The same is true of the solid proteins like 

 fibrin, etc. The coagulated proteins are insoluble in water 

 and salt solutions, but may be dissolved in acids or alkalies. 



The simple proteins 



44. Composition. The simple proteins differ from the com- 

 pounds considered in the previous sections in containing, in 

 addition to carbon, hydrogen and oxygen, the elements nitro- 

 gen and sulphur. Notwithstanding the considerable variation 

 in the properties of the different simple proteins and the notable 

 differences which have been shown to exist in their chemical 

 structure, their elementary composition differs but little. 

 Cohnheim 1 quotes the following figures from Michel for the 

 composition of serum albumin, which is in many respects a 

 typical animal protein. 



Carbon 53-o8 



Hydrogen 7.10 



Nitrogen 15.93 



Sulphur 1.90 



Oxygen 21.99 



100.00 



The variations in the percentages of the principal elements 

 as stated by Cohnheim 1 and by Plimmer 2 are : 



1 Chemie der Eiweisskorper, 2d Ed., p. 151. 



* The Chemical Constitution of the Proteins, Part I, p. 2. 



