SEPARATION OF PROTEIDS 161 



of proteids. They are exceedingly soluble in water and are not 

 precipitated by sodium chloride, acids or alkalies. They are pre- 

 cipitated by tannic acid, but not by ammonium sulphate. They 

 are not coagulated by heat. Germinating seeds often furnish a 

 large percentage of peptone. 



Inasmuch as proteids are plastic substances and are constantly 

 undergoing tranformation in the metabolism of the plant, a mix- 

 ture of proteids will generally be found in the analysis of any veg- 

 etable organism in various stages of hydrolytic change. Proteids 

 occur to the greatest extent as reserve foods in the various special- 

 ized parts of plants such as tubers, roots and seeds, and very often 

 are found in solid form especially in the cereals. 



227. Extraction of Proteids. The material to be extracted 

 should be very finely divided and then treated first with benzine 

 to remove the bulk of fats, pigments, etc. It is well to follow this 

 treatment with the same quantity of 95 per-cent. alcohol, for a few 

 hours and lastly extract with pure sulphuric ether. In the case 

 of certain cereals in which the quantity of fat or oil is practically 

 nothing, this process may be omitted. 



The extractions should all be carried on at a temperature not 

 to exceed 35 C. in order to avoid coagulating any of the proteids 

 present. The last traces of ether may be removed by spreading 

 out the tissue in shallow dishes and allowing it to dry at room 

 temperature. 



The material should now be covered with at least twice its 

 volume of dilute solution of common salt (about 10 per-cent.) and 

 the extraction allowed to continue with repeated stirring until all 

 the proteids soluble in such a solution are removed. This may 

 be determined by adding fresh solvent from day to day. The 

 extracts which are drawn off are preserved separately, and a little 

 powdered thymol added to each to preserve from mould. 



228. Separation of Proteids. The extract should be filtered and 



the filtrate dialyzed. For this purpose the solution is placed in 



a bag of vegetable parchment and suspended in running water. 



The dialysis should be continued for several days, or until the 



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