CARBOHYDRATES 



29 



A generalized outline of the procedures described above is shown below. Obviously 

 each plant is a special case, and this outline cannot be followed rigidly, but it may be a 

 useful guide. Consultation of the literature will fill in experimental details for known 

 plants and carbohydrates, but no cut-and-dried procedure can substitute for an understand- 

 ing of the principles involved or for constant checking of each step in the isolation by char- 

 acterization of the product obtained. 



RAW MATERIAL 

 -(extract with boiling water)s 



insoluble 



(extract with 17. 5% of NaOH) 



/ \ 



insoluble soluble 



Q-cellulose (acidify) 



precipitate soluble 



high molecular 



weight 



hemicellu loses 



soluble 



low molecular 



weight 

 hemicelluloses 



(add ethanol to 80%) 



/ \ 



insoluble soluble 



pectic substances (anion exchange 



starch resin) 



gums removed effluent 



mucilages phosphates, monosaccharides, 



fructans acids oligosaccharides 



CHARA CTERIZA TION 



Aside from the standard methods of organic chemistry, innumerable color tests 

 have been developed for the different classes of carbohydrates. A general reaction given 

 by all carbohydrates is the formation of a color when heated with sulfuric acid and a phenol 

 such as resorcinol, anthrone, a-naphthol, thymol, etc. Cellulose is distinguished by its 

 lack of solubility in all but the strongest acids and alkalies but ready solubility in cupr am- 

 monium hydroxide (Schweizer's reagent). Starch gives the well-known blue color with 

 iodine due to its amylose component. Amylopectin gives a red-purple color with iodine. 

 Pentoses and polysaccharides containing them give a red-violet color withphloroglucinol in 

 hydrochloric acid. Uronic acids also give a positive test; but they may be distinguished by 

 not giving the Bial reaction (a blue color on heating with orcinoland FeClsin hydrochlorc acid). 

 Ketoses are detected by heating with hydrochloric acid and resorcinol when they give a 

 red color (Seliwanoff test). A blue-green color with diazouracil (Raybin reaction) is given 

 by sucrose and other oligosaccharides containing a sucrose moiety such as raffinose and 

 stachyose (24). Fructose and fructans give a red color on heating with urea in concen- 

 trated hydrochloric acid (25). The presence of uronic acids and their polymers can be 

 detected by the evolution of carbon dioxide on heating with 12% hydrochloric acid. Other 

 tests for certain classes of carbohydrates depend on their non-specific reducing power 

 as in the reaction with Fehling's solution, Benedict's solution, ammoniacal silver nitrate, 

 alkaline dinitrosalicyclic acid, etc. Ascorbic acid is distinguished by its especially strong 

 reducing action as shown by reduction of the dye 2, 6-dichlorophenolindophenol, or of 

 silver nitrate in acidic solution. 



With pure compounds measurement of optical rotation is useful in identification. 

 This method can be applied even to polysaccharides which give cloudy aqueous solutions 

 by adding calcium chloride to cause clarification. 



