CARBOHYDRATES 27 



buds, they may be concerned with imbibition and holding of water. Gums, on the other 

 hand, are produced in response to injury and are found as exudates on various trees, 

 sometimes associated with triterpenoid compounds as gum-resins. 



Gums and mucilages are all either soluble in water or strongly hydrophilic. Solu- 

 tions are levorotatory. They frequently occur with some of the glucuronic acid groups 

 as sodium, potassium, or calcium salts, but enough free carboxyl groups usually remain 

 to produce a slightly acidic reaction. The monosaccharide components of some plant 

 gums and mucilages are listed in Table 2. It must be noted that hydrolysis normally re- 

 sults in the appearance of aldobiouronic acids which are cleaved to the monosaccharides 

 only under more drastic treatment. Occasionally hydroxyl groups may be methylated or 

 acetylated. 



ISOLATION 



Many of the carbohydrates are soluble in water. Prolonged extraction of defatted 

 material with neutral boiling water will leave undissolved the cell-wall polysaccharides, 

 as well as non-carbohydrate materials. Lignin can be removed from the insoluble cell- 

 wall material by treatment with chlorine dioxide (19). Extraction with cold alkali now re- 

 moves hemicelluloses leaving a residue of cellulose. The strength of alkali can be varied 

 if selective extraction of hemicelluloses is desired. 24% KOH (or 17. 5% NaOH) is com- 

 monly used and leaves a residue known as "a-cellulose, " which may, however, contain 

 as much as 40% other polysaccharides such as xylans, mannans, etc. , depending on the 

 source. It is therefore apparent that alkaline extraction does not quantitatively remove all 

 hemicelluloses. Further purification of the cellulose involves solution in 85% phosphoric 

 acid and precipitation of cellulose by adding three volumes of distilled water (20). Cellu- 

 lose so obtained has suffered considerable degradation (e. g. to a chain length of about 

 160 glucose units as compared to several thousand in native cellulose). Unfortunately 

 purification is necessarily attended by degradation, and pure, native cellulose can only 

 be obtained from a plant source like cotton which is almost pure cellulose already. 



Returning to the alkaline solution of hemicelluloses, this may be fractionated making 

 it acidic (pH ca. 4. 5) whereupon the hemicelluloses of high molecular weight precipitate. 

 The hemicelluloses remaining in solution may then be precipitated by adding alcohol, ace- 

 tone, etc. , to the supernatant solution. Further separations take advantage of special 

 properties of the different types of compounds. Xylans and mannans may be separated 

 from other cell wall polysaccharides by virtue of the fact that they form an insoluble copper 

 complex in alkaline solution (21). This can be separated and the polysaccharides regen- 

 erated by acidification. It is not normally required to separate xylan from mannan since 

 they do not usually occur together. 



The hot water extract contains low molecular weight compounds in free solution as 

 well as colloidal suspensions of the hydrophilic polysaccharides (and non-carbohydrate 

 material such as organic acids and amino acids). If fructans are known to be present, a 

 cold water extract may be preferred since some of them are readily hydrolyzed. Pectic 

 substances are usually extracted using dilute acid (pH 2. 5) or ammonium oxalate solution 

 (0. 5%). Depending on the source, the polysaccharides in the water extract might be gums, 

 mucilages, starch, fructans, pectic substances, galactomannans, etc. There is no need 

 of general methods to separate any one of these from all the others since in a given ma- 

 terial only two or three types at most will be present, and methods chosen will depend 

 on previous characterization. Generally all of them can be precipitated by adding ethanol 

 to reach a concentration of 80%. (0. 5% ammonium sulfate may aid the ethanol precipita- 

 tion. ) However, some of the shorter chain fructans are soluble in this concentration of 

 ethanol. Amylose is separated from amylopectin by adding compounds such as butanol, 

 thymol or nitrobenzene to a hot starch suspension. Amylose is precipitated as an insol- 

 uble complex which can be decomposed by ethanol to recover the amylose. 



