194 THE CARBOHYDRATES 



at about 80-90° ; on addition of 2-3 times its volume of 95 

 per cent alcohol, the solution gives a precipitate of the pectin 

 in a yield of approximately o-i per cent, or, in the case of the 

 turnip, almost double this quantity. 



Besides being extracted by ammonium oxalate, pectin 

 may also be extracted by means of warm solutions of sodium 

 or ammonium salts whose anions form insoluble salts with 

 calcium, such as sodium carbonate or ammonium tartrate. 



If the dried and ground tissues are extracted with 8 per cent 

 sodium hydroxide, free from carbonate, previous to extraction 

 with the ammonium oxalate, the sodium hydroxide solution 

 will be found to contain no pectin (provided the caustic soda 

 used was free from carbonate) ; it contains instead a mixture 

 of substances which can be precipitated by an equal bulk of 

 95 per cent alcohol ; this material reduces Fehling's solution 

 only after hydrolysis and is coloured blue by iodine ; the 

 substances comprising this mixture yield furfural equivalent 

 to pentose contents ranging from 40-85 per cent ; they 

 are presumably hemicelluloses and are described as Cyto- 

 pectins * ; though extracted from the tissues by alkali, they 

 are not all precipitated from these solutions on addition of 

 acid. 



If the residue remaining after extraction of the tissues with 

 caustic soda is washed free from alkali and extracted with 

 warm 0-5 per cent ammonium oxalate, the resulting extract, 

 on treatment with hydrochloric acid, yields a precipitate of 

 pectic acid. 



{b) Hot Water Method. — Ehrlich extracts sugar beet residues, 

 which contain about 25 per cent of pectin, by heating with 

 water in an autoclave under 1-2 atmospheres pressure; this 

 treatment yields a solution of what is described as hydro- 

 pectin. Hydropectin when extracted with 70 per cent alcohol 

 yields an extract which contains an araban, while the residue 

 insoluble in alcohol is a water-soluble calcium magnesium salt 

 of pectic acid ; a careful study of the products of the hydrolysis 

 of this substance has shown it to be a galacturonic acid 

 derivative (see p. 196). 



* Clayson, Norris, and Schryver : " Biochem. Journ.," 1921, 15, 643. 



