PECTINS 193 



unripe fruit, whereby the acid juices exercise a hydrolytic 

 effect upon the insoluble precursor and soluble pectin results. 

 Prolonged boiling alters the pectin, with the result that its 

 power to form a jelly is reduced ; similarly, over-ripe fruit 

 loses its coherence owing to the loss of the jellying qualities 

 characteristic of the soluble neutral pectin. 



When the water-soluble pectin is treated with sodium 

 hydroxide it undergoes hydrolysis almost instantaneously, 

 giving off methyl alcohol and leaving the sodium salt of an 

 acid from which, on the addition of a mineral acid, the insoluble 

 pectic acid is precipitated ; this latter substance has lost all 

 power of forming jellies which was the characteristic of the 

 soluble pectin. 



It thus becomes possible to distinguish three stages in the 

 history of the pectins, which are represented in the following 

 classification adopted at the Pectin Symposium of the American 

 Chemical Society in 1925 : — 



1. Protopectin (equivalent to the older term pectose of 

 Fremy). This represents the insoluble precursor of the true 

 pectins and is the form in which these substances occur in the 

 unripe material. 



2. Pectin is the soluble substance capable of forming jellies 

 which occurs free in the plant or is produced from protopectin 

 in ripening or by chemical hydrolysis. Pectin is the methyl 

 ester of pectic acid. 



3. Pectic acid is demethylated pectin and is incapable of 

 forming a jelly. 



Isolation of Pectins from the Tissues. 



Two methods of separating pectins from tissues have been 

 adopted : Extraction by means of ammonium oxalate, and 

 extraction by means of hot water. 



[a) Ammonium Oxalate Method.— This is the method fol- 

 lowed by Schryver and his fellow-workers : the material se- 

 lected—turnips, strawberries, rhubarb petioles, apples, onions, 

 and cabbage — is first ground and pressed to remove soluble 

 pectins ; the residue, after drying and further grinding, is 

 extracted with warm 0-5 per cent ammonium oxalate solution 



13 



