THE PECTIC SUBSTANCES 65 



Haas and Tollens (1895) leave no doubt as to its carbo- 

 hydrate nature, though, owing to the great difficulty of 

 purifying such gelatinous material, there is a certain amount 

 of fluctuation in the ratio of hydrogen to oxygen. Tollens 

 (1914) thinks it possible that the small amount of oxygen 

 found in excess of that required for a carbohydrate may be 

 due to the existence of one or more carboxyl groups. He 

 suggests for pectin the formula 5(C 10 H 18 9 ) + C 10 H 18 O 10 . 



To sum up, three enzymes are said to be concerned in 

 the changes undergone by pectic substances. Pectose, 

 which occurs as the middle lamella of cell walls, is split up 

 by pectosinase to form pectin. Pectin, a soluble sub- 

 stance, is transformed into a pectic acid by pectase. In 

 presence of traces of salts this gelatinous substance be- 

 comes aggregated into clumps which sink in water. Soluble 

 pectin may also be acted upon by pectinase, with the pro- 

 duction of sugars. 



For the obvious reason that there is no ready chemical 

 method for studying the changes brought about by these 

 enzymes, it has resulted that their action is but little known. 

 The role played by calcium salts in the bringing about of 

 the coagulation of pectin by pectase was demonstrated by 

 Bertrand and Mallevre (1894, 1895), as was the fact 

 that calcium could be replaced by strontium or barium. 

 They also ascertained that acids have a retarding effect 

 upon coagulation, 0-088 per cent, hydrochloric acid pro- 

 longing the time required for the process from one to forty 

 hours, whilst 0-1 per cent, completely stopped all action. 

 The behaviour of other mineral and organic acids was 

 similar. With increased concentrations of calcium salts 

 or of enzyme the effect of the acid is not so powerful. The 

 time necessary for coagulation was thus found to depend 

 upon the proportions of pectase, pectin, calcium salts, and 

 free'acids, 



