THE CHEMISTRY OF FRUIT-RIPENING. 467 



Why this fermentation occurs just at the ripening-time, and not 

 earlier or later, we do not precisely know. It may be that the pec- 

 tose has just then become capable of fermentation, or the pectase then 

 acquires potency for its office, or then, and not before, are other con- 

 ditions of the change established. We know only that the fermenta- 

 tion gives us the before-mentioned pectous substances, which, more- 

 over, succeed each other, during ripening, by repeated changes. It 

 must be confessed that these products have been but imperfectly de- 

 fined, but as a class their chief properties are known. They are given 

 by chemists as follows (distinctions having value only in analysis be- 

 ing omitted) : 



Pectine: readily soluble in hot or cold water, gelatinizing when concen- 

 trated, and more perfectly by addition of sugar; changed by very long boiling 

 to parapectine. 



Pectic acid: gelatinous, insoluable in cold water, and but slightly soluble in 

 hot water; hardened in jelly by solution of sugar, slowly changed by boiling 

 to parapeptic acid, and afterward to metapeptic acid. Pectine and pectic acid 

 result from long boiling of the crude pectose. 



Parapectine: soluble in water, capable of gelatinizing slightly, changed by 

 boiling to metapectine. 



Parapeptic acid : soluble in water, the solution changing into one of meta- 

 peptic acid. Not gelatinous. 



Metapectine: soluble in water, not gelatinous. (Found in overripe fruits.) 



Metapectic acid: soluble in water, incapable of gelatinizing. (Found in 

 overripe fruits; produced by fermentation in overripening from all the other 

 pectous substances. Also produced, from most of the other pectous substances, 

 by long boiling, much more readily if acids are present.) 



Alkalies change pectine and parapectine and metapectine to salts 

 of pectic acid. 



The properties of the separated pectous compounds represent cer- 

 tain well-known characteristics of fruits, as these are found in cooking. 

 Moist heat, as In any mode of cooking, produces upon these sub- 

 stances the chief results of ripening, and, if continued long enough, 

 the results of overripening. Unripe fruits are made more edible and 

 wholesome by cooking, owing to its artificial (imperfect) ripening of 

 pectose. Fruit-jellies owe their substance to pectic acid, pectine, 

 and slightly to pai'apectine, the products of early maturity, with the 

 cooperation of sugar. For jellies, it is well known, the use of over- 

 ripe fruits must be avoided, and too long boiling in the preparation 

 must be avoided. If the fruit be underripe, the juice should be 

 boiled much longer than if the fruit be fully ripe, and if the fruit be 

 overripe, boiling should be maintained no longer than necessary to 

 clarify, and standing in hot solution should be avoided. Grapes bear 

 full ripening for jellies. 



The following statements of the quantities of pectous substances 

 and of pectose are compiled from the reports of Fresenius. It should 



