Why this fermentation occurs just at the ripening-time, and not earlier or later, we do not precisely know. It may be that the pectose has just then become capable of fermentation, or the pectase then acquires potency for its office, or then, and not before, are other conditions of the change established. We know only that the fermentation gives us the before-mentioned pectous substances, which, moreover, succeed each other, during ripening, by repeated changes. It must be confessed that these products have been but imperfectly defined, but as a class their chief properties are known. They are given by chemists as follows (distinctions having value only in analysis being omitted):
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 metapeptic 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 certain well-known characteristics of fruits, as these are found in cooking. Moist heat, as In any mode of cooking, produces upon these substances 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 parapectine, the products of early maturity, with the coöperation 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