Page:Popular Science Monthly Volume 12.djvu/481

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THE CHEMISTRY OF FRUIT-RIPENING.
Sugar. Malic Acid.
June 20th 16.52 2.76 (per cent. in the pulp).
June 24th 16.64 2.46 ""
June 30th 16.78 2.16 ""
July 4th 17.05 1.57 ""
July 12th 17.38 0.82 ""

The green plums contain tannin, which commenced to diminish as soon as the fruit began to emit carbonic acid in the daylight, wholly disappearing by June 20th, the date at which the malic acid began to diminish. It is well known to every one that many green fruits are very astringent, and that their tannin decreases and sometimes disappears during ripening. Also, it is a familiar fact in the chemistry of tannin that it readily undergoes changes producing sugar. This, then, is the source of a portion of the sugar of many fruits. The formation of sugar from tannin will be discussed under the head of the glucosides of fruits.

Several chemists have reported the presence of sugar-producing substances peculiar to fruits. Buignet describes a fruit constituent, astringent like tannin, and combining with iodine like starch, and serving as the source of sugar.

The proportion of cane-sugar, in most fruits, is generally believed to diminish by transformation into glucose, as fruits become fully ripe or overripe. But Berthelot and Buignet (Comptes Bendus, li., 1094) found that, in oranges, the proportion of cane-sugar increased during ripening, the quantity of glucose remaining unchanged.

The increase of weight of fruits, during ripening, is no doubt largely owing to deposition of sugar. Berard found that 100 parts of unripe summer peaches yielded 179 parts of ripe fruit, and 100 parts of unripe apricots increased in ripening to 200 parts.

The maturity of fruit is the period of its maximum quantity of sugar. Sooner or later, the quantity of sugar begins to diminish, and then the fruit is overripe. It is safe to say that the sugar often begins to decompose during the life of the fruit; that is to say, fruit becomes overripe during its life. It would be difficult, however, to fix on the termination of the life of fruit. We certainly cannot say that life ceases when the circulation with the plant is cut off; and we cannot say that life continues in the sarcocarp until it is wholly disintegrated. Now it is within the limits of our subject to inquire by what changes the sugar begins to disappear.

In general terms, sugar suffers oxidation in ripe fruits, small portions being oxidized away even during the production of larger portions, and before perfect maturity. We do not know what fruit constituents, if any, result in this oxidation. The final products of oxidation, carbonic acid and water, are exhaled during ripening, and with greater rapidity after maturity has been passed.

It seems to be established that sugar in fruits is liable to traces of