Aside from the skin and seeds, all fruits consist essentially of two parts,—the cellulose structure containing the juice, and the juice itself. The latter is water, with a small proportion of fruit sugar (from one to twenty per cent in different varieties), and vegetable acids. These acids are either free, or combined with potash and lime in the form of acid salts. They are mallic, citric, tartaric, and pectic acids. The last-named is the jelly-producing principle.
While the juice, as we commonly find it, is readily transformable for use in the system, the cellular structure of the fruit is not so easily digested. In some fruits, as the strawberry, grape, and banana, the cell walls are so delicate as to be easily broken up; but in watermelons, apples, and oranges, the cells are coarser, and form a larger bulk of the fruit, hence are less easily digested. As a rule, other points being equal, the fruits which yield the richest and largest quantity of juices, and also possess a cellular framework the least perceptible on mastication, are the most readily digested. A certain amount of waste matter is an advantage, to give bulk to our food; but persons with weak stomachs, who cannot eat certain kinds of fruit, are often able to digest the juice when taken alone.
Unripe fruits differ from ripe fruits in that they contain, starch, which during ripening is changed into sugar, and generally some proportion of tannic acid, which gives them their astringency. The characteristic constituent of unripe fruit, however, is pectose, an element insoluble in water, but which, as maturation proceeds, is transformed into pectic and pectosic acids. These are soluble in boiling water, and upon cooling, yield gelatinous solutions. Their presence makes it possible to convert the juice of ripe fruits into jelly. Raw starch in any form is indigestible, hence unripe fruit should never be eaten uncooked. As fruit matures, the changes it undergoes are such as best fit for consumption and digestion. The following table shows the composition of the fruits in common use:—
Water. Albumen. Sugar. Free Acid. Pectose. Cellulose Mineral Matter. Apples 83.0 0.4 6.8 1.0 5.2 3.2 0.4 Pears 84.0 0.3 7.0 0.1 4.6 3.7 0.3 Peaches 85.0 0.5 1.8 0.7 8.0 3.4 0.6 Grapes 80.0 0.7 Glucose. Tartaric. 3.1 2.0 0.4 13.0 0.8 Plums 82.0 0.2 3.6 0.5 5.7 ... 0.6 Gooseberries 86.0 0.4 7.0 1.5 1.9 2.7 0.5 Strawberries 87.6 0.5 4.5 1.3 0.1 ... 0.6 Raspberries 86.+ 0.5 4.7 1.3 1.7 ... 0.4 Currants 85.2 0.4 6.4 1.8 0.2 ... 0.5 Blackberries 86.4 0.5 4.4 1.1 1.4 ... 0.4