Fruits, speaking generally, are composed largely of water, with starches, sugar, organic acids, pectin, cellulose, essential oils, and ethers. Some fruits contain a little protein, chiefly as albumin. Fruit sugars are sucrose and invert sugar, containing dextrose and levulose in equal amounts.

Fruits are valued chiefly for the water in them and the vegetable acids, free or in combination, upon which depend their valuable properties as antiscorbutics. These acids are chiefly malic acid in apples, tartaric acid in grapes, and citric acid, found in lemons, oranges, and kindred fruits. They are generally combined with potash or soda so as to form acid salts. The organic acids in the case of fruit are rich in potash salts, and contain also those of lime, magnesia, and iron; apples, and more especially strawberries, have also much soda. Fruits all contain pectin, or vegetable jelly, cellulose, and an insoluble substance - pectose - which, by the action of a ferment in the plant, is converted into pectin in the process of ripening. This pectin forms the main constituent of fruit jellies.

Fruits may be classified as follows: -

1. Apple, pear, quince.

2. Orange, lemon, lime.

3. Stone fruits - e.g., plum, peach, apricot, cherry, olive, date.

4. Acid fruits - e.g., grape, gooseberry, currant, cranberry.

5. Fleshy fruits - e.g., strawberry, raspberry, blackberry, mulberry.

6. Pineapple, banana.

7. Nuts.

The composition of the edible part of fruits is given in the following table. The carbohydrates include sugar and cellulose and vegetable gums. As a general rule, it may be taken that about Co per cent, of the total carbohydrates consists of fruit sugar or levulose: -

In the following table there are given the proportions between acid, sugar, and pectin in some common fruits: -