The composition of fruits is a matter of considerable importance, for on it the food value of the fruits depends. To a certain extent, the composition of all fruits is the same but the varieties of this food differ in their food values almost as greatly as do vegetables. small quantities of protein and fat are contained in fruits that very little attention need be given to these substances.
Exceptions are found in avocados or alligator pears and in ripe olives, both of which are high in fat. Whatever food value fruits may have, whether it be high or low is due to the carbohydrate they contain. Some green fruits and bananas contain a very small amount of starch but on the whole the carbohydrate of fruits is in the form of sugar and is in solution in the fruit juices. The chief form of this carbohydrate is known as 'levulose' or 'fruit sugar'. However, 'glucose', another form of sugar, is also found in nearly all fruits, grapes and dried fruits such as figs, raisins, etc., containing an unusually large amount.
All fruits contain a certain percentage of mineral salts. The quantity varies in the different kinds of fruits. These salts have the opposite effect on the blood from those found in meats and cereals but they act in much the same way as the minerals of vegetables. The minerals commonly found in fruits are iron, lime, sodium, magnesium, potash and phosphorus. These are in solution in the fruit juices to a very great extent and when the juices are extracted the minerals remain in them.
Some fruits contain only a small amount of acid while others contain larger quantities. It is these acids, together with the sugar and the volatile oils of fruits, that constitute the entire flavor of this food. Most ripe fruits contain less acid than unripe ones and cooked fruits are often higher in acid than the same fruits when raw. Numerous kinds of acid are found in the different varieties of fruits. For example, lemons, oranges, grapefruit and a few other fruits belonging to the class known as citrus fruits contain 'citric acid'; peaches, plums, apricots, and apples, 'malic acid'; and grapes and many other fruits, 'tartaric acid'.
The water content of fresh fruits is very high reaching 94 per cent in some varieties. Dried fruits on the other hand contain much less water, their content being in some cases as low as 15 to 20 per cent. It naturally follows that the fruits low in water are high in food value, while those containing considerable water have in their composition less of the material that adds food value.
The high percentage of water in fresh fruits together with the acids they contain, accounts for the fact that these fruits are so refreshing. In fruits, as in vegetables, cellulose is found in varying quantities. The larger the quantity, the lower will be the food value of the fruit, except where the water has been evaporated as in the case of dried fruits. The digestibility of this cellulose, however, is not worth considering because while it is possible that small amounts of very young and tender cellulose from fruits may be digested, on the whole this characteristic may be disregarded