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. Many of them are extremely low in this respect, while a few of them are rather high. In order to determine the place that fruit should have in a meal, it is necessary to obtain a definite idea of the composition as well as the food value of the different varieties.
Such 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. Then, too, there is a small amount of protein in grapes and some other fruits, but it is not sufficient to merit consideration.
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. In addition, cane sugar is contained in the majority of fruits. Pectin is also a carbohydrate that is found in large quantities in some fruits, while in other fruits it is lacking. This substance is related to the gums and to cellulose. Although it is one of the carbohydrates from which no food value is derived, it is of considerable importance, because it is responsible for the jelly-making properties of fruits.
In fruits that are not fully matured, or, in other words, green fruits, the sugar has not developed to so great an extent as it has in perfectly ripe fruits. Consequently, such fruits are not so high in food value as they are when they become ripe. As is well known, it is the sugar of fruits that accounts for their sweet taste, for the sweeter the fruits, the more sugar and the less acid they contain. The quantity of this substance varies from 1 per cent. in lemons to 20 per cent. in some other fresh fruits, such as plums. In dried fruits, the amount of sugar is much higher, reaching as high as 60 per cent. or even more in such fruits as figs, dates, and raisins.
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, for, 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. The skins and seeds of fruits, as well as the coarse material that helps to make up the pulp, are known as refuse and are treated as such by the human digestive tract; but it is to this waste material, or cellulose, that the laxative quality of fruit is largely due.
In cases where there are digestive or intestinal troubles, it is often necessary to remove the cellulose before the fruit is eaten. The coarse material may be removed and that which is more tender may be broken up by pressing the fruit through a sieve or a strainer of some kind. The cooking of fruits is another means of making the cellulose in them more easily digested, for it softens, or disintegrates, the various particles of the indigestible material. When fruit is taken for its laxative effect and the irritation of the cellulose needs no consideration, the skins of the fruits may be eaten instead of being rejected. However, to avoid any trouble, they should be well chewed.
All fruits contain a certain percentage of mineral salts. The quantity varies in the different kinds of fruits, but it averages about 1 per cent. 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. In other words, they have a tendency to render the blood more alkaline and less acid. They are therefore one of the food constituents that help to make fruit valuable in the diet and should be retained as far as possible in its preparation. In fact, any method that results in a loss of minerals is not a good one to adopt in the preparation of fruits.
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 juice of fruits that contain very little sugar and a large quantity of acid, such as the lemon, may be used for the seasoning of food in much the same way that vinegar is used. It may also be diluted with other liquids and used for a beverage. Then, again, various kinds of fruit juices are subjected to a process of fermentation and, through the production of another acid, are made into vinegar and wines. When apples are treated in this way, the fermentation produces acetic acid and, in addition, a certain amount of alcohol. It is on this principle that the making of wines depends.
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. Fruits of this kind, in addition to having this refreshing quality, help to provide the necessary liquid in the diet.