11. 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.
12. PROTEIN AND FAT IN FRUITS.--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.
13. CARBOHYDRATE IN FRUIT.--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.
14. 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.
15. CELLULOSE IN FRUIT.--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.
16. Minerals in Fruit.--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.
17. Acids in Fruit.--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_.