9. STRUCTURE OF VEGETABLES.--Although vegetables vary greatly in
composition and consequently in food value, they are similar so far as
physical structure is concerned. In general, they consist of a skeleton
framework that is made up of cellulose. Their digestible part is
composed of tiny cells having thin walls that confine the actual food
material in the form of a liquid or semiliquid. As the vegetables grow
old, the cellulose material and the cell walls gradually toughen, with
the result that old vegetables are less easily made tender than young
ones and are not so agreeable to the taste as those which have not grown
hard. The total food value of vegetables, as well as of cereals, meats,
and, in fact, all foods, varies with the quantity of water and cellulose
they contain. Therefore, the vegetables that contain the least coarse
material are the ones that have the highest food value.
10. The green color that characterizes many vegetables is due to a
substance called _chlorophyl_. This substance is essential to the normal
growth of plants and is present in the correct amount in only those
which are properly exposed to the sunlight. Sufficient proof of this is
seen in the case of vegetables that form heads, as, for instance,
cabbage and head lettuce. As is well known, the outside leaves are
green, while the inside ones are practically white. Since it is exposure
to the light that produces the green color, a vegetable or plant of any
kind can be bleached by merely covering it in order to keep out the
sunlight. This procedure also enables the plants to remain more tender
than those which have been allowed to grow in the normal way and become
green. For instance, the inside leaves of a head of lettuce are always
very much more tender than the green outside leaves. In fact, the center
of any kind of plant, that is, the leaves and the stem that appear last,
are more tender, possess a lighter color, and have a more delicate
flavor than the older ones.
11. PROTEIN IN VEGETABLES.--Taken as a whole, vegetables are not high in
protein. Some of them contain practically none of this food substance
and others contain a comparatively large amount, but the average is
rather low. Vegetables that are high in water, such as lettuce, celery,
tomatoes, and cucumbers, contain so little protein that the quantity is
not appreciable. Such vegetables as potatoes, beets, carrots, etc.
contain slightly larger quantities. Dried vegetables, such as beans,
peas, and lentils, contain comparatively large amounts of this
substance, and for this reason may be substituted for such high-protein
foods as meat and fish.
12. The composition of vegetable protein is only slightly different from
that of animal protein. In fact, the experiments of scientists show that
animal protein may be readily replaced by vegetable protein. One of
these proteins is sometimes called _vegetable albumin_, but the chief
protein of vegetables containing the largest amount of this substance,
namely, beans, peas, and lentils, is called _legumin_, from the term
_legumes_, the name of this class of vegetables. It is generally agreed
that vegetable protein is not so digestible as animal protein, but this
disadvantage is offset by the fact that it does not bring about so much
intestinal trouble as does the protein of animal foods and is less
likely to cause disturbances that are usually attributed to foods high
in this substance. Vegetable protein is affected by heat in much the
same way as other protein.
When any of the dry vegetables high in protein are served at a meal,
meat should be eliminated, or the result will be an oversupply of
protein. As this condition is not only harmful but wasteful, it is one
that should receive proper consideration from the housewife.
13. FAT IN VEGETABLES.--As vegetables as a class are low in protein, so
are they low in fat. In the case of some vegetables, the quantity of fat
they contain is so small that it is never considered in discussing the
food value of these vegetables, while in others slightly larger
quantities are to be found. However, on the whole, vegetables are so
nearly lacking in this food substance that it is necessary to supply fat
in their preparation and in the serving of meals in which they are
included. This is done in a variety of ways, depending on the nature of
the vegetable. For instance, in order that baked beans may take the
place of meat entirely, fat in the form of salt pork is usually added
when they are prepared. The pork, of course, also supplies a very small
amount of protein, but it is not used with the beans for this purpose.
Practically all cooked vegetables are served with butter or with a sauce
that contains fat. Green vegetables that require no cooking but are
served as a salad, are supplied with fat by the salad dressing that is
used with them. The fat varies greatly, depending on the kind of
dressing used. 14. CARBOHYDRATES IN VEGETABLES.--When the composition
of vegetables is considered chemically, the most striking thing about
them is the carbohydrates they contain. It is this that distinguishes
this class of foods from animal foods. The carbohydrate of vegetables is
found in both its forms, starch and sugar. It is in the form of sugar in
many of the vegetables when they are young or immature, but it turns
into starch as they mature. This change can be easily observed in the
case of peas. As is well known, young green peas are rather sweet
because of the sugar they contain, while mature or dried peas have lost
their sweetness and are starchy. The sugar that is found in large
quantities in such vegetables as peas, carrots, turnips, etc. is largely
cane sugar. The starch that vegetables contain occurs in tiny granules,
just as it is found in cereals, and is affected by cooking in the same
way. The mature vegetables in which the starch has developed, although
less tender and less sweet than young ones, have a higher food value. In
fact, the carbohydrate that vegetables contain constitutes a large
proportion of their food value.
One of the chief sources of starch among vegetables is the potato, in
which the starch grains are large and, if properly cooked, easily
digested. Irish, or white, potatoes contain very little carbohydrate in
the form of sugar, but in the sweet potato much of the carbohydrate is
sugar. In either of these two forms--starch and sugar--vegetable
carbohydrate is easily digested.
15. MINERAL MATTER, OR ASH, IN VEGETABLES.--The mineral matter in
vegetables is found in comparatively large quantities, the average
amount being slightly over 1 per cent. The presence of this substance is
of great value, because the mineral salts of both fruits and vegetables
are essential in the diet of adults in order to keep their health in a
normal condition. The mineral salts of vegetables render the blood more
alkaline instead of more acid, as do those contained in cereals and
meat. A large number of vegetables, particularly those low in food
value, such as greens, celery, etc., are very valuable for their mineral
salts. In reality, this substance and the cellulose they contain are the
things that recommend the use of these vegetables in the diet. Minerals
of all kinds are found in solution in the water contained in vegetables,
but chief among them are calcium, sodium, iron, phosphorus, and sulphur.
Greens and salad vegetables are particularly high in iron, the element
that assists in keeping the blood in good condition. These minerals are
easily lost if the method of cookery is not planned to retain them.
16. CELLULOSE IN VEGETABLES.--The special use of cellulose, as has
already been learned, is to serve as bulk in the food containing it. In
vegetables, the cellulose varies greatly as to quantity, as well as to
texture and the amount that can be digested. In young vegetables, it is
very soft and perhaps digestible to a certain extent, but as they grow
older it hardens and they become tough. This fact is clearly
demonstrated in the case of beets. Those which are pulled from the
garden in the summer and cooked are tender and soft, but those which are
allowed to mature in the ground and are then put away for winter are,
when cooked in the late winter or early spring, so hard and tough that
it is almost impossible to make them soft. The quantity of cellulose
that vegetables contain therefore depends largely on their age and
condition. Those low in total food value contain, as a rule, larger
quantities of it than those high in food value. This is due to the fact
that both water and cellulose, which are usually found together in large
quantities, help to detract from the fuel, or food, value of foods.
Very young persons or those who are ill sometimes find it impossible to
take in its original form a vegetable that contains a large amount of
bulk, or cellulose. In such a case, the vegetable may be put through a
colander or a sieve in order to break up the cellulose and make it
easier to digest. Under ordinary conditions, cellulose should not be
avoided, but should be included in large quantities in the diet through
the vegetables that are consumed daily.
17. WATER IN VEGETABLES.--The majority of vegetables contain a large
quantity of water. Such vegetables as lettuce, cucumbers, tomatoes,
etc., which are low in total food value, contain the most water, the
average percentage being about 95. The dry vegetables, which are high in
food value, average only about 10 per cent. of water. The water that is
found in vegetables, whether it is much or little, is contained in
cell-like structures surrounded by cellulose, and it holds in solution
the mineral salts and much of the nutriment of the vegetables. In
addition, the water holds in solution to a certain extent the material
that gives vegetables their distinctive flavor. When any of this water
is lost in the preparation of vegetables, the substances that it
contains are also lost. It is therefore essential that correct methods
of preparation be chosen for the cooking of this food, so as to prevent
the waste of valuable food materials.
18. DIGESTIBILITY OF VEGETABLES.--The digestibility of vegetables is
largely an individual matter; that is, a vegetable that agrees with one
person may not agree with another. The fact that there appears to be no
apparent reason for such a condition would lead to the conclusion that
it is due to the peculiarities of the person. Because of this, it is not
fair to make the general statement that a particular vegetable is easy
to digest and another one is hard to digest.
The chief cause for difficulty in the digestion of vegetables lies in
their volatile oils, which give them their flavor, but which are
irritating to many persons. Vegetables having a strong flavor, such as
radishes, onions, cucumbers, cabbage, and cauliflower, are the ones that
disagree most frequently with persons who eat them; but sometimes the
way in which some of them are cooked has more to do with this than the
vegetables themselves.
Vegetables containing considerable cellulose and water do not of
themselves give trouble in digestion, because they contain practically
nothing to digest; but they are sometimes responsible for interfering
with the digestion of other foods. Vegetables that are extremely high in
starch, such as potatoes, are easily digested by most persons, provided
they are properly cooked. For instance, a plain baked potato is easily
digested, but the same potato sauted in fat is more difficult of
digestion.
19. TABLE SHOWING COMPOSITION AND FOOD VALUE OF VEGETABLES.--As
vegetables vary considerably in the amount of the food substances they
contain, so do they differ greatly in their food value. This is clearly
shown in Table I, which gives the percentage of the food substances of
vegetables, as well as the food value per pound, in calories, that these
vegetables contain. The figures in this table are taken from Atwater's
Table of American Food Materials, and refer to the edible portion of the
material. In the case of several vegetables, no figures are given by
this authority, but in the table here presented the percentages and the
calories for the vegetables most similar are used. For example, the
figures for lettuce are used for endive, as the composition and food
value of this vegetable are not included and it resembles lettuce very
closely. Constant reference should be made to Table I as progress is
make with the study of vegetables and their preparation. Noting the
difference in the composition of the different vegetables, as well as
the variation in their food value, will be not only interesting but
instructive. For instance, when the housewife realizes that lettuce and
celery furnish only 85 to 90 calories to the pound, while dried beans
and peas average more than 1,700 calories to the pound, she will
understand better the place that these foods occupy in the dietary.