11. Although, as has just been stated, food may be considered as
anything that the human engine can make over into tissue or use in
living and working, not all foods are equally desirable any more than
all materials are equally good in the construction of a steam engine and
in the production of its working power. Those food substances which are
the most wholesome and healthful are the ones to be chosen, but proper
choice cannot be made unless the buyer knows of what the particular food
consists and what it is expected to do. To aid in the selection of food,
therefore, it is extremely necessary to become familiar with the five
substances, constituents, or principles of which foods are made up;
namely, water, mineral matter, or ash, protein, fat, and carbohydrate. A
knowledge of these will help also in determining the cooking methods to
adopt, for this depends on the effect that heat has on the various
substances present in a food. Of course, so far as flavor is concerned,
it is possible for the experienced cook to prepare many dishes
successfully without knowing the effect of heat on the different food
constituents; but to cook intelligently, with that success which makes
for actual economy and digestibility, certain facts must be known
concerning the food principles and the effect of dry and moist heat
on foods.
12. Water.--Of the various constituents that are found in the human
body, water occurs in the largest quantity. As a food substance, it is
an extremely important feature of a person's diet. Its chief purpose is
to replenish the liquids of the body and to assist in the digestion of
food. Although nature provides considerable amounts of water in most
foods, large quantities must be taken in the diet as a beverage. In
fact, it is the need of the body for water that has led to the
development of numerous beverages. Besides being necessary in building
up the body and keeping it in a healthy condition, water has a special
function to perform in cooking, as is explained later. Although this
food substance is extremely essential to life, it is seldom considered
in the selection of food, because, as has just been mentioned, nearly
all foods contain water.
13. Mineral Matter.--Ranking next to water in the quantity contained in
the human body is mineral matter. This constituent, which is also called
_ash_ or _mineral salts_, forms the main part of the body's framework,
or skeleton. In the building and maintaining of the body, mineral salts
serve three purposes--to give rigidity and permanence to the skeleton,
to form an essential element of active tissue, and to provide the
required alkalinity or acidity for the digestive juices and other
secretions.
The origin and distribution of these mineral substances are of interest.
Plants in their growth seize from the earth the salts of minerals and
combine them with other substances that make up their living tissue.
Then human beings, as well as other living creatures, get their supply
of these needed salts from the plants that they take as food, this being
the only form in which the salts can be thoroughly assimilated. These
salts are not affected by cooking unless some process is used that
removes such of them as are readily soluble in water. When this occurs,
the result is usually waste, as, for instance, where no use is made of
the water in which some vegetables are boiled. As is true of water,
mineral matter, even though it is found in large quantities in the body,
is usually disregarded when food is purchased. This is due to the fact
that this important nutritive material appears in some form in nearly
all foods and therefore does not necessitate the housewife's stopping to
question its presence.
14. Protein.--The food substance known as protein is a very important
factor in the growth and repair of the body; in fact, these processes
cannot be carried on unless protein is present in the diet. However,
while a certain quantity of protein is essential, the amount is not very
large and more than is required is likely to be harmful, or, since the
body can make no use of it, to be at least waste material. The principal
sources of protein are lean meat, eggs, milk, certain grains, nuts, and
the legumes, which include such foods as beans and peas. Because of the
ease with which they are digested, meat, fish, eggs, and milk are more
valuable sources of protein than bread, beans, and nuts. However, as the
foods that are most valuable for proteins cost more than others, a mixed
diet is necessary if only a limited amount of money with which to
purchase foods is available.
15. So much is involved in the cooking of foods containing protein that
the effect of heat on such foods should be thoroughly understood. The
cooking of any food, as is generally understood, tends to break up the
food and prepare it for digestion. However, foods have certain
characteristics, such as their structure and texture, that influence
their digestibility, and the method of cooking used or the degree to
which the cooking is carried so affects these characteristics as to
increase or decrease the digestibility of the food. In the case of foods
containing protein, unless the cooking is properly done, the application
of heat is liable to make the protein indigestible, for the heat first
coagulates this substance--that is, causes it to become thick--and
then, as the heat increases, shrinks and hardens it. This fact is
clearly demonstrated in the cooking of an egg, the white of which is the
type of protein called _albumin_. In a raw egg, the albumin is nearly
liquid, but as heat is applied, it gradually coagulates until it becomes
solid. If the egg is cooked too fast or too long, it toughens and
shrinks and becomes less palatable, less attractive, and less
digestible. However, if the egg is properly cooked after the heat has
coagulated the albumin, the white will remain tender and the yolk will
be fine and mealy in texture, thus rendering it digestible.
Similar results, although not so evident to the sight, are brought about
through the right or wrong way of cooking practically all other foods
that contain much protein. Milk, whose principal ingredient is a protein
known as _casein_, familiar as the curd of cheese, illustrates this fact
very plainly. When it is used to make cottage cheese, heating it too
long or to too high a degree will toughen the curd and actually spoil
the texture of the product, which will be grainy and hard, instead of
smooth and tender.
16. FATS.--The food substances just discussed--water, mineral matter,
and protein--yield the materials required for building and repairing the
tissues of the body, but, as has been explained, the body also requires
foods that produce energy, or working power. By far the greater part of
the total solids of food taken into the body serve this purpose, and of
these fats form a large percentage. Although fats make up such a large
proportion of the daily food supply, they enter into the body
composition to a less extent than do the food substances that have been
explained. The fats commonly used for food are of both animal and
vegetable origin, such as lard, suet, butter, cream, olive oil, nut oil,
and cottonseed oil. The ordinary cooking temperatures have comparatively
little effect on fat, except to melt it if it is solid. The higher
temperatures decompose at least some of it, and thus liberate substances
that may be irritating to the digestive tract.
17. CARBOHYDRATES.--Like fats, the food substances included in the term
carbohydrates supply the body with energy. However, fats and
carbohydrates differ in the forms in which they supply energy, the
former producing it in the most concentrated form and the latter in the
most economical form.
So that the term _carbohydrate_ may be clearly understood and firmly
fixed in the mind, it is deemed advisable to discuss briefly the
composition of the body and the food that enters it. Of course, in a
lesson on cookery, not so much attention need be given to this matter as
in a lesson on _dietetics_, which is a branch of hygiene that treats of
diet; nevertheless, it is important that every person who prepares food
for the table be familiar with the fact that the body, as well as food,
is made up of a certain number of chemical elements, of which nitrogen,
carbon, hydrogen, and oxygen form a large part. Protein owes its
importance to the fact that of the various food substances it alone
contains the element nitrogen, which is absolutely essential to the
formation of any plant or animal tissue. The other three elements,
carbon, hydrogen, and oxygen, go to make up the carbohydrates; in fact,
it is from the names of these three elements that the term carbohydrate
is derived. The carbohydrates include the starches and sugars that are
used and eaten in so many forms, and these contain the three elements
mentioned, the hydrogen and oxygen contained in them being in the
proportion that produces water. Thus, as will readily be seen, by
separating the name into its parts--_carbo_ (carbon) and _hydrate_
(hydrogen and oxygen in the proportion of two parts of hydrogen and one
of oxygen, that is, in the form of water)--carbohydrate is simply carbon
united with water. While the facts just brought out have much to do with
food economy, they are of interest here chiefly because they help to
make clear the term carbohydrate, which, as will be admitted, is the
only correct name for the food substance it represents.
18. STARCH, one of the chief forms of carbohydrates, is found in only
the vegetable kingdom. It is present in large quantities in the grains
and in potatoes; in fact, nearly all vegetables contain large or small
amounts of it. It is stored in the plant in the form of granules that
lie within the plant cells.
Cooking applied to starch changes it into a form that is digestible.
Moist heat cooks the granules until they expand and burst and thus
thicken the mass. Dry heat changes starch first into a soluble form and
finally into what is called _dextrine_, this being the intermediate step
in the changing of starch into sugar.
19. SUGAR, another important form of carbohydrate, is mainly of
vegetable origin, except that which is found in milk and called
_lactose_. This, together with the fat found in milk, supplies the child
with energy before it is able to digest a variety of foods. The sap of
various plants contains such large quantities of sugar that it can be
crystalized out and secured in dry form. The liquid that remains is
valuable as food, for, by boiling it down, it forms molasses. Sugar is
also present in considerable amounts in all fruits, and much of it is in
a form that can be assimilated, or taken up by the body, quickly. A
sugar very similar to this natural fruit sugar is made from the starch
of corn and is called _glucose_. Much of the carbohydrate found in
vegetables, especially young, tender vegetables, is in the form of
sugar, which, as the vegetables grow older, changes to starch.
Sugar melts upon the application of heat or, if it is in a melted
condition, as sirup or molasses, it boils down and gives off water. When
all the water has boiled away, the sugar begins to caramelize or become
brown, and develops a characteristic flavor. If the cooking is continued
too long, a dark-brown color and a bitter taste are developed. Because
the sugar in fruits and vegetables is in solution, some of it is lost
when they are boiled, unless, of course, the water in which they are
cooked is utilized.
20. CELLULOSE is a form of carbohydrate closely related to starch. It
helps to form the structure of plants and vegetables. Very little
cellulose is digested, but it should not be ignored, because it gives
the necessary bulk to the food in which it occurs and because strict
attention must be paid to the cooking of it. As cellulose usually
surrounds nutritive material of vegetable origin, it must be softened
and loosened sufficiently by cooking to permit the nutritive material to
be dissolved by the digestive juices. Then, too, in old vegetables,
there is more starch and the cellulose is harder and tougher, just as an
old tree is much harder than a sapling. This, then, accounts for the
fact that rapid cooking is needed for some vegetables and slow cooking
for others, the method and the time of cooking depending on the presence
and the consistency of the cellulose that occurs in the food.
21. IMPORTANCE OF A VARIETY OF FOODS.--Every one of the five food
substances just considered must be included in a person's diet; yet,
with the exception of milk, no single food yields the right amounts of
material necessary for tissue building and repair and for heat and
energy. Even milk is in the right proportion, as far as its food
substances are concerned, only for babies and very young children. It
will thus be seen that to provide the body with the right foods, the
diet must be such as to include all the food substances. In food
selection, therefore, the characteristics of the various food substances
must be considered well. Fats yield the most heat, but are the most
slowly digested. Proteins and carbohydrates are more quickly digested
than fats, but, in equal amounts, have less than half as much food
value. Water and mineral salts do not yield heat, but are required to
build tissue and to keep the body in a healthy condition. In addition,
it is well to note that a well-balanced diet is one that contains all of
the five food substances in just the right proportion in which the
individual needs them to build up the body, repair it, and supply it
with energy. What this proportion should be, however, cannot be stated
offhand, because the quantity and kind of food substances necessarily
vary with the size, age, and activity of each person.