THE CHEMISTRY OF FOOD.
_The Source of Food--Inorganic Substances--Water--Salt--Lime--Iron--Organic
Substances--Albumen, Fibrin, and Casein--The Fats or Oils--The Sugars,
Starch, and Gum--Stimulating Substances--Necessity of a Regulated Diet._
[Sidenote: 1. The term food? Source of food? Need of preparing food?]
1. THE SOURCE OF FOOD.--The term _food_ includes all those substances,
whether liquid or solid, which are necessary for the nourishment of the
body. The original source of all food is the earth, which the poet has
fitly styled the "Mother of all living." In her bosom, and in the
atmosphere about her, are contained all the elements on which life depends.
But man is unable to obtain nourishment directly from such crude chemical
forms as he finds in the inorganic world. They must, with a few exceptions,
be prepared for his use, by being transformed into new and higher
combinations, more closely resembling the tissues of his own body.
[Sidenote: 2. Usefulness and hurtfulness of plants? What then must man do?
Parts of the same plant or tree?]
2. This transformation is effected, first, by the vegetable world. But all
plants are not alike useful to man; while some are absolutely hurtful.
Accordingly, he must learn to discriminate between that which is poisonous
and that which is life-supporting. Again, all parts of the same plant or
tree are not alike beneficial: in some, the fruit, in others, the leaves,
and in others, the seeds only are sufficiently refined for his use. These
he must learn to select; he must also learn the proper modes of preparing
each kind for his table, whether by cooking or other processes. {54}
[Sidenote: 3. Certain forms of vegetable creation? Example of the bee?
Cattle? The inference?]
3. Again, certain forms of the vegetable creation which are unfit, in their
crude state, for man's food, and which he rejects, are chosen as food by
some of the lower animals, and are, by them, made ready for his use. Thus
the bee takes the clover, that man cannot eat, and from it collects honey.
The cattle eat the husks of corn and the dried grass, that are by far too
coarse for man, and in their own flesh convert them into tissues closely
resembling his muscular tissue. In this way, by the aid of the transforming
processes of the vegetable and animal creations, the simple chemical
elements of the mineral kingdom are elaborated into our choice articles of
food.
[Sidenote: 4. What classification? Define organic substances. Inorganic.
Organic, how spoken of? The inorganic? Water and salt?]
4. INORGANIC SUBSTANCES.--The substances we use as food are classified as
_organic_ and _inorganic_. By organic substances are meant those derived
from living forms, such as vegetables and animals. Inorganic substances are
those simpler inanimate forms which belong to the mineral kingdom. The
former alone are commonly spoken of as food, but the latter enter very
largely into the constitution of the body, and must therefore be present in
our food. With the exception of two articles, water and common salt, these
substances only enter the system when blended with organic substances.
[Sidenote: 5. Water in physiology? Where found? Computation? Water in the
teeth? Muscle, tendons, and ligaments? How ascertained? Water in the fluids
of the body? What is the advantage?]
5. WATER.--Water, from a physiological point of view, is the most important
of all the articles of food. It is everywhere found in the body, even in
the bones and the teeth. It has been computed that as large a proportion as
two-thirds of the body is water. The teeth, the densest of the solids in
the human system, contain ten per cent. of water. The muscles, tendons, and
ligaments are more than half water; for it is found that they lose more
than half their {55} weight when dried with moderate heat. But it is in the
_fluids_ of the body that water is found most abundantly. It gives to them
the power of holding a great variety of substances in solution, and is the
great highway by which new supplies are conveyed to the point where they
are required, and by which old particles of matter, that have served their
uses, are brought to the outlets of the body to be thus removed from the
system.
[Sidenote: 6. Length of time man can do without food or water? Give the
comparison? Bulk of drinks? Constituent of meats, etc.? Fruits?]
6. Man can remain a longer time without solid food than without water. He
may be deprived of the former for ten to twelve hours without great
suffering, but deprivation of water for the same length of time will
produce both severe pain and great weakness. The food should contain not
less than two parts of water to one of solid nutriment. Water constitutes
the great bulk of all our drinks, and is also a large constituent of the
meats, vegetables, and fruits which come upon the table. Fruits,
especially, contain it in great abundance, and, in their proper season,
furnish most agreeable and refreshing supplies of the needed fluid.
[Sidenote: 7. Salt, how obtained? Where found? In the human body?
Importance of salt? What else can you state of the value of salt?]
7. COMMON SALT.--Salt, or sodium chloride, as an article of food, is
obtained chiefly from the mineral kingdom; although plants contain it in
small quantities, and it is also found in the tissues of nearly all animals
used as food. In the human body, it is an ingredient of all the solids and
fluids. The importance of salt to animal life in general, is shown by the
great appetite for it manifested by domestic animals, and also by the
habitual resort of herds of wild beasts to the "salt-licks" or springs. In
those parts of the world where salt is obtained with difficulty, man places
a very high price upon it.
[Sidenote: 8. Experiments upon animals?]
8. Experiments upon domestic animals show that the withdrawal of salt from
their food, not only makes their {56} hides rough and causes the hair to
fall out, but also interferes with the proper digestion of food. If it be
withheld persistently, they become entirely unable to appropriate
nourishment, and die of starvation.
[Sidenote: 9. Salt, how taken into the system? Its use in cooking?
Consumption?]
9. Salt is usually taken into the system in sufficient quantities in our
food. Even the water we drink often has traces of it. The habitual use of
much salt in cooking, or as a seasoning at the table, is not wise; and
while it may not lead to consumption, as some writers declare, it is a bad
habit in itself, and leads to the desire for other and more injurious
condiments.
[Sidenote: 10. Lime in the bones? What does it impart? Chief ingredient of
the bones and teeth? Where else found?]
10. LIME.--This is the mineral substance which we have spoken of before as
entering very largely into the composition of the bones. It is the
important element which gives solidity and permanence to the framework upon
which the body is built. Calcium tri-phosphate, or "bone-earth," is the
chief ingredient of the bones and teeth, but is found in the cartilages and
other parts of the body in smaller quantities.
[Sidenote: 11. How does lime find its way into the body? Early life? Effect
of its derivation?]
11. How does this substance find its way into the body? Meat, milk, and
other articles obtained from the animal kingdom contain it, and it is
abundantly stored away also in the grains from which our bread is made, in
wheat, rye, and Indian corn. In early life, while the body is growing, the
supplies of this substance should be carefully provided. The evil effects
of the deprivation of it are too often and painfully evident in the
softening of the bones, and in the predisposition to curvature of the
spine--deformities which are most deplorable and which continue through
life.
[Sidenote: 12. Iron, its abundance and diffusion? Where found? What part of
the blood is it? How supplied to the system? In case of loss of blood or
wasting disease?]
12. IRON.--This substance is probably the most abundant and widely diffused
of the metals. It is found in {57} most of the vegetables, and is a very
important component of animal tissues. It enters into the composition of
human blood in about one part per thousand. Ordinarily, the food conveys to
the system enough iron for its use, but it must sometimes be introduced
separately as a remedy, especially after great loss of blood, or after some
wasting disease. Under its influence the blood seems to be rapidly
restored, and a natural color of the lips and skin replaces the pallor
caused by disease.
[Sidenote: 13. Soda, potash, and magnesia? How do they occur?]
13. OTHER INORGANIC SUBSTANCES.--In addition to the substances mentioned,
the mineral kingdom supplies compounds of soda, potash, and magnesia, which
are essential for the use of the body. They occur in small quantities in
the body, and enter it in combination with the various articles of diet.
[Sidenote: 14. Organic substances, whence derived? What do they comprise?
Groups?]
14. ORGANIC SUBSTANCES.--These substances are derived from the vegetable
and animal creations. They comprise all those articles which are commonly
spoken of as "food," and which are essential to sustain the body in life
and strength. They are divided into three groups, namely: the Albuminoid
substances, the Fats, and Sugars.
[Sidenote: 15. The Albuminoid class, includes what? These compounds
constitute what? The food? Their importance? Their properties?]
15. THE ALBUMINOIDS.--This class includes three important nutritive
substances--(1) _Albumen_, which gives it its name; (2) _Fibrin_, including
_gluten_; and (3) _Casein_. These compounds constitute a large part of the
human body, and the food contains them in proportionally large quantities.
Their importance is so great, and the system so promptly suffers from their
absence, that they have been styled the "_nutritious_ substances." The
properties which they hold in common are, that they do not crystallize, and
have a jelly-like form, except when heat is applied to them, when they
harden, or _coagulate_. {58}
[Sidenote: 16. Decomposition? Effect of cold? Illustrations? Elephants?]
16. They likewise decompose, or _putrefy_, under the influence of warmth
and moisture. Hence the decay of all dead animal tissues. Cold arrests this
process. It is well known that milk, eggs, and the like, "keep" much longer
in winter than at other seasons. The bodies of elephants, caught in the ice
many hundred years ago, are occasionally borne by the icebergs to the coast
of Siberia, completely frozen, but preserved almost perfectly in form and
limb.
[Sidenote: 17. In what substances does albumen exist? What further is said
of the egg?]
17. ALBUMEN exists in milk, meat, the grains, and the juices of many
plants; but the purest form is obtained from the white of egg. When we
consider that an egg is composed chiefly of albumen and water--namely, six
parts in seven; and when we also consider the numerous, diverse, and
complex tissues--the muscles, bones, internal organs, bill, claws, and
feathers--with which the chick is equipped on leaving his shell, we are
impressed with the importance of these apparently simple constituents of
the food and body.
[Sidenote: 18. Fibrin, gluten, clotting of the blood?]
18. FIBRIN is derived from meats, and exists in the blood both of man and
the lower animals. _Gluten_, or vegetable fibrin, resembles closely true
fibrin, and is abundantly furnished in wheat and other grains from which
flour is commonly made. Animal fibrin coagulates spontaneously when it is
removed from the body, and thus causes the "clotting" of the blood.
[Sidenote: 19. Casein? Its coagulation? Effect of rennet? Making of
cheese?]
19. CASEIN is the curdy ingredient of milk, and a highly important
food-substance. Its coagulation in milk takes place not from heat, but by
the addition of an acid, and also when milk becomes sour from exposure to
the air. It is commonly effected, however, by introducing a piece of
_rennet_, a preparation made from a calf's stomach. The _curds_, or casein,
may then be separated from the _whey_, {59} and made into cheese, by
pressing it sufficiently to drive off the water.
[Sidenote: 20. What are the fats? The oils? How supplied? How alike?
Emulsifying? Example? How do we know it?]
20. THE FATS OR OILS.--This is the second group of organic foods. Those
which are more solid are called _fats_: the more fluid ones are the _oils_.
Oleaginous substances are supplied in both animal and vegetable food; but,
from whatever source derived, they are chemically much alike. They are
insoluble in water, and yet they unite readily with the watery fluids of
the body, and are by them conveyed to its various parts for their
nourishment. This is due to their property of "emulsifying;" that is, they
are held in suspension, in a finely divided state, in water. Ordinary milk
is an example of an _emulsion_. We know that it contains fat; for butter is
obtained from it, and, under the microscope, the minute oil-globules may be
distinctly seen.
[Sidenote: 21. Whence are fatty articles of food derived?]
21. In our country and climate, and also in colder climates, fatty articles
of food are principally derived from the animal creation, such as meat or
flesh, milk and butter. But most of the bread-stuffs contain more or less
fat or oil; Indian meal as much as nine parts in a hundred.
[Sidenote: 22. Appetite of persons in cold climates? What do they require?
Upon what must they rely? Why? The Esquimaux? Laplander? Olive and palm?]
22. Among persons living in cold climates, the appetite for oleaginous food
is especially eager; and they require large quantities of it to enable them
to resist the depressing influences of cold. Since vegetation is scanty and
innutritious, and the waters of the frozen regions abound in animal life,
they must rely wholly upon a diet derived from the latter source. The
Esquimaux consumes daily from ten to fifteen pounds of meat or blubber, a
large proportion of which is fat. The Laplander will drink train-oil, and
regards tallow-candles as a great delicacy. In hot climates, on the
contrary, where flourish the olive {60} and the palm, this kind of food may
be obtained from vegetable sources in abundant quantities.
[Sidenote: 23. Which are the third of the organic groups? What do they
embrace? Points of resemblance?]
23. THE SUGARS, OR THE SACCHARINE SUBSTANCES.--These constitute the third,
and last, group of the organic substances, which are employed as food. This
group embraces, in addition to the different kinds of _Sugar_, the
varieties of starch and gum, from whatever source derived. The two
substances last named do not, at first sight, present many points of
similarity to sugar; but they closely resemble it in respect to their
ultimate chemical composition, being made up of the same elements, in
nearly the same proportions. And their office in the system is the same,
since they are all changed into sugar by the processes of digestion.
[Sidenote: 24. Origin of the sugars? Ordinary sugar? Beetroot? Maple-sugar?
Grape-sugar? Cane-sugar?]
24. SUGAR is chiefly of vegetable origin; the animal varieties being
obtained from honey and milk. The most noticeable characteristic of this
substance is its agreeable, sweet taste, which makes it everywhere a
favorite article of food. But this quality of sweetness is not possessed by
all the varieties of sugar in the same degree; that obtained from milk, for
instance, has a comparatively feeble taste, but rather imparts a gritty
feeling to the tongue. The other important properties of sugar are, its
power to crystallize when evaporated from watery solutions, such as the
juices of many plants; a tendency to ferment, by which process alcohol is
produced; and a ready solubility in water. This latter quality renders it
very easy of digestion, and more so than any other of the saccharine group.
It is computed that the annual production of sugar, in all parts of the
world, is more than one million of tons. The kind of sugar that is in
ordinary use, in this country, is prepared from the juice of the
sugar-cane, which contains eighteen per cent. of sugar. In France it is
manufactured from the {61} beet root, which holds about nine per cent.; the
maple-tree of our climate yields a similar sugar. The sweet taste of fruits
is due to the presence of grape-sugar: the white grains seen on raisins
belong to this variety. Cane-sugar is more soluble than the latter, and has
twice the sweetening power.
[Illustration: FIG. 15.--GRANULES OF POTATO STARCH.]
[Sidenote: 25. Starch, how widely distributed? Its qualities? Its
constituents? Its solubility?]
25. STARCH.--This is the most widely distributed of the vegetable
principles. It is tasteless, inodorous, and does not crystallize. It
consists of minute rounded granules, which, under the microscope, reveal a
somewhat uniform structure (Fig. 15). Starch will not dissolve in cold
water, but in boiling water the small grains burst open, and may then be
dissolved and digested.
[Sidenote: 26. How much starch in bread-stuffs? In rice? Unripe fruits?
Ripe fruits?]
26. The bread stuffs, wheat, corn, and rye flours, are more than one-half
starch. Rice, which is the "staff of life" to one-third of the human
family, contains eighty per cent. Unripe fruits have much starch in them,
which renders them indigestible when eaten uncooked; for the grains of raw
starch are but slightly acted upon within the body. But, under the potent
chemistry of the sun's ray, this crude material is converted into sugar.
Thus are the fruits prepared by the careful hand of Nature, so that when
ripe they may be freely used without further preparation.
[Sidenote: 27. Gum, where found? Its composition? Gum Arabic?]
27. GUM is commonly found in those articles which {62} also contain starch;
and has the same chemical composition as the latter, but is much less
nutritious. In the East, gum-arabic and similar substances are largely
employed as food. Persons who travel by caravan across vast, sandy deserts,
find such substances well adapted to their wants, since they are not
perishable, and are easily packed and carried.
[Sidenote: 28. The three classes of food principles? What besides? What is
said of them? Name the articles not nutritious.]
28. STIMULATING SUBSTANCES.--The three classes of food-principles already
considered--the Albuminoids, the Fats, and the Sugars--comprise all the
more important organic ingredients of our food. There are, besides, a great
variety of coloring and flavoring matters that stimulate or increase the
appetite for food by appealing to the eye and taste; but they are not
nutritious, and are quickly separated from the truly useful substances, and
do not long remain in the body. Among these may be classed spices, flavors
of fruits, tea, coffee, and vegetable acids.
[Sidenote: 29. What is said of experiments that have been tried?]
29. NECESSITY OF A REGULATED DIET.--A great variety of experiments have
been tried in order to test the relative value of the different nutritive
principles. They have been practised to some extent upon man, but chiefly
upon those inferior animals which require a similar diet to man.
[Sidenote: 30. What has been demonstrated in the first place? Example?
Second demonstration? Example? Give the illustration in relation to
convertibility.]
30. By this means it has been demonstrated that--first, when any one of
these substances is eaten exclusively, the body is imperfectly nourished,
and life is shortened. Dogs fed exclusively upon either albumen, fat, or
sugar, soon die of starvation. Second, a diet long deprived of either of
these principles, is a fertile cause of disease; for example, on
ship-board, where fresh vegetables are not dealt out for a long period,
_scurvy_ becomes prevalent among the sailors. They are, however, to a
certain extent mutually convertible, and thus the missing article is
indirectly supplied. For {63} instance, sugar changes to fat in the body;
and hence, as is well known, the "hands" on a sugar plantation grow fat
during the sugar season, by partaking freely of the ripened juices of the
cane.
31. That is the best diet therefore which contains some of each of these
principles, in due proportion; and that is the worst which excludes the
most of them. The cravings and experience of man had unerringly guided him
to a correct regulation of his diet, long before the chemistry of food was
understood; so that his ordinary meals long ago combined these various
principles, the necessity and value of which are now explained.
QUESTIONS FOR TOPICAL REVIEW.
PAGE
1. What is understood by the term food? 53
2. What can you state in relation to the source of food? 53
3. What discriminations and selections are necessary? 53, 54
4. How can you tell the organic from the inorganic substances? 54
5. What relative position does water hold as an article of food? 54
6. In what parts of the body is water found? 54, 55
7. In what articles that we eat is it found? 55
8. If you were required to go without water or solid food for a
number of days, which would you prefer to have, and why? 55
9. What can you state of the importance of salt as an article
of food? 55, 56
10. How abundant is salt, and how does it find its way into the
human system? 55, 56
11. What can you state of the importance of lime in the body? 56
12. What, of the importance of iron? 56, 57
13. What further is stated of other inorganic substances? 57
14. What in relation to organic substances? 57
15. What can you state in relation to the albuminoids? 57, 58
16. What, in relation to albumen? 58
17. What, in relation to casein? 58, 59
18. In relation to the fats or oils, and how generally consumed? 59, 60
19. What do we understand by the sugars or saccharine substances? 60
20. State what you can of sugar--its origin and various qualities. 60, 61
21. Of starch--its varieties and qualities. 61
22. Of the abundance of starch, and its importance as a food
principle. 61
23. What is stated in relation to stimulating substances? 62
24. Of the necessity for regulation in diet? 62, 63
* * * * *
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