LABORATORY PRACTICE 299
Object of Laboratory Practice; Laboratory Note-book
and Suggestions for Laboratory Practice; List of Apparatus
Used; Photograph of Apparatus Used; Directions
for Weighing; Directions for Measuring; Use of Microscope;
Water in Flour; Water in Butter; Ash in Flour;
Nitric Acid Test for Nitrogenous Organic Matter; Acidity
of Lemons; Influence of Heat on Potato Starch Grains;
Influence of Yeast on Starch Grains; Mechanical Composition
of Potatoes; Pectose from Apples; Lemon Extract;
Vanilla Extract; Testing Olive Oil for Cotton Seed Oil;
Testing for Coal Tar Dyes; Determining the Per Cent of
Skin in Beans; Extraction of Fat from Peanuts; Microscopic
Examination of Milk; Formaldehyde in Cream or
Milk; Gelatine in Cream or Milk; Testing for Oleomargarine;
Testing for Watering or Skimming of Milk; Boric
Acid in Meat; Microscopic Examination of Cereal Starch
Grains; Identification of Commercial Cereals; Granulation
and Color of Flour; Capacity of Flour to absorb
Water; Acidity of Flour; Moist and Dry Gluten; Gliadin
from Flour; Bread-making Test; Microscopic Examination
of Yeast; Testing Baking Powders for Alum; Testing
Baking Powders for Phosphoric Acid; Testing Baking
Powders for Ammonia; Vinegar Solids; Specific Gravity
of Vinegar; Acidity of Vinegar; Deportment of Vinegar
with Reagents; Testing Mustard for Turmeric; Examination
of Tea Leaves; Action of Iron Compounds upon
Tannic Acid; Identification of Coffee Berries; Detecting
Chicory in Coffee; Comparative Amounts of Soap Necessary
with Hard and Soft Water; Solvent Action of Water
on Lead; Suspended Matter in Water; Organic Matter
in Water; Deposition of Lime by Boiling Water; Qualitative
Tests for Minerals in Water; Testing for Nitrites
in Water.
REVIEW QUESTIONS 323
REFERENCES 350
INDEX 357
HUMAN FOODS AND THEIR NUTRITIVE VALUE
CHAPTER I
GENERAL COMPOSITION OF FOODS
1. Water.--All foods contain water. Vegetables in their natural
condition contain large amounts, often 95 per cent, while in meats there
is from 40 to 60 per cent or more. Prepared cereal products, as flour,
corn meal, and oatmeal, which are apparently dry, have from 7 to 14 per
cent. In general the amount of water in a food varies with the
mechanical structure and the conditions under which it has been
prepared, and is an important factor in estimating the value, as the
nutrients are often greatly decreased because of large amounts of water.
The water in substances as flour and meal is mechanically held in
combination with the fine particles and varies with the moisture
content, or hydroscopicity, of the air. Oftentimes foods gain or lose
water to such an extent as to affect their weight; for example, one
hundred pounds of flour containing 12 per cent of water may be reduced
in weight three pounds or more when stored in a dry place, or there may
be an increase in weight from being stored in a damp place. In tables
of analyses the results, unless otherwise stated, are usually given on
the basis of the original material, or the dry substance. Potatoes, for
example, contain 2-1/2 per cent of crude protein on the basis of 75 per
cent of water; or on a dry matter basis, that is, when the water is
entirely eliminated, there is 10 per cent of protein.
The water of foods is determined by drying the weighed material in a
water or air oven at a temperature of about 100° C, until all of the
moisture has been expelled in the form of steam, leaving the dry matter
or material free from water.[1] The determination of dry matter, while
theoretically a simple process, is attended with many difficulties.
Substances which contain much fat may undergo oxidation during drying;
volatile compounds, as essential oils, are expelled along with the
moisture; and other changes may occur affecting the accuracy of the
work. The last traces of moisture are removed with difficulty from a
substance, being mechanically retained by the particles with great
tenacity. When very accurate dry matter determinations are desired, the
substance is dried in a vacuum oven, or in a desiccator over sulphuric
acid, or in an atmosphere of some non-oxidizing gas, as hydrogen.
2. Dry Matter.--The dry matter of a food is a mechanical mixture of
the various compounds, as starch, sugar, fat, protein, cellulose, and
mineral matter, and is obtained by drying the material. Succulent
vegetable foods with 95 per cent of water contain only 5 per cent of
dry matter, while in flour with 12 per cent of water there is 88 per
cent, and in sugar 99 per cent. The dry matter is obtained by
subtracting the per cent of water from 100, and in foods it varies from
5 per cent and less in some vegetables to 99 per cent in sugar.
[Illustration: FIG. 1.--APPARATUS USED FOR THE DETERMINATION OF DRY
MATTER AND ASH IN FOODS.
1, desiccator; 2, muffle furnace for combustion of foods and obtaining
ash; 3, water oven for drying food materials.]
3. Ash.--The ash, or mineral matter, is that portion obtained by
burning or igniting the dry matter at the lowest temperature necessary
for complete combustion. The ash in vegetable foods ranges from 2 to 5
per cent and, together with the nitrogen, represents what was taken from
the soil during growth. In animal bodies, the ash is present mainly in
the bones, but there is also an appreciable amount, one per cent or
more, in all the tissues. Ash is exceedingly variable in composition,
being composed of the various salts of potassium, sodium, calcium,
magnesium, and iron, as sulphates, phosphates, chlorides, and silicates
of these elements. There are also other elements in small amounts. In
the plant economy these elements take an essential part and are
requisite for the formation of plant tissue and the production in the
leaves of the organic compounds which later are stored up in the seeds.
Some of the elements appear to be more necessary than others, and
whenever withheld plant growth is restricted. The elements most
essential for plant growth are potassium, calcium, magnesium, iron,
phosphorus, and sulphur.[1]
In the animal body minerals are derived, either directly or indirectly,
from the vegetable foods consumed. The part which each of the mineral
elements takes in animal nutrition is not well understood. Some of the
elements, as phosphorus and sulphur, are in organic combination with the
nitrogenous compounds, as the nucleated albuminoids, which are very
essential for animal life. In both plant and animal bodies, the mineral
matter is present as mineral salts and organic combinations. It is held
that the ash elements which are in organic combination are the forms
mainly utilized for tissue construction. While it is not known just what
part all the mineral elements take in animal nutrition, experiments show
that in all ordinary mixed rations the amount of the different mineral
elements is in excess of the demands of the body, and it is only in rare
instances, as in cases of restricted diet, or convalescence from some
disease, that special attention need be given to increasing the mineral
content of the ration. An excess of mineral matter in foods is equally
as objectionable as a scant amount, elimination of the excess entailing
additional work on the body.
The composition of the ash of different food materials varies widely,
both in amount, and form of the individual elements. When for any reason
it is necessary to increase the phosphates in a ration, milk and eggs do
this to a greater extent than almost any other foods. Common salt, or
sodium chloride, is one of the most essential of the mineral
constituents of the body. It is necessary for giving the blood its
normal composition, furnishing acid and basic constituents for the
production of the digestive fluids, and for the nutrition of the cells.
While salt is a necessary food, in large amounts, as when the attempt is
made to use sea water as a beverage, it acts as a poison, suggesting
that a material may be both a food and a poison. When sodium chloride is
entirely withheld from an animal, death from salt starvation ensues.
Many foods contain naturally small amounts of sodium chloride.
4. Organic Matter.--That portion of a food material which is converted
into gaseous or volatile products during combustion is called the
organic matter. It is a mechanical mixture of compounds made up of
carbon, hydrogen, oxygen, nitrogen, and sulphur, and is composed of
various individual organic compounds, as cellulose, starch, sugar,
albumin, and fat. The amount in a food is determined by subtracting the
ash and water from 100. The organic matter varies widely in composition;
in some foods it is largely starch, as in potatoes and rice, while in
others, as forage crops consumed by animals, cellulose predominates. The
nature of the prevailing organic compound, as sugar or starch,
determines the nutritive value of a food. Each has a definite chemical
composition capable of being expressed by a formula. Considered
collectively, the organic compounds are termed organic matter. When
burned, the organic compounds are converted into gases, the carbon
uniting with the oxygen of the air to form carbon dioxide, hydrogen to
form water, sulphur to form sulphur dioxide, and the nitrogen to form
oxides of nitrogen and ammonia.
5. Classification of Organic Compounds.--All food materials are
composed of a large number of organic compounds. For purposes of study
these are divided into classes. The element nitrogen is taken as the
basis of the division. Compounds which contain this element are called
nitrogenous, while those from which it is absent are called
non-nitrogenous.[2] The nitrogenous organic compounds are composed of
the elements nitrogen, hydrogen, carbon, oxygen, and sulphur, while the
non-nitrogenous compounds are composed of carbon, hydrogen, and oxygen.
In vegetable foods the non-nitrogenous compounds predominate, there
being usually from six to twelve parts of non-nitrogenous to every one
part of nitrogenous, while in animal foods the nitrogenous compounds are
present in larger amount.
NON-NITROGENOUS COMPOUNDS
6. Occurrence.--The non-nitrogenous compounds of foods consist mainly
of cellulose, starch, sugar, and fat. For purposes of study, they are
divided into subdivisions, as carbohydrates, pectose substances or
jellies, fats, organic acids, essential oils, and mixed compounds. In
plants the carbohydrates predominate, while in animal tissue the fats
are the chief non-nitrogenous constituents.
7. Carbohydrates.--This term is applied to a class of compounds
similar in general composition, but differing widely in structural
composition and physical properties. Carbohydrates make up the bulk of
vegetable foods and, except in milk, are found only in traces in animal
foods. They are all represented by the general formula CH_2n_2n, there
being twice as many hydrogen as oxygen atoms, the hydrogen and oxygen
being present in the same proportion as in water. As a class, the
carbohydrates are neutral bodies, and, when burned, form carbon dioxide
and water.
[Illustration: FIG. 2.--CELLULAR STRUCTURE OF PLANT CELL.]
8. Cellulose is the basis of the cell structure of plants, and is
found in various physical forms in food materials.[3] Sometimes it is
hard and dense, resisting digestive action and mechanically inclosing
other nutrients and thus preventing their being available as food. In
the earlier stages of plant growth a part of the cellulose is in
chemical combination with water, forming hydrated cellulose, a portion
of which undergoes digestion and produces heat and energy in the body.
Ordinarily, however, cellulose adds but little in the way of nutritive
value, although it is often beneficial mechanically and imparts bulk to
some foods otherwise too concentrated. The mechanical action of
cellulose on the digestion of food is discussed in Chapter XV.
Cellulose usually makes up a very small part of human food, less than 1
per cent. In refined white flour there is less than .05 of a per cent;
in oatmeal and cereal products from .5 to 1 per cent, depending upon the
extent to which the hulls are removed, and in vegetable foods from .1 to
1 per cent. The cellulose content of foods is included in the crude
fiber of the chemist's report.
9. Starch occurs widely distributed in nature, particularly in the
seeds, roots, and tubers of some plants. It is formed in the leaves of
plants as a result of the joint action of chlorophyll and protoplasm,
and is generally held by plant physiologists to be the first
carbohydrate produced in the plant cell. Starch is composed of a number
of overlapping layers separated by starch cellulose; between these
layers the true starch or amylose is found. Starch from the various
cereals and vegetables differs widely in mechanical structure; in wheat
it is circular, in corn somewhat angular, and in parsnips exceedingly
small, while potato starch granules are among the largest.[4] The nature
of starch can be determined largely from its mechanical structure as
studied under the microscope. It is insoluble in cold water because of
the protecting action of the cellular layer, but on being heated it
undergoes both mechanical and chemical changes; the grains are partially
ruptured by pressure due to the conversion into steam of the moisture
held mechanically. The cooking of foods is beneficial from a mechanical
point of view, as it results in partial disintegration of the starch
masses, changing the structure so that the starch is more readily acted
upon by the ferments of the digestive tract. At a temperature of about
120° C. starch begins to undergo chemical change, resulting in the
rearrangement of the atoms in the molecule with the production of
dextrine and soluble carbohydrates. Dextrine is formed on the crust of
bread, or whenever potatoes or starchy foods are browned. At a still
higher temperature starch is decomposed, with the liberation of water
and production of compounds of higher carbon content. When heated in
contact with water, it undergoes hydration changes; gelatinous-like
products are formed, which are finally converted into a soluble
condition. In cooking cereals, the hydration of the starch is one of the
main physical and chemical changes that takes place, and it simply
results in converting the material into such a form that other chemical
changes may more readily occur. Before starch becomes dextrose,
hydration is necessary. If this is accomplished by cooking, it saves the
body just so much energy in digestion. Many foods owe their value
largely to the starch. In cereals it is found to the extent of 72 to 76
per cent; in rice and potatoes in still larger amounts; and it is the
chief constituent of many vegetables. When starch is digested, it is
first changed to a soluble form and then gradually undergoes oxidation,
resulting in the production of heat and energy, the same
products--carbon dioxide and water--being formed as when starch is
burned. Starch is a valuable heat-producing nutrient; a pound yields
1860 calories. See Chapter XV.
10. Sugar.--Sugars are widely distributed in nature, being found
principally in the juices of the sugar cane, sugar beet, and sugar
maple. They are divided into two large classes: the sucrose group and
the dextrose group, the latter being produced from sucrose, starch, and
other carbohydrates by inversion and allied chemical changes. Because of
the importance of sugar in the dietary, Chapter V is devoted to the
subject.
11. Pectose Substances are jelly-like bodies found in fruits and
vegetables. They are closely related in chemical composition to the
carbohydrates, into which form they are changed during digestion; and in
nutrition they serve practically the same function. In the early stages
of growth the pectin bodies are combined with organic acids, forming
insoluble compounds, as the pectin in green apples. During the ripening
of fruit and the cooking of vegetables, the pectin is changed to a more
soluble and digestible condition. In food analysis, the pectin is
usually included with the carbohydrates.
12. Nitrogen-free-extract.--In discussing the composition of foods,
the carbohydrates other then cellulose, as starch, sugar, and pectin,
are grouped under the name of nitrogen-free-extract. Methods of
chemical analysis have not yet been sufficiently perfected to
enable accurate and rapid determination to be made of all these
individual carbohydrates, and hence they are grouped together as
nitrogen-free-extract. As the name indicates, they are compounds which
contain no nitrogen, and are extractives in the sense that they are
soluble in dilute acid and alkaline solutions. The nitrogen-free-extract
is determined indirectly, that is, by the method of difference. All the
other constituents of a food, as water, ash, crude fiber (cellulose),
crude protein, and ether extract, are determined; the total is
subtracted from 100, and the difference is nitrogen-free-extract. In
studying the nutritive value of foods, particular attention should be
given to the nature of the nitrogen-free-extract, as in some instances
it is composed of sugar and in others of starch, pectin, or pentosan
(gum sugars). While all these compounds have practically the same fuel
value, they differ in composition, structure, and the way in which they
are acted upon by chemicals and digestive ferments.[1]
[Illustration: FIG. 3.--APPARATUS USED FOR THE DETERMINATION OF
FAT.]
13. Fat.--Fat is found mainly in the seeds of plants, but to some
extent in the leaves and stems. It differs from starch in containing
more carbon and less oxygen. In starch there is about 44 per cent of
carbon, while in fat there is 75 per cent. Hence it is that when fat is
burned or undergoes combustion, it yields a larger amount of the
products of combustion--carbon dioxid and water--than does starch. A
gram of fat produces 2-1/4 times as much heat as a gram of starch. Fat
is the most concentrated non-nitrogenous nutrient. As found in food
materials, it is a mechanical mixture of various fats, among which are
stearin, palmitin, and olein. Stearin and palmitin are hard fats,
crystalline in structure, and with a high melting point, while olein is
a liquid. In addition to these three, there are also small amounts of
other fats, as butyrin in butter, which give character or individuality
to materials. There are a number of vegetable fats or oils which are
used for food purposes and, when properly prepared and refined, have a
high nutritive value. Occasionally one fat of cheaper origin but not
necessarily of lower nutritive value is substituted for another. The
fats have definite physical and chemical properties which enable them to
be readily distinguished, as iodine number, specific gravity, index of
refraction, and heat of combustion. By iodine number is meant the
percentage of iodine that will unite chemically with the fat. Wheat oil
has an iodine number of about 100, meaning that one pound of wheat oil
will unite chemically with one pound of iodine. Fats have a lower
specific gravity than water, usually ranging from .89 to .94, the
specific gravity of a fat being fairly constant. All fats can be
separated into glycerol and a fatty acid, glycerol or glycerine being
common constituents, while each fat yields its own characteristic acid,
as stearin, stearic acid; palmitin, palmitic acid; and olein, oleic
acid. The fats are soluble in ether, chloroform, and benzine. In the
chemical analysis of foods, they are separated with ether, and along
with the fat, variable amounts of other substances are extracted, these
extractive products usually being called "ether extract" or "crude
fat."[5] The ether extract of plant tissue contains in addition to fat
appreciable amounts of cellulose, gums, coloring, and other materials.
From cereal products the ether extract is largely fat, but in some
instances lecithin and other nitrogenous fatty substances are present,
while in animal food products, as milk and meat, the ether extract is
nearly pure fat.
14. Organic Acids.--Many vegetable foods contain small amounts of
organic acids, as malic acid found in apples, citric in lemons, and
tartaric in grapes. These give characteristic taste to foods, but have
no direct nutritive value. They do not yield heat and energy as do
starch, fat, and protein; they are, however, useful for imparting flavor
and palatability, and it is believed they promote to some extent the
digestion of foods with which they are combined by encouraging the
secretion of the digestive fluids. Many fruits and vegetables owe their
dietetic value to the organic acids which they contain. In plants they
are usually in chemical combination with the minerals, forming compounds
as salts, or with the organic compounds, producing materials as acid
proteins. In the plant economy they take an essential part in promoting
growth and aiding the plant to secure by osmotic action its mineral food
from the soil. Organic acids are found to some extent in animal foods,
as the various lactic acids of meat and milk. They are also formed in
food materials as the result of ferment action. When seeds germinate,
small amounts of carbohydrates are converted into organic acids. In
general the organic acids are not to be considered as nutrients, but as
food adjuncts, increasing palatability and promoting digestion.
15. Essential Oils.--Essential or volatile oils differ from fats, or
fixed oils, in chemical composition and physical properties.[6] The
essential oils are readily volatilized, leaving no permanent residue,
while the fixed fats are practically non-volatile. Various essential
oils are present in small amounts in nearly all vegetable food
materials, and the characteristic flavor of many fruits is due to them.
It is these compounds which are used for flavoring purposes, as
discussed in Chapter IV. The amount in a food material is very small,
usually only a few hundredths of a per cent. The essential oils have no
direct food value, but indirectly, like the organic acids, they assist
in promoting favorable digestive action, and are also valuable because
they impart a pleasant taste. Through poor methods of cooking and
preparation, the essential oils are readily lost from some foods.
16. Mixed Compounds.--Food materials frequently contain
compounds which do not naturally fall into the five groups
mentioned,--carbohydrates, pectose substances, fats, organic acids, and
essential oils. The amount of such compounds is small, and they are
classed as miscellaneous or mixed non-nitrogenous compounds. Some of
them may impart a negative value to the food, and there are others which
have all the characteristics, as far as general composition is
concerned, of the non-nitrogenous compounds, but contain nitrogen,
although as a secondary rather than an essential constituent.
17. Nutritive Value of Non-nitrogenous Compounds.--The non-nitrogenous
compounds, taken as a class, are incapable alone of sustaining life,
because they do not contain any nitrogen, and this is necessary for
producing proteid material in the animal body. They are valuable for
the production of heat and energy, and when associated with the
nitrogenous compounds, are capable of forming non-nitrogenous reserve
tissue. It is equally impossible to sustain life for any prolonged
period with the nitrogenous compounds alone. It is when these two
classes are properly blended and naturally united in food materials that
their main value is secured. For nutrition purposes they are mutually
related and dependent. Some food materials contain the nitrogenous and
non-nitrogenous compounds blended in such proportion as to enable one
food alone to practically sustain life, while in other cases it is
necessary, in order to secure the best results in the feeding of animals
and men, to combine different foods varying in their content of these
two classes of compounds.[7]
NITROGENOUS COMPOUNDS
18. General Composition.--The nitrogenous compounds are more complex
in composition than the non-nitrogenous. They are composed of a larger
number of elements, united in different ways so as to form a much more
complex molecular structure. Foods contain numerous nitrogenous organic
compounds, which, for purposes of study, are divided into four
divisions,--proteids, albuminoids, amids, and alkaloids. In addition to
these, there are other nitrogenous compounds which do not naturally fall
into any one of the four divisions.
[Illustration: FIG. 4.--APPARATUS USED FOR DETERMINING
TOTAL NITROGEN AND CRUDE PROTEIN IN FOODS.
The material is digested in the flask (3) with sulphuric acid and the
organic nitrogen converted into ammonium sulphate, which is later
liberated and distilled at 1, and the ammonia neutralized with standard
acid (2).]
Also in some foods there are small amounts of nitrogen in mineral forms,
as nitrates and nitrites.
19. Protein.--The term "protein" is applied to a large class of
nitrogenous compounds resembling each other in general composition, but
differing widely in structural composition. As a class, the proteins
contain about 16 per cent of nitrogen, 52 per cent of carbon, from 6 to
7 per cent of hydrogen, 22 per cent of oxygen, and less than 2 per cent
of sulphur. These elements are combined in a great variety of ways,
forming various groups or radicals. In studying the protein molecule a
large number of derivative products have been observed, as amid
radicals, various hydrocarbons, fatty acids, and carbohydrate-like
bodies.[8] It would appear that in the chemical composition of the
proteins there are all the constituents, or simpler products, of the
non-nitrogenous compounds, and these are in chemical combination with
amid radicals and nitrogen in various forms. The nitrogen of many
proteids appears to be present in more than one form or radical. The
proteids take an important part in life processes. They are found more
extensively in animal than in plant bodies. The protoplasm of both the
plant and animal cell is composed mainly of protein.
Proteids are divided into various subdivisions, as albumins, globulins,
albuminates, proteoses and peptones, and insoluble proteids. In plant
and animal foods a large amount of the protein is present as insoluble
proteids; that is, they are not dissolved by solvents, as water and
dilute salt solution. The albumins are soluble in water and coagulated
by heat at a temperature of 157° to 161° F. Whenever a food material is
soaked in water, the albumin is removed and can then be coagulated by
the action of heat, or of chemicals, as tannic acid, lead acetate, and
salts of mercury. The globulins are proteids extracted from food
materials by dilute salt solution after the removal of the albumins.
Globulins also are coagulated by heat and precipitated by chemicals. The
amount of globulins in vegetable foods is small. In animal foods myosin
in meat and vitellin, found in the yolk of the egg, and some of the
proteids of the blood, are examples of globulins. Albuminates are
casein-like proteids found in both animal and vegetable foods. They are
supposed to be proteins that are in feeble chemical combination with
acid and alkaline compounds, and they are sometimes called acid and
alkali proteids. Some are precipitated from their solutions by acids and
others by alkalies. Peas and beans contain quite large amounts of a
casein-like proteid called legumin. Proteoses and peptones are proteins
soluble in water, but not coagulated by heat. They are produced from
other proteids by ferment action during the digestion of food and the
germination of seeds, and are often due to the changes resulting from
the action of the natural ferments or enzymes inherent in the food
materials. As previously stated, the insoluble proteids are present in
far the largest amount of any of the nitrogenous materials of foods.
Lean meat and the gluten of wheat and other grains are examples of the
insoluble proteids. The various insoluble proteids from different food
materials each has its own composition and distinctive chemical and
physical properties, and from each a different class and percentage
amount of derivative products are obtained.[1] While in general it is
held that the various proteins have practically the same nutritive
value, it is possible that because differences in structural composition
and the products formed during digestion there may exist notable
differences in nutritive value. During digestion the insoluble proteids
undergo an extended series of chemical changes. They are partially
oxidized, and the nitrogenous portion of the molecule is eliminated
mainly in the form of amids, as urea. The insoluble proteins constitute
the main source of the nitrogenous food supply of both humans and
animals.
20. Crude Protein.--In the analysis of foods, the term "crude protein"
is used to designate the total nitrogenous compounds considered
collectively; it is composed largely of protein, but also includes the
amids, alkaloids, and albuminoids. "Crude protein" and "total
nitrogenous compounds" are practically synonymous terms. The various
proteins all contain about 16 per cent of nitrogen; that is, one part of
nitrogen is equivalent to 6.25 parts of protein. In analyzing a food
material, the total organic nitrogen is determined and the amount
multiplied by 6.25 to obtain the crude protein. In some food materials,
as cereals, the crude protein is largely pure protein, while in others,
as potatoes, it is less than half pure protein, the larger portion being
amids and other compounds. In comparing the crude protein content of one
food with that of another, the nature of both proteids should be
considered and also the amounts of non-proteid constituents. The factor
6.25 for calculating the protein equivalent of foods is not strictly
applicable to all foods. For example, the proteids of wheat--gliadin and
glutenin--contain over 18 per cent of nitrogen, making the nitrogen
factor about 5.68 instead of 6.25. If wheat contains 2 per cent of
nitrogen, it is equivalent to 12.5 per cent of crude protein, using the
factor 6.25; or to 11.4, using the factor 5.7. The nitrogen content of
foods is absolute; the protein content is only relative.[9]
21. Food Value of Protein.--Because of its complexity in composition,
protein is capable of being used by the body in a greater variety of
ways than starch, sugar, or fat. In addition to producing heat and
energy, protein serves the unique function of furnishing material for
the construction of new muscular tissue and the repair of that which is
worn out. It is distinctly a tissue-building nutrient. It also enters
into the composition of all the vital fluids of the body, as the blood,
chyme, chyle, and the various digestive fluids. Hence it is that protein
is required as a nutrient by the animal body, and it cannot be produced
from non-nitrogenous compounds. In vegetable bodies, the protein can be
produced synthetically from amids, which in turn are formed from
ammonium compounds. While protein is necessary in the ration, an
excessive amount should be avoided. When there is more than is needed
for functional purposes, it is used for heat and energy, and as foods
rich in protein are usually the most expensive, an excess adds
unnecessarily to the cost of the ration. Excess of protein in the ration
may also result in a diseased condition, due to imperfect elimination of
the protein residual products from the body.[10]
22. Albuminoids differ from proteids in general composition and, to
some extent, in nutritive value. They are found in animal bodies mainly
in the connective tissue and in the skin, hair, and nails. Some of the
albuminoids, as nuclein, are equal in food value to protein, while
others have a lower food value. In general, albuminoids are capable of
conserving the protein of the body, and hence are called "protein
sparers," but they cannot in every way enter into the composition of the
body, as do the true proteins.
23. Amids and Amines.--These are nitrogenous compounds of simpler
structure than the proteins and albuminoids. They are sometimes called
compound ammonia in that they are derived from ammonia by the
replacement of one of the hydrogen atoms with an organic radical. In
plants, amids are intermediate compounds in the production of the
proteids, and in some vegetables a large portion of the nitrogen is
amids. In animal bodies amids are formed during oxidation, digestion,
and disintegration of proteids. It is not definitely known whether or
not a protein in the animal body when broken down into amid form can
again be reconstructed into protein. The amids have a lower food value
than the proteids and albuminoids. It is generally held that, to a
certain extent, they are capable, when combined with proteids, of
preventing rapid conversion of the body proteid into soluble form. When
they are used in large amounts in a ration, they tend to hasten
oxidation rather than conservation of the proteids.
24. Alkaloids.--In some plant bodies there are small amounts of
nitrogenous compounds called alkaloids. They are not found to any
appreciable extent in food plants. The alkaloids, like ammonia, are
basic in character and unite with acids to form salts. Many medicinal
plants owe their value to the alkaloids which they contain. In animal
bodies alkaloids are formed when the tissue undergoes fermentation
changes, and also during disease, the products being known as ptomaines.
Alkaloids have no food value, but act physiologically as irritants on
the nerve centers, making them useful from a medicinal rather than from
a nutritive point of view. To medical and pharmaceutical students the
alkaloids form a very important group of compounds.
[Illustration: FIG. 5.--GRAPHIC COMPOSITION OF FLOUR.
1, flour; 2, starch; 3, gluten; 4, water; 5, fat; 6, ash.]
25. General Relationship of the Nitrogenous Compounds.--Among the
various subdivisions of the nitrogenous compounds there exists a
relationship similar to that among the non-nitrogenous compounds. From
proteids, amids and alkaloids may be formed, just as invert sugars and
their products are formed from sucrose. Although glucose products are
derived from sucrose, it is not possible to reverse the process and
obtain sucrose or cane sugar from starch. So it is with proteins, while
the amid may be obtained from the proteid in animal nutrition, as far
as known the process cannot be reversed and proteids be obtained from
amids. In the construction of the protein molecule of plants, nitrogen
is absorbed from the soil in soluble forms, as compounds of nitrates and
nitrites and ammonium salts. These are converted, first, into amids and
then into proteids. In the animal body just the reverse of this process
takes place,--the protein of the food undergoes a series of changes, and
is finally eliminated from the body as an amid, which in turn undergoes
oxidation and nitrification, and is converted into nitrites, nitrates,
and ammonium salts. These forms of nitrogen are then ready to begin
again in plant and animal bodies the same cycle of changes. Thus it is
that nitrogen may enter a number of times into the composition of plant
and animal tissues. Nature is very economical in her use of this
element.[5]
CHAPTER II
CHANGES IN COMPOSITION OF FOODS DURING COOKING AND PREPARATION
26. Raw and Cooked Foods Compared.--Raw and cooked foods differ in
chemical composition mainly in the content of water. The amount of
nutrients on a dry matter basis is practically the same, but the
structural composition is affected by cooking, and hence it is that a
food prepared for the table often differs appreciably from the raw
material. Cooked meat, for example, has not the same percentage and
structural composition as raw meat, although the difference in nutritive
value between a given weight of each is not large. During cooking, foods
are acted upon chemically, physically, and bacteriologically, and it is
usually the joint action of these three agencies that brings about the
desirable changes incident to their preparation for the table.
27. Chemical Changes during Cooking.--Each of the chemical compounds
of which foods are composed is influenced to a greater or less extent by
heat and modified in composition. The chemistry of cooking is mainly a
study of the chemical changes that take place when compounds, as
cellulose, starch, sugar, pectin, fat, and the various proteids, are
subjected to the joint action of heat, moisture, air, and ferments. The
changes which affect the cellulose are physical rather than chemical. A
slight hydration of the cellular tissue, however, does take place. In
human foods cellulose is not found to any appreciable extent. Many
vegetables, as potatoes, which are apparently composed of cellular
substances, contain but little true cellulose. Starch, as previously
stated, undergoes hydration in the presence of water, and, at a
temperature of 120° C., is converted into dextrine. At a higher
temperature disintegration of the starch molecule takes place, with the
formation of carbon monoxid, carbon dioxid, and water, and the
production of a residue richer in carbon than is starch. On account of
the moisture, the temperature in many cooking operations is not
sufficiently high for changes other than hydration and preliminary
dextrinizing. In Chapter XI is given a more extended account of the
changes affecting starch which occur in bread making.
During the cooking process sugars undergo inversion to a slight extent.
That is, sucrose is converted into levulose and dextrose sugars. At a
higher temperature, sugar is broken up into its constituents--water and
carbon dioxide. The organic acids which many fruits and vegetables
contain hasten the process of inversion. When sugar is subjected to dry
heat, it becomes a brown, caramel-like material sometimes called barley
sugar. During cooking, sugars are not altered in solubility or
digestibility; starches, however, are changed to a more soluble form,
and pectin--a jelly-like substance--is converted from a less to a more
soluble condition, as stated in Chapter I. Changes incident to the
cooking of fruits and vegetables rich in pectin, as in the making of
jellies, are similar to those which take place in the last stages of
ripening.
The fats are acted upon to a considerable extent by heat. Some of the
vegetable oils undergo slight oxidation, resulting in decreased
solubility in ether, but since there is no volatilization of the fatty
matter, it is a change that does not materially affect the total fuel
value of the food.[11]
There is a general tendency for the proteids to become less soluble by
the action of heat, particularly the albumins and globulins. The protein
molecule dissociates at a high temperature, with formation of volatile
products, and therefore foods rich in protein should not be subjected to
extreme heat, as losses of food value may result. During cooking,
proteids undergo hydration, which is necessary and preliminary to
digestion, and the heating need be carried only to this point, and not
to the splitting up of the molecule. Prolonged high temperature in the
cooking of proteids and starches is unnecessary in order to induce the
desired chemical changes. When these nutrients are hydrated, they are in
a condition to undergo digestion, without the body being compelled to
expend unnecessary energy in bringing about this preliminary change.
Hence it is that, while proper cooking does not materially affect the
total digestibility of proteids or starches, it influences ease of
digestion, as well as conserves available energy, thereby making more
economical use of these nutrients.
[Illustration: FIG. 6.--CELLS OF A PARTIALLY COOKED
POTATO. (After KÖNIG.)]
28. Physical Changes.--The mechanical structure of foods is influenced
by cooking to a greater extent than is the chemical composition. One of
the chief objects of cooking is to bring the food into better mechanical
condition for digestion.[12] Heat and water cause partial disintegration
of both animal and vegetable tissues. The cell-cementing materials are
weakened, and a softening of the tissues results. Often the action
extends still further in vegetable foods, resulting in disintegration of
the individual starch granules. When foods are subjected to dry heat,
the moisture they contain is converted into steam, which causes bursting
of the tissues. A good example of this is the popping of corn. Heat may
result, too, in mechanical removal of some of the nutrients, as the
fats, which are liquefied at temperatures ranging from 100° to 200° F.
Many foods which in the raw state contain quite large amounts of fat,
lose a portion mechanically during cooking, as is the case with bacon
when it is cut in thin slices and fried or baked until crisp. When foods
are boiled, the natural juices being of somewhat different density from
the water in which they are cooked, slight osmotic changes occur. There
is a tendency toward equalization of the composition of the juices of
the food and the water in which they are cooked. In order to achieve the
best mechanical effects in cooking, high temperatures are not necessary,
except at first for rupturing the tissues; softening of the tissues is
best effected by prolonged and slow heat. At a higher temperature many
of the volatile and essential oils are lost, while at lower temperatures
these are retained and in some instances slightly developed. The cooking
should be sufficiently prolonged and the temperature high enough to
effectually disintegrate and soften all of the tissues, but not to cause
extended chemical changes.
[Illustration: FIG. 7.--CELLS OF RAW POTATO, SHOWING STARCH
GRAINS. (After KÖNIG.)]
There is often an unnecessarily large amount of heat lost through faulty
construction of stoves and lack of judicious use of fuels, which greatly
enhances the cost of preparing foods. Ovens are frequently coated with
deposits of soot; this causes the heat to be thrown out into the room or
lost through the chimney, rather than utilized for heating the oven. In
an ordinary cook stove it is estimated that less than 7 per cent of the
heat and energy of the fuel is actually employed in bringing about
physical and chemical changes incident to cooking.[13]
29. Bacteriological Changes.--The bacterial organisms of foods are
destroyed in the cooking, provided a temperature of 150° F. is reached
and maintained for several minutes. The interior of foods rarely reaches
a temperature above 200° F., because of the water they contain which is
not completely removed below 212°. One of the chief objects in cooking
food is to render it sterile. Not only do bacteria become innocuous
through cooking, but various parasites, as trichina and tapeworm, are
destroyed, although some organisms can live at a comparatively high
temperature. Cooked foods are easily re-inoculated, in some cases more
readily than fresh foods, because they are in a more disintegrated
condition.
In many instances bacteria are of material assistance in the preparation
of foods, as in bread making, butter making, curing of cheese, and
ripening of meat. All the chemical compounds of which foods are
composed are subject to fermentation, each compound being acted upon by
its special ferment body. Those which convert the proteids into soluble
form, as the peptonizing ferments, have no action upon the
carbohydrates. A cycle of bacteriological changes often takes place in a
food material, one class of ferments working until their products
accumulate to such an extent as to prevent their further activity, and
then the process is taken up by others, as they find the conditions
favorable for development. This change of bacterial flora in food
materials is akin to the changes in the vegetation occupying soils. In
each case, there is a constant struggle for possession. Bacteria take a
much more important part in the preparation of foods than is generally
considered. As a result of their workings, various chemical products, as
organic acids and aromatic compounds, are produced. The organic acids
chemically unite with the nutrients of foods, changing their composition
and physical properties. Man is, to a great extent, dependent upon
bacterial action. Plant life also is dependent upon the bacterial
changes which take place in the soil and in the plant tissues. The
stirring of seeds into activity is apparently due to enzymes or soluble
ferments which are inherent in the seed. A study of the bacteriological
changes which foods undergo in their preparation and digestion more
properly belongs to the subject of bacteriology, and in this work only
brief mention is made of some of the more important parts which
microörganisms take in the preparation of foods.
30. Insoluble Ferments.--Insoluble ferments are minute, plant-like
bodies of definite form and structure, and can be studied only with the
microscope.[1] They are developed from spores or seeds, or from the
splitting or budding of the parent cells. Under suitable conditions they
multiply rapidly, deriving the energy for their life processes from the
chemical changes which they induce. For example, in the souring of milk
the milk sugar is changed by the lactic acid ferments into lactic acid.
In causing chemical changes, the ferment gives none of its own material
to the reacting substance. These ferment bodies undergo life processes
similar to plants of a higher order.
[Illustration: FIG. 8.--LACTIC ACID BACTERIA, MUCH
ENLARGED. (After RUSSELL.)]
All foods contain bacteria or ferments. In fact, it is impossible for a
food stored and prepared under ordinary conditions, unless it has been
specially treated, to be free from them. Some of them are useful, some
are injurious, while others are capable of producing disease. The
objectionable bacteria are usually destroyed by the joint action of
sunlight, pure air, and water.
31. Soluble Ferments.--Many plant and animal cells have the power of
secreting substances soluble in water and capable of producing
fermentation changes; to these the term "soluble ferments," or
"enzymes," is applied. These ferments have not a cell structure like
the organized ferments. When germinated seed, as malted barley, is
extracted, a soluble and highly nitrogenous substance, called the
diastase ferment, is secured that changes starch into soluble forms. The
soluble ferments induce chemical change by causing molecular disturbance
or splitting up of the organic compounds, resulting in the production of
derivative products. They take an important part in animal and plant
nutrition, as by their action insoluble compounds are brought into a
soluble condition so they can be utilized for nutritive purposes. In
many instances ferment changes are due to the joint action of soluble
and insoluble ferments. The insoluble ferment secretes an enzyme which
induces a chemical change, modified by the further action of the soluble
ferment. Many of the enzymes carry on their work at a low temperature,
as in the curing of meat and cheese in cold storage.[14]
32. General Relationship of Chemical, Physical, and Bacteriological
Changes.--It cannot be said that the beneficial results derived from
the cooking of foods are due to either chemical, physical, or
bacteriological change alone, but to the joint action of the three. In
order to secure a chemical change, a physical change must often precede,
and a bacteriological change cannot take place without causing a change
in chemical composition; the three are closely related and
interdependent.
33. Esthetic Value of Foods.--Foods should be not only of good
physical texture and contain the requisite nutrients, but they should
also be pleasing to the eye and served in the most attractive manner.
Some foods owe a part of their commercial value to color, and when they
are lacking in natural color they are not consumed with a relish. There
is no objection to the addition of coloring matter to foods, provided it
is of a non-injurious character and does not affect the amount of
nutrients, and that its presence and the kind of coloring material are
made known. Some foods contain objectionable colors which are eliminated
during the process of manufacture, as in the case of sugar and flour. As
far as removal of coloring matter from foods during refining is
concerned, there can be no objection, so long as no injurious reagents
or chemicals are retained, as the removal of the color in no way affects
the nutritive value or permits fraud, but necessitates higher
purification and refining. The use of chemicals and reagents in the
preparation and refining of foods is considered permissible in all cases
where the reagents are removed by subsequent processes. In the food
decisions of the United States Department of Agriculture, it is stated:
"Not excluded under this provision are substances properly used in the
preparation of food products for clarification or refining and
eliminated in the further process of manufacture." [15]
CHAPTER III
VEGETABLE FOODS
34. General Composition.--Vegetable foods, with the exception of
cereals, legumes, and nuts, contain a smaller percentage of protein than
animal food products. They vary widely in composition and nutritive
value; in some, starch predominates, while in others, sugar, cellulose,
and pectin bodies are most abundant. The general term "vegetable foods"
is used in this work to include roots, tubers, garden vegetables,
cereals, legumes, and all prepared foods of vegetable origin.
35. Potatoes contain about 75 per cent of water and 25 per cent of dry
matter, the larger portion being starch. There is but little nitrogenous
material in the potato, only 2.25 per cent, of which about half is in
the form of proteids. There are ten parts of non-nitrogenous substance
to every one part of nitrogenous; or, in other words, the potato has a
wide nutritive ratio, and as an article of diet needs to be supplemented
with foods rich in protein. The mineral matter, cellular tissue, and
fat in potatoes are small in amount, as are also the organic acids.
Mechanically considered, the potato is composed of three parts,--outer
skin, inner skin, and flesh. The layer immediately beneath the outer
skin is slightly colored, and is designated the fibro-vascular layer.
The outer and inner skins combined make up about 10 per cent of the
weight of the potato.
[Illustration: FIG. 9.--TRANSVERSE SECTION OF POTATO.
(After COWDEN and BUSSARD.) _a_, skin; _b_, cortical
layer; _c_, outer medullary layer; _d_, inner medullary layer.]
A large portion of the protein of the potato is albumin, which is
soluble in water. When potatoes are peeled, cut in small pieces, and
soaked in water for several hours before boiling, 80 per cent of the
crude protein, or total nitrogenous material, is extracted, rendering
the product less valuable as food. When potatoes are placed directly in
boiling water, the losses of nitrogenous compounds are reduced to about
7 per cent, and, when the skins are not removed, to 1 per cent.
Digestion experiments show that 92 per cent of the starch and 72 per
cent of the protein are digested.[12] Compared with other foods,
potatoes are often a cheap source of non-nitrogenous nutrients. If used
in excessive amounts, however, they have a tendency to make the ration
unbalanced and too bulky.
MECHANICAL COMPOSITION OF THE POTATO
================================================
|Per Cent
Unpeeled potatoes | 100.0
Outer, or true skin | 2.5
Inner skin, or fibro-vascular layer[A] | 8.5
Flesh | 89.0
================================================
CHEMICAL COMPOSITION OF THE POTATO
================================================================
| | | | CARBOHYDRATES
|-----|-------|---|------------------------
|Water| Crude |Fat|Nitrogen-free-| |
| |Protein| | extract |Fiber|Ash
| % | % | % | % | % |%
---------------------|-----|-------|---|--------------|-----|---
Outer, or true skin | 80.1| 2.7 |0.8| 14.|6 |1.8
Inner skin, or | | | | | |
fibro-vascular | | | | | |
layer | 83.2| 2.3 |0.1| 12.6 | 0.7 |1.1
Flesh | 81.1| 2.0 |0.1| 15.7 | 0.3 |0.8
Average of 86 | | | | | |
American analyses[B]| 78.0| 2.2 |0.1| 18.|8 |0.9
Average of 118 | | | | | |
European analyses[C]| 75.0| 2.1 |0.1| 21.0 | 0.7 |1.1
================================================================
[Footnote A: Including a small amount of flesh.]
[Footnote B: From an unpublished compilation of analyses of American
food products.]
[Footnote C: König, "Chemie der Nahrungs-und Genussmittel," 3d ed., II,
p. 626.]
36. Sweet Potatoes contain more dry matter than white potatoes, the
difference being due mainly to the presence of about 6 per cent of
sugar. There is approximately the same starch content, but more fat,
protein, and fiber. As a food, they supply a large amount of
non-nitrogenous nutrients.
37. Carrots contain about half as much dry matter as potatoes, and
half of the dry matter is sugar, nearly equally divided between sucrose
and levulose, or fruit sugar. Like the potato, carrots have some organic
acids and a relatively small amount of proteids. In carrots and milk
there is practically the same per cent of water. The nutrients in each,
however, differ both as to kind and proportion. Experiments with the
cooking of carrots show that if a large amount of water is used, 30 per
cent or more of the nutrients, particularly of the more soluble sugar
and albumin, are extracted and lost in the drain waters.[12] The color
of the carrot is due to the non-nitrogenous compound carrotin,
C_{26}H_{38}. Carrots are valuable in a ration not because of the
nutrients they supply, but for the palatability and the mechanical
action which the vegetable fiber exerts upon the process of digestion.
38. Parsnips contain more solid matter than beets or carrots, of which
3 to 4 per cent is starch. The starch grains are very small, being only
about one twentieth the size of the potato starch grains. There is 3 per
cent of sugar and an appreciable amount of fat, more than in any other
of the vegetables of this class, and seven times as much as in the
potato. The mineral matter is of somewhat different nature from that in
potatoes; in parsnips one half is potash and one quarter phosphoric
acid, while in potatoes three quarters are potash and one fifth
phosphoric acid.
39. Cabbage contains very little dry matter, usually less than 10 per
cent. It is proportionally richer in nitrogenous compounds than many
vegetables, as about two of the ten parts of dry matter are crude
protein, which makes the nutritive ratio one to five. During cooking 30
to 40 per cent of the nutrients are extracted. Cabbage imparts to the
ration bulk but comparatively little nutritive material. It is a
valuable food adjunct, particularly used raw, as in a salad, when it is
easily digested and retains all of the nutrients.[12]
[Illustration: FIG. 10.--GRAPHIC COMPOSITION OF CABBAGE.]
40. Cauliflower has much the same general composition as cabbage, from
which it differs mainly in mechanical structure.
41. Beets.--The garden beet contains a little more protein than
carrots, but otherwise has about the same general composition, and the
statements made in regard to the losses of nutrients in the cooking of
carrots and to their use in the dietary apply also to beets.
42. Cucumbers contain about 4 per cent of dry matter. The amount of
nutrients is so small as to scarcely allow them to be considered a food.
They are, however, a valuable food adjunct, as they impart palatability.
43. Lettuce contains about 7 per cent of solids, of which 1.5 is
protein and 2.5 starch and sugar. While low in nutrients, it is high in
dietetic value, because of the chlorophyll which it contains. It has
been suggested that it is valuable, too, for supplying iron in an
organic form, as there is iron chemically combined with the chlorophyll.
44. Onions are aromatic bulbs, valuable for condimental rather than
nutritive purposes. They contain essential and volatile oils, which
impart characteristic odor and flavor. In the onion there are about 1.5
per cent of protein and 9.5 per cent of non-nitrogenous material. Onions
are often useful in stimulating the digestive tract to action.
45. Spinach is a valuable food, not to be classed merely as a relish.
Its composition is interesting; for, although there is 90 per cent
water, and less than 10 per cent dry matter, it still possesses high
food value. Spinach contains 2.1 per cent crude protein, or about one
part to every four parts of carbohydrates. In potatoes, turnips, and
beets there are ten or more parts of carbohydrates to every one part of
protein.
46. Asparagus is composed largely of water, about 93 per cent. The dry
matter, however, is richer in protein than that of many vegetables.
Asparagus contains, too, an amid compound, asparagin, which gives some
of the characteristics to the vegetable.
47. Melons.--Melons contain from 8 to 10 per cent of dry matter, the
larger portion of which is sugar and allied carbohydrates. The flavor is
due to small amounts of essential oils and to organic acids associated
with the sugars. Melons possess condimental rather than nutritive value.
[Illustration: FIG. 11.--GRAPHIC COMPOSITION OF TOMATO.]
48. Tomatoes.--The tomato belongs to the night-shade family, and for
this reason was long looked upon with suspicion. It was first used for
ornamental purposes and was called "love-apple." Gradually, as the idea
of its poisonous nature became dispelled, it grew more and more popular
as a food, until now in the United States it is one of the most common
garden vegetables. It contains 7 per cent of dry matter, 4 per cent of
which is sucrose, dextrose, and levulose. It also contains some malic
acid, and a small amount of proteids, amids, cellulose, and coloring
material. In the canning of tomatoes, if too much of the juice is
excluded, a large part of the nutritive material is lost, as the sugars
and albumins are all soluble and readily removed.[16] If the seeds are
objectionable, they may be removed by straining and the juice added to
the fleshy portion. The product then has a higher nutritive value than
if the juice had been discarded with the seeds.
49. Sweet Corn.--Fresh, soft, green, sweet corn contains about 75 per
cent of water. The dry matter is half starch and one quarter sugar. The
protein content makes up nearly 5 per cent, a larger proportional amount
than is found in the ripened corn, due to the fact that the proteids are
deposited in the early stages of growth and the carbohydrates mainly in
the last stages. Sweet corn is a vegetable of high nutritive value and
palatability.
50. Eggplant contains a high per cent of water,--90 per cent. The
principal nutrients are starch and sugar, which make up about half the
weight of the dry matter. It does not itself supply a large amount of
nutrients, but the way in which it is prepared, by combination with
butter, bread crumbs, and eggs, makes it a nutritious and palatable
dish, the food value being derived mainly from the materials with which
it is combined, the eggplant giving the flavor and palatability.
51. Squash and Pumpkin.--Squash has much the same general composition
and food value as beets and carrots, although it belongs to a different
family. Pumpkins contain less dry matter than squash. The dry matter of
both is composed largely of starch and sugar and, like many other of the
vegetables, they are often combined with food materials containing a
large amount of nutrients, as in pumpkin and squash pies, where the food
value is derived mainly from the milk, sugar, eggs, flour, and butter or
other shortening used.
52. Celery.--The dry matter of celery is comparatively rich in
nitrogenous material, although the amount is small, and the larger
proportion is in non-proteid form. When grown on rich soil, celery may
contain an appreciable quantity of nitrates and nitrites, which have not
been converted into amids and proteids. The supposed medicinal value is
probably due to the nitrites which are generally present. Celery is
valuable from a dietetic rather than a nutritive point of view.
53. Sanitary Condition of Vegetables.--The conditions under which
vegetables are grown have much to do with their value, particularly from
a sanitary point of view. Uncooked vegetables often cause the spread of
diseases, particularly those, as cholera and typhoid, affecting the
digestive tract. Particles of dirt containing the disease-producing
organisms adhere to the uncooked vegetable and find their way into the
digestive tract, where the bacteria undergo incubation. When sewage has
been used for fertilizing the land, as in sewage irrigation, the
vegetables are unsound from a sanitary point of view. Such vegetables
should be thoroughly cleaned and also well cooked, in order to render
them sterile. Vegetables to be eaten in the raw state should be dipped
momentarily into boiling water, to destroy the activity of the germs
present upon the surface. They may then be immediately immersed in
ice-cold water, to preserve the crispness.
54. Miscellaneous Compounds in Vegetables.--In addition to the general
nutrients which have been discussed, many of the vegetables contain some
tannin, glucosides, and essential oils; and occasionally those grown
upon rich soils have appreciable amounts of nitrogen compounds, as
nitrates and nitrites, which have not been built up into proteids.
Vegetables have a unique value in the dietary, and while as a class they
contain small amounts of nutrients, they are indispensable for promoting
health and securing normal digestion of the food.
55. Canned Vegetables.--When sound vegetables are thoroughly cooked to
destroy ferments, and then sealed in cans while hot, they can be kept
for a long time without any material impairment of nutritive value.
During the cooking process there is lost a part of the essential oils,
which gives a slightly different flavor to the canned or tinned
goods.[17] In some canned vegetables preservatives are used, but the
enactment and enforcement of national and state laws have greatly
reduced their use. When the cans are made of a poor quality of tin, or
the vegetables are of high acidity, some of the metal is dissolved in
sufficient quantity to be objectionable from a sanitary point of
view.[18]
56. Edible Portion and Refuse of Vegetables.--Many vegetables have
appreciable amounts of refuse,[19] or non-edible parts, as skin, pods,
seeds, and pulp, and in determining the nutritive value, these must be
considered, as in some cases less than 50 per cent of the weight of the
material is edible portion, which proportionally increases the cost of
the nutrients. Ordinarily, the edible part is richer in protein than the
entire material as purchased. In some cases, however, the refuse is
richer in protein, but the protein is in a less available form. See
comparison of potatoes and potato skins.
CHAPTER IV
FRUITS, FLAVORS, AND EXTRACTS
57. General Composition.--Fruits are characterized by containing a
large amount of water and only a small amount of dry matter, which is
composed mainly of sugar and non-nitrogenous compounds. Fruits contain
but little fatty material and protein. A large portion of the total
nitrogen is in the form of amid compounds. Organic acids, as citric,
tartaric, and malic, are found in all fruits, and the essential oils
form a characteristic feature. The taste of fruits is due mainly to the
blending of the various organic acids, essential oils, and sugars.
Although fruits contain a high per cent of water, they are nevertheless
valuable as food.[20] The constituents present to the greatest extent
are sugars and acids. The sugar is not all like the common granulated
sugar, but in ripe fruits a part is in the form known as levulose or
fruit sugar, which is two and a half times sweeter than granulated
sugar. Sugars are valuable for heat-and fat-producing purposes, but not
for muscle repairing. Proteids are the muscle-forming nutrients. The
organic acids, as malic acid in apples, citric acid in lemons and
oranges, and tartaric acid in grapes, have characteristic medicinal
properties. The sugar, proteid, and acid content of some of our more
common fruits is given in the following table:[21]
COMPOSITION OF FRUITS
==============================================================
| WATER |PROTEIDS| SUGAR |ACID IN |KIND OF
| | | | JUICE | ACID
----------------|--------|--------|--------|--------|---------
|Per Cent|Per Cent|Per Cent|Per Cent|
Apples (Baldwin)| 85.0 | 0.50 | 10.75 | 0.92 |Malic
Apples, sweet | 86.0 | 0.50 | 11.75 | 0.20 |Malic
Blackberries | 88.9 | 0.90 | 11.50 | 0.75 |Malic
Currants | 86.0 | -- | 1.96 | 5.80 |Tartaric
Grapes | 83.0 | 1.50 |10 to 16|1.2 to 5|Tartaric
Strawberries | 90.8 | 0.95 | 5.36 | 1.40 |Malic
Oranges | 85.0 | 1.10 | 10.00 | 1.30 |Citric
Lemons | 84.0 | 0.95 | 2.00 | 7.20 |Citric
==============================================================
In addition to sugars, acids, and proteids, there are a great many other
compounds in fruits. Those which give the characteristic taste are
called essential or volatile oils.
58. Food Value.--When the nutrients alone are considered, fruits
appear to have a low food value, but they should not be judged entirely
on this basis, because they impart palatability and flavor to other
foods and exercise a favorable influence upon the digestive process. In
the human ration fruits are a necessary adjunct.
59. Apples.--Apples vary in composition with the variety and physical
characteristics of the fruit. In general they contain from 10 to 16 per
cent of dry matter, of which 75 per cent, or more, is sugar or allied
carbohydrates. Among the organic acids malic predominates, and the
acidity ranges from 0.1 to 0.8 per cent. Apples contain but little
protein, less than 1 per cent. There is some pectin, or jelly-like
substance closely related to the carbohydrates. The flavor of the apple
varies with the content of sugar, organic acids, and essential oils.
During storage some apples appear to undergo further ripening, resulting
in partial inversion of the sucrose, and there is a slight loss of
weight, due to the formation of carbon dioxide. The apple is an
important and valuable adjunct to the dietary.[22]
[Illustration: FIG. 12.--GRAPHIC COMPOSITION OF APPLE.]
[Illustration: FIG. 13.--GRAPHIC COMPOSITION OF ORANGE.]
60. Oranges contain nearly the same proportion of dry matter as
apples, the larger part of which is sugar. Citric acid predominates and
ranges in different varieties from 1 to 2.5 per cent. The amounts of
protein, fat, and cellulose are small. In some varieties of oranges
there is more iron and sulphur than is usually found in fruits. All
fruits, however, contain small amounts, but not as much as is found in
green vegetables. The average composition of oranges is as follows:
===========================================================
PHYSICAL COMPOSITION|CHEMICAL COMPOSITION OF EDIBLE PORTION
-----------------------------------------------------------
Per Cent| Per Cent
Rind 20 to 30| Solids 10 to 16
Pulp 25 to 35| Sugars 8 to 12
Juice 35 to 50| Citric acid 1 to 2.5
| Ash 0.5
===========================================================
61. Lemons differ from oranges in containing more citric acid and less
sucrose, levulose, and dextrose. The ash of the lemon is somewhat
similar in general composition to the ash of the orange, but is larger
in amount. The average composition of the lemon is as follows:
===========================================================
PHYSICAL COMPOSITION|CHEMICAL COMPOSITION OF EDIBLE PORTION
-----------------------------------------------------------
Per Cent| Per Cent
Rind 25 to 35| Solids 10 to 12
Pulp 25 to 35| Sugar 2 to 4
Juice 40 to 55| Citric acid 6 to 9
===========================================================
62. Grape Fruit.--The rind and seed of this fruit make up about 25 per
cent, leaving 75 per cent as edible portion. The juice contains 14 per
cent solids, of which nearly 10 per cent is sugar and 2.5 per cent is
citric acid. There is more acid in grape fruit than in oranges and
appreciably less than in lemons. The characteristic flavor is due to a
glucoside-like material. Otherwise the composition and food value are
about the same as of oranges.
[Illustration: FIG. 14.--GRAPHIC COMPOSITION OF STRAWBERRY.]
63. Strawberries contain from 8 to 12 per cent of dry matter, mainly
sugar and malic acid. The protein, fat, and ash usually make up less
than 2 per cent. Essential oils and coloring substances are present in
small amounts. It has been estimated that it would require 75 pounds of
strawberries to supply the protein for a daily ration. Nevertheless they
are valuable in the dietary. It has been suggested that the malic and
other acids have antiseptic properties which, added to the appearance
and palatability, make them a desirable food adjunct. Strawberries have
high dietetic rather than high food value.
64. Grapes contain more dry matter than apples or oranges. There is no
appreciable amount of protein or fat, and while they add some nutrients,
as sugar, to the ration, they do not contribute any quantity. Their
value, as in the case of other fruits, is due to palatability and
indirect effect upon the digestibility of other foods. In the juice of
grapes there is from 10 to 15 per cent or more of sugar, as sucrose,
levulose, and dextrose. Grapes contain also from 1 to 1.5 per cent of
tartaric acid, which, during the process of manufacture into wine, is
rendered insoluble by the alcohol formed, and the product, known as
argole, is used in the preparation of cream of tartar. Differences in
flavor and taste of grapes are due to variations in the sugar, acid, and
essential oil content.
65. Peaches contain about 12 per cent of dry matter, of which over 10
per cent is sugar and other carbohydrates. There is less than 1.5 per
cent of protein, fat, and mineral matter and about 0.5 per cent of acid.
The peach contains also a very small amount of hydrocyanic acid, which
is more liberally present in the kernel than in the fruit. Flavor is
imparted mainly by the sugar and essential oils. Peaches vary in
composition with variety and environment.[23]
66. Plums contain the most dry matter of any of the fruits, about 22
per cent, mainly sugar. About one per cent is acid and about 0.5 per
cent are protein and ash. There are a great many varieties of plums,
varying in composition. Dried plums (prunes) have mildly laxative
properties.
67. Olives.--The ripe olive contains about 15 per cent of oil,
exclusive of the pit, which makes up 20 per cent of the weight. In
green, preserved olives there is considerably less oil. Because of the
oil the olive has food value. Olive oil is slightly laxative and assists
mechanically in the digestion of foods.
68. Figs.--Dried figs contain about 50 per cent of sugar and 3.5 per
cent of protein. The fig has a mildly laxative action.
69. Dried Fruits.--Many fruits are prepared for market by drying. The
dried fruit has a slightly different composition from the fresh fruit
because of loss of the volatile and essential oils, and minor chemical
changes which take place during the drying process. When free from
preservatives, dried fruits are valuable adjuncts to the dietary and can
be advantageously used when fresh fruits are not obtainable.
70. Canning and Preservation of Fruits.--To obtain the best results in
canning, the fruit should not be overripe. After the ripened state has
been reached fermentation and bacterial changes occur, and it is more
difficult to preserve the fruit than when not so fully matured.[24] When
a fruit has begun to ferment, it is hard to destroy the ferment bodies
and their spores so as to prevent further ferment action. The chemical
changes that occur in the last stages of ripening are similar to those
which take place during the cooking process whereby the pectin or
jelly-like substances are rendered more soluble and digestible.
71. Adulterated Canned Fruits.--Analyses of a number of canned fruits,
made by various Boards of Health, show the presence of small amounts of
arsenic, tin, lead, and other poisonous metals. The quantity dissolved
depends upon the kind, age, and condition of the canned goods and the
state of the fruit when canned. The longer a can of fruit or vegetable
has been kept in stock, the larger is the amount of tin or metal that
has been dissolved. When fresh canned, there is usually very little
dissolved tin, but in old goods the amount may be comparatively large.
The tin used for the can is occasionally of poor quality and may contain
some arsenic, which also is dissolved. The occasional use of canned
goods preserved in tin is not objectionable, but they should not be used
continually if it can be avoided. Preservatives, as borax, salicylic
acid, benzoic acid, and sodium sulphate, are sometimes added to prevent
fermentation and to preserve the natural appearance of the fruit or
vegetable.[18]
72. Fruit Flavors and Extracts.--Formerly all fruit extracts and
flavors were obtained from vegetable sources; at present many are made
in the chemical laboratory by synthetic methods; that is, by combining
simpler organic compounds and radicals to produce the material having
the desired flavor and odor. The various fruit flavors are definite
chemical compounds, and can be produced in the laboratory as well as in
the cells of plants. When properly made, there is no difference in
chemical composition between the two. As prepared in the laboratory,
however, traces of acids, alkalies, and other compounds, used in
bringing about the necessary chemical combination, are often present,
not having been perfectly removed. Hence it is that natural and
artificial flavors differ mainly in the impurities which the artificial
flavors may contain.
Some of the flavoring materials have characteristic medicinal
properties, as the flavor of bitter almond, which contains hydrocyanic
acid, a poisonous substance. Flavors and extracts should not be
indiscriminately used. In small amounts they often exert a favorable
influence upon the digestion of foods, and the value of some fruits is
in a large measure due to the special flavors they contain. A study of
the separate compounds which impart flavor to fruits, as the various
aldehydes, ethers, and organic salts, belongs to organic chemistry
rather than to foods. Some of the simpler compounds of which flavors are
composed may exist in entirely different form or combination in food
products; as for example, pineapple flavoring is ethyl butrate. This can
be prepared by combination of butyric acid from stale butter with
alcohol which supplies the ethyl radical. The chemical union of the two
produces the new compound, ethyl butrate, the distinctive flavoring
substance of the pineapple. Banana flavor can be made from stale
butter, caustic soda, and chloroform. None of these materials, as such,
go into the flavor, but an essential radical is taken from each. These
manufactured products, when properly made, are in every essential
similar to the flavor made by the plant and stored up in the fruit. The
plant combines the material in the laboratory of the plant cell, and the
manufacturer of essences puts together these same constituents in a
chemical laboratory. In the fruit, however, the essential oil is
associated with a number of other compounds.
CHAPTER V
SUGARS, MOLASSES, SYRUP, HONEY, AND CONFECTIONS
73. Composition of Sugars.--The term "sugar" is applied to a large
class of compounds composed of the elements carbon, hydrogen, and
oxygen. Sugars used for household purposes are derived mainly from the
sugar cane and the sugar beet.[25] At the present time about two fifths
are obtained from the cane and about three fifths from the beet. When
subjected to the same degree of refining, there is no difference in the
chemical composition of the sugars from the two sources; they are alike
in every respect and the chemist is unable to determine their origin.
The production of sugar is an agricultural industry; the methods of
manufacture pertain more to industrial chemistry than to the chemistry
of foods, and therefore a discussion of them is omitted in this
work.[26]
[Illustration: FIG. 15.--SUGAR CRYSTALS.]
74. Commercial Grades of Sugar.--Sugars are graded according to the
size of the granule, the color and general appearance of the crystals,
and the per cent of sucrose or pure sugar. Common granulated sugar is
from 98.5 to 99.7 per cent pure sucrose. The impurities consist mainly
of moisture and mineral matter. In the process of refining, sulphur
fumes are frequently used for bleaching and clarifying the solution.[26]
The sulphurous acid formed is neutralized with lime, which is rendered
insoluble and practically all removed in subsequent filtrations. There
are, however, traces of sulphates and sulphites in ordinary sugar, but
these are in such small amounts as not to be injurious to health. When
sugar is burned, as in the bomb calorimeter, so as to permit collection
of all of the products of combustion, granulated sugar yields about 0.01
of a per cent of sulphur dioxid.[13] Occasionally coloring substances,
as a small amount of indigo, are added to yellow tinged sugars to impart
a white color, much on the same principle as the bluing of clothes. The
amount used is usually extremely small, and the effect on health has
never been determined. Occasionally, however, bluing is used to such an
extent that a blue scum appears when the sugar is boiled with water.
Sugar has high value for the production of heat and energy. Digestion
experiments show that when it is used in the dietary in not excessive
amounts, it is directly absorbed by the body and practically all
available. It can advantageously be combined with other foods to form a
part of the ration.[27] When a ration contains the requisite amount of
protein, sugar is used to the best advantage. Alone it is incapable of
sustaining life, because it does not contain any nitrogen. When sugar
was substituted for an excess of protein in a ration, it was found to
produce heat and energy at much less expense. Many foods, as apples,
grapes, and small fruits, contain appreciable amounts of sugar and owe
their food value almost entirely to their sugar content. In the dietary,
sugar is too frequently regarded as a condiment instead of a nutrient,
to be used for imparting palatability rather than for purposes of
nutrition. While valuable for improving the taste of foods, the main
worth of sugar is as a nutritive substance; used in the preparation of
foods it adds to the total heat and energy of the ration. Sugar is
sometimes used in excessive amounts and, as is the case with any food or
nutrient, when that occurs, nutrition disturbances result, due to misuse
of the food. Statistics show that the average consumption of sugar in
the United States is nearly 70 pounds a year per capita. In the dietary
of the adult, sugar to the extent of four ounces per day can be consumed
advantageously. The exclusion of sugar from the diet of children is a
great mistake, as they need it for heat and energy and to conserve the
protein for growth.
"Sugar is one of the most important forms in which carbohydrates
can be added to the diet of children. The great reduction in the
price of sugar which has taken place in recent years is probably
one of the causes of the improved physique of the rising
generation. The fear that sugar may injure children's teeth is,
largely illusory. The negroes who live largely on sugar cane have
the finest teeth the world can show. If injudiciously taken, sugar
may, however, injure the child's appetite and digestion. The
craving for sweets which children show is no doubt the natural
expression of a physiological need, but they should be taken with,
and not between, meals."[28]
[Illustration: FIG. 16.--NUTRIENTS OF A RATION WITH SUGAR.
The hacket parts represent the proportion of nutrients not digested.]
75. Sugar in the Dietary.--Sugar has an important place in the
dietary. It not only serves for the production of heat and energy in the
body, but is also valuable in enabling the proteids to be used more
economically. In reasonable amounts, it is particularly valuable in the
dietary of growing children, as the proteids of the food are then
utilized to better advantage for growth. The unique value of sugar
depends upon its intelligent use and its proper combination with other
foods, particularly with those rich in the nitrogenous compounds or
proteids. Sugar alone is incapable of sustaining life, but combined with
other foods is a valuable nutrient. The amount which can be
advantageously used depends largely upon the individual. Ordinarily
three to five ounces per day is sufficient, although some persons cannot
safely consume as much as this. In the case of diabetes mellitus, the
amount of sugar in the ration must be materially reduced. Persons in
normal health and engaged in outdoor work can use sugar to
advantage.[29] Many of the "harvest drinks," made largely from molasses
with a little ginger, and used extensively in some localities, are not
without merit, as they contain an appreciable amount of nutrients. Milk
contains more sugar as lactose or milk sugar than any other nutrient.
[Illustration: FIG. 17.--NUTRIENTS OF A RATION WITHOUT SUGAR.
The hacket parts represent the proportion of nutrients not digested.]
The craving for sugar by growing children and athletes is natural.
Sugar, however, is often injudiciously used, and a perverted taste may
be established which can be satisfied only by excessive amounts. This
results in impaired digestion and malnutrition.
76. Maple Sugar.--Sugar obtained by evaporation from the sap of the
maple tree (_Acer saccharinum_) is identical, except for the foreign
substances which it contains, with that from the beet and sugar cane.
The mottled appearance and characteristic color and taste of maple sugar
are due to the various organic acids and other compounds present in the
maple sap and recovered in the sugar. Maple sugar, as ordinarily
prepared, has 0.4 of a per cent or more of ash or mineral matter, while
refined cane sugar contains less than one tenth as much.[30] Hence, when
maple sugar is adulterated with cane and beet sugars, the ash content is
noticeably lowered, as is also the content of organic acids. It is
difficult, however, to determine with absolute certainty pure high grade
maple sugar from the impure low grade to which a small amount of
granulated sugar has been added.
77. Adulteration of Sugar.--Sugar at the present time is not
materially adulterated. Other than the substances mentioned which are
used for clarification and color, none are added during refining which
remain in the sugar in appreciable amounts. Sugar does not readily lend
itself to adulteration, as it has a definite crystalline structure, and
materials that would be suitable for its adulteration are of entirely
different physical character.[31] Cane sugar is not easily blended with
glucose, or starch sugar, because of the physical differences between
the two. The question of the kind of sugar to use in the household, as
granulated, loaf, or pulverized, is largely one of personal choice, as
there is no appreciable difference in the nutritive value or purity of
the different kinds.
78. Dextrose Sugars.--Products known as glucose and dextrose sugars
are made from corn and other starches; they can also be prepared from
cane sugar by the use of heat, chemicals, or ferments for carrying on
the process known as inversion. The dextrose sugars differ from cane
sugar in containing a dissimilar number of carbon, hydrogen, and oxygen
atoms in the molecule. The formula of the dextrose sugars is
C_{6}H_{12}_O{6}, while that of cane sugar is C_{12}H_{22}O_{11}. By the
addition of one molecule of water, H_{2}O, to a molecule of sucrose, two
molecules of invert sugar (dextrose and glucose) are produced:[1]
C_{12}H_{22}O_{11} + H_{2} = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}. In
bringing about this change, acids are employed, but the acid in no way
enters into the chemical composition of the final product; it is removed
as described during the process of sugar manufacture. The action of the
acid brings about a catalytic change, the acid being necessary only as a
presence reagent to start the chemical reaction. When properly prepared
and the acid product thoroughly removed, dextrose and glucose have
practically the same food value as sugar. When they are digested, heat
and energy are produced, and a given weight has about the same fuel
value as an equal weight of sugar. Some of the glucose-yielding products
can be made at less expense than sugar, and when they are sold under
their right names there is no reason why they should not be used in the
dietary, as they serve the same nutritive purpose.
79. Molasses is a by-product obtained in the refining of sugar. It is
a mixture of cane sugar and invert sugars, as levulose and dextrose.
When in sugar making the sucrose is removed by crystallization, a point
is finally reached where the solution, or mother liquid, as it is
called, refuses to give up any further crystals;[31] then this product,
consisting of various sugars and small amounts of organic acids and ash,
is partially refined and clarified to form molasses. The term "New
Orleans" molasses was formerly applied to the product obtained by the
use of open kettles for the manufacture of sugar, but during recent
years the vacuum pan process has been introduced, and "New Orleans"
molasses is now an entirely different article. The terms first, second,
and third molasses are applied to the liquids obtained after the removal
of the first, second, and third crops of sugar crystals; first molasses
being richer in sucrose, while third molasses is richer in dextrose and
invert sugars. The ash in molasses ranges from 4 to 6.5 per cent. Some
of the low grades of molasses are used in the preparation of animal
foods.
The taste and physical characteristics of molasses are due largely to
the organic acids and impurities that are present, as well as to the
proportion in which the various sugars occur. When used with soda in
cooking and baking operations, the organic acid of the molasses
liberates carbon dioxide gas, which acts as a leavening agent. Because
of the organic acids, molasses should not be stored in tin or metalware
dishes, as the solvent action results in producing poisonous tin and
other metallic salts.
The food value of molasses is dependent entirely upon the amount of dry
matter and the per cent of sugar. A large amount of water is considered
an adulterant; ordinarily molasses contains from 20 to 33 per cent. If a
sample of molasses contains 75 per cent of dry matter, it has slightly
less than three fourths of the nutritive value of the same weight of
sugar.
80. Syrups.--The term "syrup" is applied to natural products obtained
by evaporation and purification of the saccharine juices of plants.
Sorghum syrup is from the sorghum plant, which is pressed by machinery
and the juice clarified and evaporated so as to contain about 25 per
cent of water. In sorghum syrups there are from 30 to 45 per cent of
cane sugar, and from 12 to 20 per cent of glucose and invert sugars.
Cane syrup is made from the clarified juice of the sugar cane, and has
about the same general composition as sorghum syrup. Maple syrup,
prepared from the juice of the sugar maple, is characteristically rich
in sucrose and contains but little glucose or reducing sugars. The
flavor of all the syrups is due mainly to organic acids, ethereal
products, and impurities. In some instances the essential flavor can be
produced synthetically, or derived from other and cheaper materials;
and by the use of these flavors, mixed syrups can be prepared closely
resembling many of the natural products. When properly made, they are
equal in nutritive value to natural syrups. When sold under assumed
names, they are to be considered and classified as adulterated, and not
as syrups from definite and specific products. Low-grade syrups and
molasses are often used for making fuel alcohol. They readily undergo
alcoholic fermentation and are valuable for this purpose, rendering it
possible for a good grade of fuel alcohol to be produced at low cost.
The manufacture of sugar, syrups, and molasses has been brought to a
high degree of perfection through the assistance rendered by industrial
chemistry. Losses in the process are reduced to a minimum, and the
various steps are all controlled by chemical analysis. Sugar has the
physical property of deflecting a ray of polarized light, the amount of
deflection depending upon the quantity of sugar in solution. This is
measured by the polariscope, an instrument by means of which the sugar
content of sugar plants is rapidly determined.
[Illustration: FIG. 18.--GRAPHIC COMPOSITION OF
SYRUP.]
81. Honey is composed largely of invert sugars gathered by the
honeybee from the nectar of flowers. It varies in composition and flavor
according to its source. The color depends upon the flower from which it
came, white clover giving a light-colored, pleasant-flavored honey,
while that from buckwheat and goldenrod is dark and has a slightly rank
taste. The comb is composed largely of wax, which has somewhat the same
general composition as fat, but contains ethereal instead of glycerol
bodies. On account of the predominance of invert sugars, pure honey has
a levulo or left-handed rotation when examined by the polariscope. Honey
contains from 60 to 75 per cent of invert sugars, and from 12 to 20 per
cent of water, while the ash content is small, less than one tenth of
one per cent. Strained honey is easily adulterated with glucose
products. Adulteration with cane sugar is readily detected, as pure
honey contains only a very small amount of sucrose. Honey can be made by
feeding bees on sugar; the sugar undergoes inversion, with the
production of dextrose. Such honey, although not adulterated, is
inferior in quality and lacking in natural flavor.[18]
82. Confections.--By blending various saccharine products, confections
are made. Usually sucrose (cane and beet sugar) is used as the basis for
their preparation. Sucrose has definite physical properties, as
crystalline structure, and forms chemical and mechanical combinations
with acid, alkaline, and other substances; it also unites with water,
and when heated undergoes changes in structural composition. The
presence of small amounts of acid substances, or variations in the
concentration of the sugar solution, materially affect the mechanical
relation of the sugar particles to each other, and their
crystallization. Usually crystallization takes place when there is less
than 25 per cent of water present. The form, size, and arrangement of
the crystals are influenced by agitation during cooling. To secure
desired results, often small quantities of various other substances are
employed for their mechanical action. Glucose is frequently used, and is
said to be necessary for the production of some kinds of candy.
Candies are colored with various dyes and pigments, many of which are
harmless, although some are injurious. Coal tar dyes are frequently
employed for this purpose. Objection has generally been urged against
their use, as it is believed many of them are injurious to health. It
cannot be said, however, that all are poisonous, as some are known to be
harmless. The use of a few coal tar dyes is allowed by the United States
government. Mineral colors are now rarely, if ever, used.
Impure candies result from objectionable ingredients, as starch,
paraffin, and large amounts of injurious coloring substances. Coal tar
coloring materials are identified in the way described in Experiment No.
13. Confectionery, when properly prepared and unadulterated, has the
same nutritive value as sugar and the other ingredients, and is entitled
to a place in the dietary for the production of heat and energy. Much
larger amounts of candies are sold and consumed during the winter than
the summer months, suggesting that in cold weather candy is most needed
in the dietary.
83. Saccharine is an artificial sweetening, five hundred times sweeter
than cane sugar. It contains in its molecule, chemically united,
benzine, sulphuric acid, and ammonia radicals. It is employed for
sweetening purposes in cases of diabetes mellitus, where physicians
advise against the use of sugar. It has no food value. A small amount is
sometimes added to canned corn and tomatoes to impart a sweet taste. The
physiological properties of saccharine have not been extensively
investigated.
CHAPTER VI
LEGUMES AND NUTS
84. General Composition of Legumes.--Peas, beans, lentils, and peanuts
are the legumes most generally used for human food. As a class, they are
characterized by high protein content and a comparatively low per cent
of starch and carbohydrates. They contain the largest amount of
nitrogenous compounds of any of the vegetable foods, and hence are
particularly valuable in the human ration as a substitute for meats.[32]
For feeding animals the legumes are highly prized, particularly the
forage crops, clover and alfalfa. These secure their nitrogen, which is
the characteristic element of protein, from the free nitrogen of the
air, through the workings of bacterial organisms found in the nodules on
the roots of the plants. The legumes appear to be the only plants
capable of making use of the nitrogen of the air for food purposes.
85. Beans contain about 24 per cent of protein and but little fat,
less than is found in any of the grain or cereal products. The protein
of the bean differs from that of cereals in its general and structural
composition. It is a globulin known as legumin, and is acted upon
mainly by ferments working in alkaline solutions, as in the lower part
of the digestive tract. Beans have about the same amount of ash as the
cereals, but the ash is richer in potash and lime.
[Illustration: FIG. 19.--GRAPHIC COMPOSITION OF BEANS.
HACKED PART INDIGESTIBLE.]
86. Digestibility of Beans.--Beans are usually considered
indigestible, but experiments show they are quite completely digested,
although they require more work on the part of the digestive tract than
many other foods. The digestibility was found to vary with individuals,
86 per cent of the protein being digested in one case, and only 72 per
cent in another. The protein of beans is not as completely digested as
that of meats. When beans were combined with other foods, forming a part
of a ration, they were more completely digested than when used in large
amounts and with only a few other foods. The presence of the skin is in
part responsible for low digestibility. When in the preparation of beans
the skins, which contain a large amount of cellulose, are removed, the
beans are more completely digested. By cooking from twenty minutes to
half an hour in rapidly boiling water containing a small amount of soda,
the skins are softened and loosened and are then easily removed by
rubbing in cold water. Some of the soda enters into combination with the
legumin. Along with the skins a portion of the germ is lost. The germ
readily ferments, which is probably the cause of beans producing
flatulence with some individuals during digestion. After the skins are
removed the nutrients are more susceptible to the action of the
digestive fluids. Experiments show that 42 per cent of the protein of
baked skinned beans is soluble in pepsin and pancreatin solutions, while
under similar conditions there is only 3.85 per cent of the protein
soluble from beans baked without removal of the skins.
[Illustration: FIG. 20.--BEANS, RAW AND COOKED. SKINS, WET
AND DRY.]
87. Use of Beans in the Dietary.--There is no vegetable food capable
of furnishing so much protein at such low cost as beans; from a pound
costing five cents about one fifth of a pound of protein and three
fifths of a pound of carbohydrates are obtained. Beans can, to a great
extent, take the place of meats in the dietary. There is more protein in
beans than in beef. Four ounces of uncooked beans or six ounces of baked
beans are as much as can conveniently be combined in the dietary, and
these will furnish a quarter of the protein of the ration. In the case
of active out-of-door laborers over a pound of baked beans per day is
often consumed with impunity.
88. String Beans.--String beans--green beans with pod--contain a large
amount of water, 85 to 88 per cent. The dry matter is rich in protein,
nearly 20 per cent, although in the green beans as eaten, containing 85
per cent water, there is less than 2-1/2 per cent. Lima beans are richer
in protein than string beans, as the green pod is not included. String
beans are valuable both for the nutrients they contain and for the
favorable influence they exert upon the digestibility of other foods.
89. Peas.--In general composition and digestibility, peas are quite
similar to beans. They belong to the same family, Leguminosæ, and the
protein of each is similar in quantity and general properties. The
statements made in regard to the composition, digestibility, and use of
beans in the dietary apply with minor modifications to peas. When used
in the preparation of soups, they add appreciable amounts of nutrients.
[Illustration: FIG. 21.--PEA STARCH GRANULES.]
90. Canned Peas.--In order to impart a rich green color, copper
sulphate has been used in the canning of peas. Physiologists differ as
to its effect upon health. While a little may not be particularly
injurious, much interferes with normal digestion of the food and forms
insoluble copper proteids. In some countries a small amount of copper
sulphate is tolerated, while in others it is prohibited.
91. Peanuts.--Peanuts differ from peas and beans in containing more
fat. They should be considered a food, for at ordinary prices they
furnish a large amount of protein and fat. Like the other members of the
legume family, the peanut is rather slow of digestion and requires
considerable intestinal work for completion of the process.
NUTS
92. General Composition.--Nuts should be regarded as food, for they
contribute to a ration appreciable amounts of nutrients. The edible
portion of nearly all is rich in fat; pecans, for example, contain as
high as 70 per cent. In protein content nuts range from 3 per cent in
cocoanuts to 30 per cent in peanuts. The carbohydrate content is usually
comparatively low, less than 5 per cent in hickory nuts, although there
is nearly 40 per cent in chestnuts. On account of high fat content, nuts
supply a large amount of heat and energy.[33]
93. Chestnuts are characterized by containing less fat and protein and
much more carbohydrate material, especially starch, than is found in
other nuts. In southern Europe chestnuts are widely used as food; the
skins are removed, and the nuts are steamed, boiled, or roasted, and
sometimes they are dried and ground into flour. Chestnuts are less
concentrated in protein and fat, and form a better balanced food used
alone than do other nuts.
94. The Hickory Nut, which is a characteristically American nut,
contains in the edible portion about 15 per cent protein, 65 per cent
fat, and 12 per cent carbohydrates.
95. The Almonds used in the United States come chiefly from southern
Europe, although they are successfully raised in California. They
contain about 55 per cent fat and 22 per cent protein. The flavor of
almonds is due to a small amount of hydrocyanic acid.
96. Pistachio.--Some nuts are used for imparting color and flavor to
food products, as the pistachio nut, the kernel of which is greenish in
color and imparts a flavor suggestive of almonds. The pistachio has high
food value, as it is rich in both fat and protein. It is employed in the
manufacture of confectionery and in ice cream for imparting flavor and
color.
97. Cocoanuts grow luxuriantly in many tropical countries, and have a
high food value. They are characteristically rich in fat, one half of
the edible portion being composed of this nutrient. For tropical
countries they supply the fat of a ration at less expense than any other
food. When used in large amounts they should be supplemented with foods
rich in carbohydrates, as rice, and in proteids, as beans. Cocoanut milk
is proportionally richer in carbohydrates and poorer in fat and protein
than the meat of the cocoanut. In discussing the cocoanut, Woods
states:[34]
"The small, green, and immature nuts are grated fine for medicinal
use, and when mixed with the oil of the ripe nut it becomes a
healing ointment. The jelly which lines the shell of the more
mature nut furnishes a delicate and nutritious food. The milk in
its center, when iced, is a most delicious luxury. Grated cocoanut
forms a part of the world-renowned East India condiment, curry.
Dried, shredded (desiccated) cocoanut is an important article of
commerce. From the oil a butter is made, of a clear, whitish color,
so rich in fat, that of water and foreign substances combined there
are but O.0068. It is better adapted for cooking than for table
use. At present it is chiefly used in hospitals, but it is rapidly
finding its way to the tables of the poor, particularly as a
substitute for oleomargarine."
98. Use of Nuts in the Dietary.--When nuts can be secured at a low
price per pound, ten cents or less, they compare favorably in nutritive
value with other staple foods. Digestion experiments with rations
composed largely of nuts show that they are quite thoroughly digested.
Professor Jaffa of the California Experiment Station, in discussing the
nutritive value of nuts and fruits, says:[35]
"It is certainly an error to consider nuts merely as an accessory
to an already heavy meal, and to regard fruit merely as something
of value for its pleasant flavor, or for its hygienic or medicinal
virtues. The agreement of one food or another with any person is
more or less a personal idiosyncrasy, but it seems fair to say that
those with whom nuts and fruits agree, can, if they desire, readily
secure a considerable part of their nutritive material from such
sources."
AVERAGE COMPOSITION OF NUTS
(From Fifteenth Annual Report, Maine Agricultural Experiment Station.)
===========================================================================
|REFUSE|EDIBLE | EDIBLE PORTION |VALUE[A]
| | |------------------------------|
| |PORTION|Water|Prot.| Fat |Carb.| Ash | PER LB.
---------------------------------------------------------------------------
| % | % | % | % | % | % | % |Calories
Almonds | 64.8 | 35.2 | 1.7 | 7.3 |19.3 | 6.2 | 0.7 | 1065
Almonds, kernels | -- | 100.0 | 4.8 |21.0 |54.9 |17.3 | 2.0 | 3030
Brazil nuts | 49.6 | 50.4 | 2.7 | 8.6 |33.6 | 3.5 | 2.0 | 1545
Filberts | 52.1 | 47.9 | 1.8 | 7.5 |31.3 | 6.2 | 1.1 | 1575
Filberts, kernels | -- | 100.0 | 3.7 |15.6 |65.3 |13.0 | 2.4 | 3290
Hickory nuts | 62.2 | 37.8 | 1.4 | 5.8 |25.5 | 4.3 | 0.8 | 1265
Pecans | 49.7 | 50.3 | 1.4 | 5.2 |35.6 | 7.2 | 0.8 | 1733
Pecans, kernels | -- | 100.0 | 2.9 |10.3 |70.8 |14.3 | 1.7 | 3445
Walnuts | 58.0 | 42.0 | 1.2 | 7.0 |27.0 | 6.1 | 0.7 | 1385
Walnuts, kernels | -- | 100.0 | 2.8 |16.7 |64.4 |14.8 | 1.3 | 3305
Chestnuts | 16.1 | 83.9 |31.0 | 5.7 | 6.7 |39.0 | 1.5 | 1115
Acorns | 35.6 | 64.4 | 2.6 | 5.2 |24.1 |30.9 | 1.6 | 1690
Beechnuts | 40.8 | 59.2 | 2.3 |13.0 |34.0 | 7.8 | 2.1 | 1820
Butternuts | 86.4 | 13.6 | 0.6 | 3.8 | 8.3 | 0.5 | 0.4 | 430
Litchi nuts | 41.6 | 58.4 |10.5 | 1.7 | 0.1 |45.2 | 0.9 | 875
Piñon, P. edulis | 40.6 | 59.4 | 2.0 | 8.7 |36.8 |10.2 | 1.7 | 1905
Piñon, P. monophylla| 41.7 | 58.3 | 2.2 | 3.8 |35.4 |15.3 | 1.6 | 1850
Piñon, P. sabiniana | 77.0 | 23.0 | 1.2 | 6.5 |12.3 | 1.9 | 1.1 | 675
Pistachio, kernels | -- | 100.0 | 4.2 |22.6 |54.5 |15.6 | 3.1 | 3010
Peanuts, raw | 26.4 | 73.6 | 6.9 |20.6 |30.7 |13.8 | 1.6 | 1935
Peanuts, kernels | -- | 100.0 | 9.3 |27.9 |42.0 |18.7 | 2.1 | 2640
Roasted peanuts | 32.6 | 67.4 | 1.1 |20.6 |33.1 |10.9 | 1.7 | 1985
Shelled peanuts | -- | 100.0 | 1.6 |30.5 |49.2 |16.2 | 2.5 | 2955
Peanut butter | -- | -- | 2.0 |29.3 |46.6 |17.1 |[B]5.0| 2830
Cocoanuts | 48.8 | 51.2 | 7.2 | 2.9 |25.9 |14.3 | 0.9 | 1415
Cocoanuts, shredded | -- | -- | 3.5 | 6.3 |57.3 |31.6 | 1.3 | 3125
Cocoanut milk | -- | -- |92.7 | 0.4 | 1.5 | 4.6 | 0.8 | 97
=========================================================================
[Footnote A: Calculated from analyses.]
[Footnote B: Including salt, 4.1.]
CHAPTER VII
MILK AND DAIRY PRODUCTS
99. Importance in the Dietary.--There is no article of food which
enters so extensively into the dietary as milk, and it is one of the few
foods which supply all the nutrients,--fats, carbohydrates, and
proteids.[36] Milk alone is capable of sustaining life for comparatively
long periods, and it is the chief article of food during many diseases.
An exclusive milk diet for a healthy adult, however, would be
unsatisfactory; in the case of young children, milk is essential,
because the digestive tract has not become functionally developed for
the digestion of other foods.
It is necessary to consider not only the composition and nutritive value
of milk, but also its purity or sanitary condition.
100. General Composition.--Average milk contains about 87 per cent
water and 13 per cent dry matter. The dry matter is composed
approximately of:
=======================
| Per Cent
Fat | 3.5
Casein | 3.25
Albumin | 0.50
Milk sugar | 5.00
Ash | 0.75
=======================
[Illustration: FIG. 22.--MILK FAT GLOBULES.]
Fat is the most variable constituent of milk. Occasionally it is found
as low as 2 per cent and as high as 6 per cent or more. The poorest and
richest milks differ mainly in fat content, as the sugar, ash, casein,
and albumin, or "solids of the milk serum," are fairly constant in
amount and composition. Variations in the content of fat are due to
differences in feed and in the breed and individuality of the animal.
101. Digestibility.--Milk is one of the most completely digested of
foods, about 95 per cent of the protein and fat and 97 per cent of the
carbohydrates being absorbed and utilized by the body.
In a mixed ration, the nutrients of milk are practically all absorbed.
Milk also exerts a favorable influence upon the digestibility of other
foods with which it is combined. This is doubtless due to the digestive
action of the special ferments or enzymes which milk contains. In milk
there is a soluble ferment material or enzyme which has the power of
peptonizing proteids. It is this ferment which carries on the ripening
process when cheese is cured in cold storage, and it is believed to be
this body which promotes digestion of other foods with which milk is
combined.[27]
Milk is not easily digested by some persons. The tendency to costiveness
caused by a milk diet can be largely overcome by the use of salt with
the milk, or of some solid food, as toast or crackers, to prevent
coagulation and the formation of masses resistant to the digestive
fluids. Barley water and lime water in small amounts are also useful for
assisting mechanically in the digestion of milk. Milk at ordinary prices
is one of the cheapest foods that can be used.
[Illustration: FIG. 23.--DIRT IN A SAMPLE OF UNSANITARY MILK.]
102. Sanitary Condition of Milk.--Equally as important as composition
is the sanitary condition or wholesomeness of milk. Milk is a food
material which readily undergoes fermentation and is a medium for the
distribution of germ diseases. The conditions under which it is produced
and the way in which it is handled determine largely its sanitary
value, and are of so much importance in relation to public health that
during recent years city and state boards of health have introduced
sanitary inspection and examination of milk along with the chemical
tests for detecting its adulteration. Some of the more frequent causes
of contaminated and unsound milk are: unhealthy animals, poor food and
water, unsanitary surroundings of the animals, and lack of cleanliness
and care in the handling and transporting of the milk. Outbreaks of
typhoid and scarlet fevers and other germ diseases have frequently been
traced to a contaminated milk supply.[37]
103. Certified Milk.--When milk is produced under the most sanitary
conditions, the number of bacterial bodies per cubic centimeter is
materially reduced. In order to supply high grade milk containing but
few bacteria, special precautions are taken in the care of the animals,
and in the feeding and milking, and all sources of contamination of the
milk are eliminated as far as possible. Such milk, when sold in
sterilized bottles, is commonly called "certified milk," indicating that
its purity is guaranteed by the producer and that the number of bacteria
per unit does not exceed a certain standard, as 8000 per cubic
centimeter. Ordinary market milk contains upwards of 50,000.
104. Pasteurized Milk.--In order to destroy the activity of the
bacterial organisms, milk is subjected to a temperature of 157° F. for
ten minutes or longer, which process is known as pasteurization. When
milk is heated to a temperature above 180°, it is sterilized. Below
157°, the albumin is not coagulated. By pasteurizing, milk is much
improved from a sanitary point of view, and whenever the milk supply is
of unknown purity, it should be pasteurized.[38] After the milk has been
thus treated, the same care should be exercised in keeping it protected
to prevent fresh inoculation or contamination, as though it were
unpasteurized milk. For family use milk can be pasteurized in small
amounts in the following way: Before receiving the milk, the receptacle
should be thoroughly cleaned and sterilized with boiling water or dry
heat, as in an oven. The milk is loosely covered and placed in a pan of
water, a false bottom being in the pan so as to prevent unequal heating.
The water surrounding the milk is gradually heated until a temperature
of 159° F. is registered, and the milk is kept at this temperature for
about ten minutes. It is then cooled and placed in the refrigerator.
[Illustration: FIG. 24.--PASTEURIZING MILK.]
105. Tyrotoxicon.--Tyrotoxicon is a chemical compound produced by a
ferment body which finds its way into milk when kept in unsanitary
surroundings. It induces digestion disorders similar to cholera, and
when present in large amounts, may prove fatal. It sometimes develops in
cream, ice cream, or cheese, but only when they have been kept in
unclean places or produced from infected milk.
601. Color of Milk is often taken as a guide to its purity and
richness in fat. While a yellow tinge is usually characteristic of
milks rich in fat, it is not a hard and fast rule, for frequently
light-colored milks are richer in fat than yellow-tinged ones. The
coloring material is independent of the percentage of fat, and it is not
always safe to judge the richness of milk on the basis of color.
107. Souring of Milk.--Souring of milk is due to the action of the
lactic acid organism, which finds its way into the milk through
particles of dust carried in the air or from unclean receptacles which
contain the spores of the organism.[39] When milk sours, a small amount
of sugar is changed to lactic acid which reacts upon the casein,
converting it from a soluble to an insoluble condition. When milk is
exposed to the air at a temperature of from 70° to 90° F., lactic acid
fermentation readily takes place. At a low temperature the process is
checked, and at a high temperature the organisms and spores are
destroyed. In addition to lactic acid ferments, there are large numbers
of others which develop in milk, changing the different compounds of
which milk is composed. In the processes of butter and cheese making,
these fermentation changes are controlled so as to develop the flavor
and secure the best grades of butter and cheese.
108. Use of Preservatives in Milk.--In order to check fermentation,
boric acid, formalin, and other preservatives have been proposed.
Physiologists object to their use because the quantity required to
prevent fermentation is often sufficient to have a medicinal effect.
The tendency is to use excessive amounts, which may interfere with
normal digestion of the food. Milk that is cared for under the most
sanitary conditions has a higher dietetic value and is much to be
preferred to that which has been kept sweet by the use of preservatives.