Human Foods and Their Nutritive Value | Page 7

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