Human Foods and Their Nutritive Value | Page 4

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.