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 deg. 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 al