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 deg. to 200 deg.
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 KOeNIG.)]

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 br