ion 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 KOeNIG.)]

28.