the body as an amid, which in turn undergoes
oxidation and nitrification, and is converted into nitrites, nitrates,
and ammonium salts. These forms of nitrogen are then ready to begin
again in plant and animal bodies the same cycle of changes. Thus it is
that nitrogen may enter a number of times into the composition of plant
and animal tissues. Nature is very economical in her use of this
element.[5]

CHAPTER II

CHANGES IN COMPOSITION OF FOODS DURING COOKING AND PREPARATION

26. Raw and Cooked Foods Compared.--Raw and cooked foods differ in
chemical composition mainly in the content of water. The amount of
nutrients on a dry matter basis is practically the same, but the
structural composition is affected by cooking, and hence it is that a
food prepared for the table often differs appreciably from the raw
material. Cooked meat, for example, has not the same percentage and
structural composition as raw meat, although the difference in nutritive
value between a given weight of each is not large. During cooking, foods
are acted upon chemically, physically, and bacteriologically, and it is
usually the joint action of these three agencies that brings about the
desirable changes incident to their preparation for the table.

27. Chemical Changes during Cooking.--Each of the chemical compounds
of which foods are composed is influenced to a greater or less extent by
heat and modified in composition. The chemistry of cooking is mainly a
study of the chemical changes that take place when compounds, as
cellulose, starch, sugar, pectin, fat, and the various proteids, are
subjected to the joint action of heat, moisture, air, and ferments. The
changes which affect the cellulose are physical rather than chemical. A
slight hydration of the cellular tissue, however, does take place. In
human foods cellulose is not found to any appreciable extent. Many
vegetables, as potatoes, which are apparently composed of cellular
substances, contain but little true cellulose. Starch, as previously
stated, undergoes hydration in the presence of water, and, at a
temperature of 120 deg. C., is converted into dextrine. At a higher
temperature disintegration of the starch molecule takes place, with the
formation of carbon monoxid, carbon dioxid, and water, and the
production of a residue richer in carbon than is starch. On account of
the moisture, the temperature in many cooking operations is not
sufficiently high for changes other than hydrat