nous compounds. Some of
them may impart a negative value to the food, and there are others which
have all the characteristics, as far as general composition is
concerned, of the non-nitrogenous compounds, but contain nitrogen,
although as a secondary rather than an essential constituent.

17. Nutritive Value of Non-nitrogenous Compounds.--The non-nitrogenous
compounds, taken as a class, are incapable alone of sustaining life,
because they do not contain any nitrogen, and this is necessary for
producing proteid material in the animal body. They are valuable for
the production of heat and energy, and when associated with the
nitrogenous compounds, are capable of forming non-nitrogenous reserve
tissue. It is equally impossible to sustain life for any prolonged
period with the nitrogenous compounds alone. It is when these two
classes are properly blended and naturally united in food materials that
their main value is secured. For nutrition purposes they are mutually
related and dependent. Some food materials contain the nitrogenous and
non-nitrogenous compounds blended in such proportion as to enable one
food alone to practically sustain life, while in other cases it is
necessary, in order to secure the best results in the feeding of animals
and men, to combine different foods varying in their content of these
two classes of compounds.[7]

NITROGENOUS COMPOUNDS

18. General Composition.--The nitrogenous compounds are more complex
in composition than the non-nitrogenous. They are composed of a larger
number of elements, united in different ways so as to form a much more
complex molecular structure. Foods contain numerous nitrogenous organic
compounds, which, for purposes of study, are divided into four
divisions,--proteids, albuminoids, amids, and alkaloids. In addition to
these, there are other nitrogenous compounds which do not naturally fall
into any one of the four divisions.

[Illustration: FIG. 4.--APPARATUS USED FOR DETERMINING
TOTAL NITROGEN AND CRUDE PROTEIN IN FOODS.

The material is digested in the flask (3) with sulphuric acid and the
organic nitrogen converted into ammonium sulphate, which is later
liberated and distilled at 1, and the ammonia neutralized with standard
acid (2).]

Also in some foods there are small amounts of nitrogen in mineral forms,
as nitrates and nitrites.

19. Protein.--The term "protein" is applied to a large class of
nitrogenous compounds resembling each other in general compo