Human Foods and Their Nutritive Value | Page 8

acids are not to be considered as nutrients, but as food
adjuncts, increasing palatability and promoting digestion.
15. Essential Oils.--Essential or volatile oils differ from fats, or fixed
oils, in chemical composition and physical properties.[6] The essential
oils are readily volatilized, leaving no permanent residue, while the
fixed fats are practically non-volatile. Various essential oils are present
in small amounts in nearly all vegetable food materials, and the
characteristic flavor of many fruits is due to them. It is these
compounds which are used for flavoring purposes, as discussed in
Chapter IV. The amount in a food material is very small, usually only a
few hundredths of a per cent. The essential oils have no direct food
value, but indirectly, like the organic acids, they assist in promoting
favorable digestive action, and are also valuable because they impart a
pleasant taste. Through poor methods of cooking and preparation, the
essential oils are readily lost from some foods.

16. Mixed Compounds.--Food materials frequently contain compounds
which do not naturally fall into the five groups
mentioned,--carbohydrates, pectose substances, fats, organic acids,
and essential oils. The amount of such compounds is small, and they
are classed as miscellaneous or mixed non-nitrogenous 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 composition,
but differing widely in structural composition. As a class, the proteins
contain about 16 per cent of nitrogen, 52 per cent of carbon, from 6 to
7 per cent of hydrogen, 22 per cent of oxygen, and less than 2 per cent
of sulphur. These elements are combined in a great variety of ways,
forming various groups or radicals. In studying the protein molecule a
large number of derivative products have been observed, as amid
radicals, various hydrocarbons, fatty acids, and carbohydrate-like
bodies.[8] It would appear that in the chemical composition of the
proteins there are all the constituents, or simpler products, of the
non-nitrogenous compounds, and these are in chemical combination
with amid radicals and nitrogen in various forms. The nitrogen of many
proteids appears to be present in more than one form or radical. The
proteids take an important part in life processes. They are found more
extensively in animal than in plant bodies. The protoplasm of both the
plant and animal cell is composed mainly of protein.
Proteids are divided into various subdivisions, as albumins, globulins,
albuminates, proteoses and peptones, and insoluble proteids. In plant
and animal foods a large amount of the protein is present as insoluble
proteids; that is, they are not dissolved by solvents, as water and dilute
salt solution. The albumins are soluble in water and coagulated by heat
at a temperature of 157° to