r cane, sugar beet, and sugar
maple. They are divided into two large classes: the sucrose group and
the dextrose group, the latter being produced from sucrose, starch, and
other carbohydrates by inversion and allied chemical changes. Because of
the importance of sugar in the dietary, Chapter V is devoted to the
subject.

11. Pectose Substances are jelly-like bodies found in fruits and
vegetables. They are closely related in chemical composition to the
carbohydrates, into which form they are changed during digestion; and in
nutrition they serve practically the same function. In the early stages
of growth the pectin bodies are combined with organic acids, forming
insoluble compounds, as the pectin in green apples. During the ripening
of fruit and the cooking of vegetables, the pectin is changed to a more
soluble and digestible condition. In food analysis, the pectin is
usually included with the carbohydrates.

12. Nitrogen-free-extract.--In discussing the composition of foods,
the carbohydrates other then cellulose, as starch, sugar, and pectin,
are grouped under the name of nitrogen-free-extract. Methods of
chemical analysis have not yet been sufficiently perfected to
enable accurate and rapid determination to be made of all these
individual carbohydrates, and hence they are grouped together as
nitrogen-free-extract. As the name indicates, they are compounds which
contain no nitrogen, and are extractives in the sense that they are
soluble in dilute acid and alkaline solutions. The nitrogen-free-extract
is determined indirectly, that is, by the method of difference. All the
other constituents of a food, as water, ash, crude fiber (cellulose),
crude protein, and ether extract, are determined; the total is
subtracted from 100, and the difference is nitrogen-free-extract. In
studying the nutritive value of foods, particular attention should be
given to the nature of the nitrogen-free-extract, as in some instances
it is composed of sugar and in others of starch, pectin, or pentosan
(gum sugars). While all these compounds have practically the same fuel
value, they differ in composition, structure, and the way in which they
are acted upon by chemicals and digestive ferments.[1]

[Illustration: FIG. 3.--APPARATUS USED FOR THE DETERMINATION OF
FAT.]

13. Fat.--Fat is found mainly in the seeds of plants, but to some
extent in the leaves and stems. It differs from starch in containing
more carbon and less oxygen. In starch the