156

QUANTITATIVE AGRICULTURAL ANALYSIS

f ! '.&

j> 1

I

*• «»4

f '51

.1

digestible. Sugars and dextrins are the most important of these.
Starch is next in importance as it is easily made soluble in
digestive processes by hydrolytic action/ Other groups such as
the hemicelluloses (examples of which are the pentosans, galac-
tans and pectins) are made soluble with more difficulty and they
are therefore less valuable as foods. That portion of the carbo-
hydrates which does not yield soluble forms on hydrolysis is
practically worthless for feeding purposes. It is chiefly cellulose
and from the analysis it is reported as "crude fiber. "

The pentosans are widely distributed in the vegetable kingdom,
being present in the seeds, roots and leaves of all plants. One
of the most common of the pentosans is gum Arabic, which occurs
intimately associated with the other plant constituents. The
galactans also are widely distributed in plants and they occur
chemically combined with the pentosans in the plant. Agar-agar
is one of the most common of the galactans. It yields galactose
upon hydrolysis, while pentosans yield pentose sugars when
similarly treated.

Analytical Methods. — Carbohydrates in foods and feeds are
determined (a) by direct acid hydrolysis and subsequent deter-
mination of the reducing sugar thus formed, (fc) by hydrolysis of
starch by diastase, thus forming dextrins, maltose and glucose,
or (c) by difference, deducting from 100 the sum of the per cents
of crude protein, crude fat, ash, crude fiber and moisture. This
difference is reported as "nitrogen-free extract.77

True starch cannot be determined accurately by direct
hydrolysis with acids because other polysaccharides, such as
gums, pentosans and galactans, are hydrolyzed at the same time,
yielding reducing sugars which are determined along with those
that are derived from starch. The type reaction of hydrolysis is
as represented in the equation :

(C6H1005)n

nC6H120(5.

A separation from these hydrolyzable materials may be
made by first digesting with the enzyme diastase, from malt
extract, then washing out the soluble carbohydrates and hy drofyz-
ing them to glucose by boiling with dilute acids. By this pro-
cedure only the true starches are affected by the enzyme and a
series of compounds of simpler structure are formed. A large