FEEDS

165

acid, after most of the fat has been removed, the starch and some
of the pentosans are hydrolyzed to reducing sugars. It is due to
the pentosans that these results, considered as starch, are too
high when compared with the amount obtained by the diastase
method just described.
Determination of Starch: Direct Acid Hydrolysis.—Stir a quantity of
the sample, representing 2.5 to 3 gm of the dry material, in a beaker with
50 cc of cold water for an hour. Transfer to a filter and wash with 250 cc
of cold water. Heat the insoluble residue with 200 cc of water and 20 cc
of 25-per cent hydrochloric acid (specific gravity 1.125) and boil for 2.5 hours,
in a flask provided with a reflux condenser. Cool, rinse into a 250-cc volumet-
ric flask and nearly neutralize with sodium hydroxide, using methyl-orange.
Dilute to 250 cc mix and filter, and determine the dextrose in 50 cc of the
filtrate as directed on page 159, omitting the addition of water just before
mixing with Fehling's solution. The weight of the dextrose obtained
multiplied by 0.93 gives the weight of starch.
The factor 0.90 is the theoretical ratio between starch and glucose but,
according to Noyes and other investigators,^1 the factor 0.93 more nearly
represents the analytical ratio.
Arabin, Xylan and the Pentosans.—These are compounds
of unknown constitution but they all yield pentoses (aldehyde
sugars containing five carbon atoms) upon hydrolysis under the
influence of hydrochloric acid. Arabin and xylan are constitu-
ents of the plant gums. Arabin may be obtained from gum
Arabic, while xylan is found in many woods, in straw and in corn
cobs. Lignin is one of the most common of the pentosans. It
occurs with cellulose in wood, straw, bran and similar materials.
It will thus be seen that all of these substances will be probable
constituents of the rougher materials of the sort to be found in
animal feeds.
The pentoses which are formed by hydrolysis of the com-
pounds already mentioned are further converted into the alde-
hyde furfural, upon distillation with hydrochloric acid. The
type reaction is as follows:
HO   H   OHH	H   H

H—C
H   OHH   OHH
A pentose
^1 /. Am. Chem. Soc., 26, 266 (1904).

0-
Furfural

= C—C = 0 + 3H_20.
H