164

QUANTITATIVE AGRICULTURAL ANALYSIS

After hydrolysis of the starch by the enzyme, the resulting
maltose and dextrin may be further hydrolyzed to dextrose
under the influence of acid as follows :

''Hi
'- f ^fil

Cl2H220n   +  HoO
Maltose	Dextrose
The dextrose so produced is then determined by methods already
described for reducing sugars.
The work should be so planned that the determination can be
carried through without delay. If an interruption is necessary
after the completion of the enzyme action, fermentation should
be prevented by the addition of 0.2 gm of salicylic acid.
Determination of Starch: Diastase Method. — Prepare malt extract as
follows :
Grind about 10 gm of malt and add to it 200 cc of water. Allow it to
digest at the temperature of the room for about three hours, with occasional
shaking. Filter. Determine the weight of reducing sugars in 40 cc of the
extract, after treatment with hydrochloric acid as described below for the
feed.
Extract on a hardened filter 5 gm of the dry material, very finely ground,
with five successive portions of 10 cc of ether. Wash with 150 cc of 10-per
cent alcohol and then with a little 95-per cent alcohol. This removes all
fatty material and sugars. Place the residue in a beaker with 50 cc of
water, immerse the beaker in boiling water and stir constantly for 15 minutes
or until all the starch is gelatinized. Cool to 55°, add 20 cc of malt extract
and maintain at this temperature by placing in a water bath for an hour.
Heat again to boiling for a few minutes, cool to 55°, add 20 cc of malt extract
and maintain at this temperature for an hour or until particles of the residue
treated with iodine show no blue color upon microscopic examination.
Cool, make up directly to 250 cc and filter through a dry paper. Place
200 cc of the filtrate in a flask with 20 cc of 25 per cent hydrochloric acid
(specific gravity 1.125). Connect with a reflux condenser and heat in a boil-
ing water bath for 2.5 hours. Cool, nearly neutralize with sodium hydroxide
solution, finish the neutralization with sodium carbonate solution (using
methyl orange) and make up to 500 cc in a volumetric flask. Mix the solution
well, pour through a dry filter and determine the dextrose in 50 cc as
directed on page 159. Conduct a blank determination upon 40 cc of the
malt extract by hydrolyzing with acid, with subsequent determination of
copper reduced, and correct the weight of copper reduced by the feed
solution accordingly. The weight of the dextrose obtained multiplied by
0.93 gives the weight of starch. Calculate the per cent.
Direct Acid Hydrolysis, — Members of the starch group com-
prised in the " nitrogen free extract" are often determined by
direct acid hydrolysis. When the mixed feed is boiled with