FEEDS

157

number of dextrins are formed as intermediate products. Some
of these (erythrodextrins) give a red color with iodine while
others (acroodextrins) give no color. Under the influence of the
acid these dextrins finally yield maltose, a sugar having the same
molecular weight as sucrose.
Polarimetric methods are not well suited to the determination
of the carbohydrates in feeds, because of the relatively small
amounts usually occurring in such materials. Greater reliance
is placed upon chemical methods, such as those here to be
described.
Reducing Sugars.—"Reducing sugars'^7 are those that have
the power of reducing the copper from an alkaline solution
of copper tartrate to cuprous oxide, Cu_2O. Dextrose, levulose,
maltose and invert sugar are examples of common reducing
sugars while sucrose is a non-reducing sugar.
It has already been stated that reducing sugars may either
be present in the original material or they may be formed as a
result of hydrolysis of other carbohydrates, such as starch or
sucrose. Therefore the determination of original reducing
sugars may conveniently be combined with that of sucrose.
Calculation of Reducing Sugars from the Weight of Cuprous
Oxide.—When the weight of cuprous oxide is used as a basis
for calculating weights of sugars, the method of reducing and
precipitating must be definitely standardized as the formation
of cuprous oxide does not proceed according to an absolutely
definite and constant reaction, depending not only upon the kinds
and amounts of reducing sugars present but also upon the tem-
perature and concentration of the solution and upon the length
of time it is heated. Tables have been prepared for different
sugars, giving the amount of cuprous oxide reduced by each
under specified conditions. These are given in Table VII,
pages 160 and 161.
Methods for Determining the Reduced Cuprous Oxide.—A
number of methods are in use for the determination of the
cuprous oxide reduced by the sugars. Three of these will be
described.
In method (a) the solution is filtered through a Gooch crucible,
the cuprous oxide then being dried and weighed as such or
ignited and weighed as cupric oxide. Direct weighing is suitable