What are reducing sugars?
Sugars containing either aldehyde (aldoses), ketone
(ketoses) or hemiacetal groups can be oxidized and are
classified as reducing sugars. The presence of reducible
sugars can easily be proved by test reagents (Tollens’
reagent, Fehling’s reagent or Benedict’s
reagent).
These tests are commonly used to detect aldehyde functions
which can be oxidized. Under basic conditions, the interconversion
of ketoses and aldoses is facilitated. Fructose, which
is a ketose, can also be oxidized and is therefore a
reducing sugar.
Precisely, the oxidation of reducing sugars is only
possible when they are in the open and not in the cyclic
form. Whereas any cyclic sugar containing a hemiacetal
can open up and act as a reducing agent, this is not
possible with sugars being acetals (e.g. sucrose). Examples
of reducing sugars are: glucose, fructose, maltose and
lactose. The content of reducing sugars in honey is
76.65% (range: 61.40 – 83.70%).
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