Sugars form rings that involve the aldehyde or ketone group. The ring forming and opening again is reversible unless the hemiacetal or ketal hydroxyl group has become involved in another link. Rings that are locked have no aldehyde or ketone group to react (unless there are several rings, and one can open) and are non-reducing sugars. There is one glucose ring at the end of each chain in starch and cellulose, but its effect is too small to produce a positive test.
When only one sugar is present and it is a reducing sugar, one of these old tests may still be an adequate and inexpensive method for measuring a sugar concentration. For example, a flow system with a colorimeter may be used for on-line detection. However, enzymatic methods that use the specificity of the enzyme for one sugar are very common. Even better are chromatographic procedures that will measure the different sugars in a mixture.