2.1.6—Non-reducing sugar hydrolysis test

A non-reducing sugar is detected indirectly: acid hydrolysis breaks its glycosidic bonds into reducing monosaccharides, which can then give a positive Benedict’s result. The initial test is essential because it shows whether reducing sugar was already present.

1. Test the original sample with Benedict’s reagent first. Continue only if the initial result is negative for reducing sugar.
2. Add dilute hydrochloric acid to a fresh portion of sample and heat it in a boiling water bath.
3. Neutralise the acid with sodium hydrogencarbonate, checking with a suitable indicator; add a little extra so the mixture is slightly alkaline.
4. Repeat Benedict’s test: add Benedict’s reagent and heat in a boiling water bath.
5. A new orange-to-red precipitate supports the presence of a non-reducing sugar that was hydrolysed; without the initial negative control, the conclusion is inconclusive.

Acid and heat hydrolyse glycosidic bonds. The resulting monosaccharides have functional groups that can donate electrons to copper(II) ions, so heating with Benedict’s reagent can reduce copper(II) to coloured copper(I) oxide. Neutralisation is required because Benedict’s reaction works in alkaline conditions; excess acid would prevent a valid repeat test.

A negative first Benedict’s test does not mean the sample contains no sugar; it only shows that no reducing sugar was detected before hydrolysis. The positive result after hydrolysis is evidence for a previously non-reducing sugar only when the pre-hydrolysis control, neutralisation and matched Benedict procedure are all valid.