Student Question

What are the steps for testing non-reducing sugar?

 

Expert Answers

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Non-reducing sugars do not have the aldehyde group and so they cannot reduce copper sulphate like reducing sugars would do when tested with the Benedict’s reagent.

To test for non-reducing sugar, therefore, an indirect test will have to be conducted by first hydrolysing (breaking down) the non-reducing sugar to its constituent monosaccharides (reducing sugars). The sample can then be tested for the presence of reducing sugars with Benedict’s reagent.

The steps to take would be as follows. First, the reducing sugar test would need to be carried out on the sample to ensure that a reducing sugar that may confuse the result of the test is not already present in the sample (see the source link below for the reducing sugar test with Benedict’s reagent). The reducing sugar test should be negative if reducing sugar is not already present in the sample.

The next step is to hydrolyse the sample into reducing sugars. Add 1cm cubed of dilute hydrochloric acid to 2cm cubed of the sample. Mix and boil the mixture for about 2 – 3 minutes. Allow the mixture to cool and then neutralise it by adding sodium hydrogen carbonate until the solution is slightly alkaline. Benedict’s reagent will only work in an alkaline environment, so check with a blue litmus paper to detect when the solution turns alkaline.

Perform the reducing sugar test on the mixture by adding 2cm cubed of Benedict’s reagent. The solution turns clear blue. Insert the test tube in boiling water for about five minutes. If the solution remains clear blue, it means the solution does not contain any reducing sugars.

An orange-red precipitate forms if reducing sugar is present. You can then deduce that non-reducing sugar was originally present because a negative result was obtained the first time the sample was tested for reducing sugar but a positive result is now obtained when the non-reducing sugar was hydrolysed to reducing sugar after hydrochloric acid was added.

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