Testing for reducing sugars

All sugars can be grouped into two categories.

These categories are reducing sugars and non-reducing sugars:

• Reducing sugars include all monosaccharides and some disaccharides such as maltose and lactose.
• Non-reducing sugars include some disaccharides such as sucrose and all polysaccharides.

Determining the concentration of reducing sugars

A positive result will form a brick red precipitate, however the colour seen is a mixture of the precipitate and the blue Benedict's reagent.

The concentration of reducing sugar determines the colour of this mixture:

• Blue - This indicates no reducing sugar is present.
• Green - This indicates a low concentration.
• Orange - This indicates a medium concentration.
• Brick-red - This indicates a high concentration.

This allows you to compare the concentration of reducing sugar between different samples.

Testing for non-reducing sugars

Non-reducing sugars give a negative result (blue solution) for the normal reducing sugars test. To test for these types of sugars, you must first hydrolyse them into their monosaccharide components before we can do the rest of the test.

Steps to find out whether a sample contains a non-reducing sugar:

1. Carry out the test for reducing sugars, and if the result is negative (turns blue), continue with the next steps.
2. Add 2 cm³ of the food sample to 2 cm³ of dilute hydrochloric acid.
3. Heat the mixture in a gently boiling water bath for 5 minutes (the acid will hydrolyse disaccharides into monosaccharides).
4. Neutralise the mixture by adding sodium hydrogencarbonate solution.
5. Retest this mixture using the test for reducing sugars.
6. If non-reducing sugars were present at the start, the mixture will now change from a blue solution to a brick red precipitate.

Testing for starch