What are Reducing Sugars?

A sugar that serves as a reducing agent due to its free aldehyde or ketone functional groups in its molecular structure. Disaccharides are hydrolysed to their constituent monosaccharides when boiled in dilute hydrochloric acid.

The monosaccharide products of hydrolysis are reducing sugars i.e. have the aldehyde functional group and can reduce copper in the presence of alkali producing the colour changes. Examples are glucose, fructose, lactose, arabinose and maltose.

Biochemical test for Reducing Sugars: Benedict’s test

The principal reagent in Benedict’s Test for Reducing Sugars is Benedict’s Solution which contains:

  • Copper(II) Sulphate
  • Sodium Hydrogen Carbonate (NaHCO₃)
  • Sodium Citrate

Equipment:

  • 2cm3 of food sample (must be in liquidated form)
  • 2cm3 of benedict reagent (per sample)
  • Beaker
  • Clean, grease free test tube
  • Kettle/hot water source
  • mortar and pestle

Method:

A liquid food sample does not need prior preparation except dilution if viscous or concentrated.

For a solid sample prepare a test solution by crushing the food in a mortar and pestle, and adding a moderate amount of distilled water. Decant the suspension to remove large particles. Use the decanted liquid as the test solution.

  1. Add 2 cm3 of the sample solution to a test tube.
  2. Add 1 cm3 of dilute hydrochloric acid and boil for one minute.
  3. Allow the tube to cool and then neutralize the acid with Sodium Hydrogen Carbonate (NaHCO₃).
  4. Leave the test tube in a boiling water bath for about 5 minutes, or until the colour of the mixture does not change.
  5. Observe the colour changes during that time as well as the final colour.
  6. To prepare a control, repeat the steps using 2 cm3 of distilled water instead of sample solution.

Results:

Observation Interpretation
No Colour Change (Blue) No non-reducing sugars present
Green Trace amounts of non-reducing sugars present
Yellow Low amounts of reducing sugars present
Orange Moderate amounts of reducing sugars present
Brick Red Large amounts of non-reducing sugars present

 

The blue copper(II) ions from copper(II) sulphate in Benedict’s Reagent are reduced to red copper(I) ions by the aldehyde groups in the reducing sugars. This accounts for the colour changes observed.

The red copper(I) oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed.

As the concentration of reducing sugar increases, the nearer the final colour is to brick-red and the greater the precipitate formed.