Food Test for Reducing Sugar (Benedict's Test)

Aim

To test a food sample for the presence of reducing sugar, and to compare the relative amount of reducing sugar between food samples, using Benedict's solution.

What it tests for

The Benedict's test detects reducing sugars, such as glucose and fructose, in food. Reducing sugars can donate electrons to reduce copper(II) ions in Benedict's solution to copper(I) oxide, which is why the test requires heating to make the reaction happen.

Variables

• Manipulated variable: the type of food sample tested for a comparison, for example glucose solution, ripe banana extract, and milk.
• Responding variable: the final colour of the mixture after heating, read against a colour scale from blue through green, yellow and orange to brick-red.
• Controlled variables: the volume of food extract, the volume of Benedict's solution, the temperature of the water bath, and the heating time.

Materials and apparatus

• Food samples or extracts to be tested
• Benedict's solution
• Distilled water for the control and for preparing extracts
• Test tubes and a test-tube rack
• A beaker for the water bath, with a Bunsen burner, tripod stand, wire gauze and heat-resistant mat
• A measuring cylinder or syringe, a dropper, and a test-tube holder
• A mortar and pestle and a filter funnel with filter paper for solid foods

Reagent and method

1. Prepare a food extract by crushing or dissolving the food sample in water, then filter if necessary to obtain a clear liquid.
2. Add about 2 cm3 of the food extract into a clean test tube.
3. Add an equal volume of Benedict's solution (blue in colour) to the test tube.
4. Place the test tube in a water bath of boiling water for about 5 minutes.
5. Observe and record the colour change as the mixture heats.

Positive result

If reducing sugar is present, the mixture changes from blue, through green, yellow, and orange, to a brick-red precipitate, depending on how much reducing sugar is present. A higher concentration of reducing sugar produces a more complete colour change to brick-red.

If no reducing sugar is present, the solution stays blue.

Expected results

The final colour depends on how much reducing sugar the sample contains, so the results can be tabulated and ranked.

Inference and conclusion

A colour change away from blue shows that reducing sugar is present, because the sugar donates electrons that reduce the blue copper(II) ions in Benedict's solution to insoluble red copper(I) oxide. The final colour indicates the relative amount: a sample that turns brick-red contains more reducing sugar than one that only turns green or yellow, while a sample that stays blue contains none.

The test is therefore semi-quantitative, it ranks samples by colour but does not give an exact concentration. Comparing the final colours allows the food samples to be ordered by their reducing-sugar content.

Paper 3-style questions

Question 1 (hypothesis). A student wants to find out whether a riper banana contains more reducing sugar than an unripe one. State a suitable hypothesis.

Model answer. A riper banana contains more reducing sugar, so its extract produces a colour closer to brick-red with Benedict's solution than the extract of an unripe banana.

Question 2 (variables). In the same investigation, state the manipulated, responding and one controlled variable. Model answer. Manipulated variable: the ripeness of the banana.

Responding variable: the final colour of the Benedict's mixture after heating. Controlled variable: the volume of Benedict's solution (also the volume of extract, the heating time, and the temperature).

Question 3 (recording results). Design a table to record the results for three bananas of different ripeness. Model answer. A table with the headings Ripeness of banana, Colour of mixture after heating and Inference on amount of reducing sugar, with one row per banana, allows the results to be compared fairly.

Question 4 (inference). The unripe banana turned the solution green while the ripe banana turned it brick-red. What can you infer?

Model answer. The ripe banana contains more reducing sugar than the unripe banana, because a colour nearer brick-red shows that more copper(II) ions were reduced, which happens when more reducing sugar is present. This supports the hypothesis.

Safety

• Wear safety goggles, because Benedict's solution is an irritant and hot liquid can splash.
• Use a test-tube holder to move hot test tubes, and point the mouth of the tube away from yourself and others while heating.
• Heat in a water bath rather than directly in a flame, to avoid sudden boiling and splashing.
• Handle the Bunsen burner carefully and turn it off when it is not in use; place hot apparatus on a heat-resistant mat.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions