Food test 2: Non reducing sugars and starch

Step 1

Proceed with this test only after a negative benedicts test for reducing sugars on the food

Step 2

Half fill a 400cm3 beaker with tap water to act as a water bath

Use a heatproof mat, tripod, gauze and bunsen burner to heat the water to a boiling point 

Step 3

Label the top of your test tubes

Step 4

Use a pasteur pipette to dispense 2cm3 of a fresh sample into a test tube (or finely chopped/ ground food and distilled water)

Step 5

Use a dropper pipette to add 10 drops of dilute hydrochloric acid to the sample

Step 6

Use a test tube holder to transfer the test solution to the water bath

You can save time by having up to five test tubes in the water bath 

Heat to boiling point for two minutes 

This is the stage when acid hydrolysis may occur 

Step 7 

Cool the test tube in cold water 

Step 8 

Sodium hydrogencarbonate powder is needed to make the solution alkaline 

With a spatula add sodium hydrogencarbonate to the solution until the fizzing stops 

The solution must be alkaline by using a pasteur pipette to take a sample and adding it to universal indicator paper on a tile 

Compare the colour with that in the chart 

Step 9

Add 10 drops of Benedicts reagent to the test solution

Step 10

Heat the test solution to boiling point in a water bath and continue heating for two minutes

Step 11

Observe the colour changes in the test tubes over two minutes of heating at boiling point

Record your observations

Sucrose is a non reducing sugar that occurs naturally in plants 

Sucrose is a disaccharide consisting of glucose and fructose units linked by a glycosidic bond 

It is not a reducing sugar because the reducing groups of glucose and fructose are tied up in the glycosidic bond 

When a sucrose solution is acidified and heated the hydrogen ions of the acid catalyse the hydrolysis of sucrose to glucose and fructose which are both reducing sugars and cause the reduction reaction