The test for starch is easier, as it does not involve heating. You simply add iodine solution to a sample of the food. If there is starch present, a blue-black colour is obtained (Figure 4.7). If there is no starch, the iodine solution remains orange-brown.
Activity 4.1
Testing foods for sugars Skills AO3.1 Using techniques, apparatus and materials AO3.3 Observing, measuring and recording
Wear eye protection if available. If possible, heat the tubes using a water bath. If you have to heat directly over a Bunsen flame, use a test-tube holder and point the opening of the tube away from people. Take care if using a sharp blade to cut the food. All simple sugars, and some complex sugars such as maltose, are reducing sugars. This means that they will react with a blue liquid called Benedict’s solution. We can use this reaction to find out if a food or other substance contains a reducing sugar. 1 Draw a results chart. Food
Colour with Benedict’s solution
Simple sugar present
Figure 4.7 The black colour shows that the potato contains starch.
Activity 4.2
2 Cut or grind a little of the food into very small pieces. Put these into a test tube. Add some water, and shake it up to try to dissolve it. 3 Add some Benedict’s solution. Benedict’s solution is blue, because it contains copper salts. 4 Heat the tube to about 80 °C, in a water bath. If there is reducing sugar in the food, a brickred precipitate will form. 5 Record your result in your results chart. If the Benedict’s solution does not change colour, do not write ‘no change’. Write down the actual colour that you see – for example, blue. Then write down your conclusion from the result of the test. This test works because the reducing sugar reduces the blue copper salts to a red compound.
Testing foods for starch Skills AO3.1 Using techniques, apparatus and materials AO3.3 Observing, measuring and recording
There is no need to dissolve the food for this test. 1 Draw a results chart. 2 Put a small piece of the food onto a white tile. 3 Add a drop or two of iodine solution. Iodine solution is brown, but it turns blue-black if there is starch in the food. Record each of your results and conclusions.
Question
A1 How could you test a solution to see if it contained iodine?
Original material © Cambridge University Press 2014
Chapter 4: The chemicals of life
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