Skip to content

Food Test – Glucose & Starch

HOW DOES IT WORK?

Experiment 1: To test for the presence of glucose.
Time allocation: 45 Min

Glucose is an essential basic carbohydrate that provides energy for the body to function well. It is easily absorbed and stored by the body. If not used up by the cells, it can be stored as starch in plants and glycogen in animals. Glucose is made during photosynthesis from water and carbon dioxide, using energy from sunlight. Different food types containing glucose can be tested positively using the Benedict’s solution or mixture of Fehling’s A and B solutions. Substances containing glucose will change to a brick-red / orange colour when tested with Benedict’s solution.  Glucose is also known as a reducing sugar as the copper (II) ions in the Benedict’s solution are reduced to Copper (I) ions, which causes the colour change. It has the same effect as Fehling’s A and B solution.

 

Experiment 2: Test for the present of starch in the given food types
Time allocation: 45 Min

Starch is a polysaccharide which is found in cereals (e.g. Rice), grain (e.g. Maize, corn) and in tubers (e.g. potato). Plants store glucose as starch. Iodine test for starch is the method used to analyse the presence of starch in the given food material. Iodine attaches itself to the starch molecules. When the iodine solution is mixed with starch it turns from reddish-brown to blue-black / purple. Iodine solution penetrates into the starch most easily when the cell walls have been destroyed and it reacts most readily when the starch grains are swollen.

EXPERIMENT 1: TO TEST FOR THE PRESENCE OF GLUCOSE.

AIM                                                                                                

    Time allocation: 45 Min

To test for the presence of glucose.

HYPOTHESIS

Substances containing glucose will change to a brick-red / orange colour when tested with Benedict’s solution.

MATERIALS

  • Glucose Solution 250 ml
  • Sucrose solution 250 ml
  • Benedict’s solution 25 ml dropper bottle
  • 5 x Test tube
  • Disposable pipette
  • Distilled water 100 ml
  • Burner and fuel cartridge
  • Gauze wire
  • Glass beaker 600 ml
Supplied by teacher:

Food samples such as table sugar, fruits, cereals, potatoes etc.

METHOD:

  1. Half fill the beaker with water and bring it to boiling point.
  2. Label test tubes as A, B, C, D and E.
  3. Quarter fill test tube A with the glucose solution.
  4. Quarter fill test tube B with the sucrose solution.
  5. Quarter fill the rest of the test tubes with distilled water and add the following: crushed apple (test tube C), cereal (test tube D), and crushed potato (test tube E).
  6. Add a few drops of Benedict’s solution to each test tube with the pipette.
  7. Observe and record the colour in the table below.
  8. Place the test tubes into the boiling water bath, lower the flame.
  9. Wait for 5 minutes then observe and record the colour of the solutions.

CONCLUSION:

On heating the change of colour from blue to green to yellow and orange/ brick red indicates the presence of glucose.

EXPERIMENT 2: TO TEST IF STARCH IS PRESENT IN CERTAIN TYPES OF FOOD

AIM                                                                                                   

Time allocation: 45 Min

To find out if starch is present in the given types of food

MATERIALS

  • 5 x test tubes
  • Test tube rack
  • 5 x Pipette
  • Spatula
  • Iodine solution 100ml
  • Distilled water
  • Starch powder 50g
  • Grated potato
  • Boiled rice
  • Cooked oats
  • Unknown substance e.g. Glucose powder or table sugar

METHOD:

  1. Label each test tube A, B, C, D and E.
  2. Put a small amount from each of your food stuffs in test tube A, B, C and D.
  3. For your control put half a spatula-full of starch powder in test tube E. Where necessary add a little bit of distilled water in the test tube to make sure that your sample is moist which will enhance the reaction with the Iodine solution.
  4. Use the dropper to put one drop of the iodine solution on each of the types of food.
  5. Leave the experiment for 30 seconds without disturbing the test tubes.
  6. Observe the results: Observe the colour of each type of food and compare it to the colour change of starch after adding iodine solution.

OBSERVATION:

Complete a table similar to the table below by doing the following .Write the name of the type of food you are testing in the space provided. If the colour changes, like that of starch write (+); if the colour shows slight change, write (±). If the there is no colour change write (-) in the appropriate spaces.

CONCLUSION:

Items containing starch turned blue-black, confirming the presence of starch whereas samples not containing starch didn’t change colour at all.

PRECAUTIONS

  • Do not leave the bottle containing iodine open because iodine (I2) sublimes (changes from solid to a vapour once exposed to air).
  • Make sure that the iodine solution is not exposed to direct sunlight; hence you are instructed to keep it in an amber bottle.
  • Make sure that there is no spill over of the liquid from any of the test tubes.
  • Avoid touching iodine crystals and iodine solution with your bare hands.
  • Do not include any processed food among the type of food you are testing.
    • Examples: processed food; bread, yogurt, cakes, Vienna’s etc.
  • Ensure that it is safe to use a flame.
  • When boiling ensure that the mouth of the test tubes points away from people.
  • Use a test tube holder and wear goggles.
  • Always make sure that all chemicals are handled with care.
  • Do not try and eat any of the food items after or before they are tested in the lab environment, ensure these food items are disposed of correctly.