Food Tests

• Sugars are found in all sorts of foods such as biscuits, cereal and bread. There are two types of sugars: non-reducing and reducing. You can test for reducing sugars in food using the Benedict's test.
• The Benedict's test is where you:

  1. Prepare a food sample and transfer 5cm^3 to a test tube.
  2. Prepare a water bath so that it's set to 75 degrees C.
  3. Add some Benedict's solution to the test tube (about 10 drops) using a pipette.
  4. Place the test tube in the water bath using a test tube holder and leave it in there for 5 minutes. Make sure the tube is pointing away from you.
  5. If the food sample contains a reducing sugar, the solution in the test tube will change from the normal blue colour to green, yellow or brick-red - depending on how much sugar is in the food.
• You can check food samples for the presence of starch. Foods like pasta, rice and potatoes contain a lot of starch. You do this by:

  1. Make a food sample and transfer 5cm^3 of your sample to a test tube.
  2. Then add a few drops of iodine solution and gently shake the tube to mix the contents. If the sample contains starch, the colour of the solution will change from browny-orange to black or blue-black.
• You can use the biuret test to see if a type of food contains protein. Meat and cheese are protein rich and good foods to use in this test.
• The biuret test is where you:

  1. Prepare a sample of your food and transfer 2cm^3 of your sample to a test tube.
  2. Add 2cm^3 of biuret solution to the sample and mix the contents of the tube by gently shaking it.
  3. If the food sample contains protein, the solution will change from blue to purple. If no protein is present, the solution will stay blue.
• Lipids are found in foods such as olive oil, margarine and milk. You can test for the presence of lipids in a food using Sudan III stain solution.
• When testing for lipids using Sudan III solution the steps you should take are:

  1. Prepare a sample of the food you are testing. Transfer about 5cm^3 into a test tube.
  2. Use a pipette to add 3 drops of Sudan III stain solution to the test tube and gently shake the tube.
  3. Sudan III stain solution stains lipids. If the sample contains lipids, the mixture will separate out into two layers. The top layer will be bright red. If no lipids are present, no separate red layer will form at the top of the liquid.