Practical

• Required practical activity - food tests: Use qualitative reagents to test for a range of carbohydrates, lipids and proteins
• Qualitative tests for foods

  There are several qualitative tests for food chemicals. These can be used to detect the presence of food chemicals, but not how much is present.
• Test for sugars

  Benedict's test is used to detect sugars.

  Method

  
• Result for benedicts test:

  
• Sugars classed as reducing sugars will react with Benedict's solution on heating for a few minutes. Glucose is an example of a reducing sugar.

  Reducing sugars gives a red-brown precipitate with Benedict's solution. The precipitate takes a while to settle in the tube – you're more likely to see simply a red or brown colour. If there's not much glucose present, the final colour may be green or yellow, or orange if there's a little more
• Test for starch

  Add iodine solution to the food being tested.

  Foods containing starch will turn a blue-black colour.

  The iodine test can also be used with a microscope to stain starch grains in plant cells.

  
• Test for proteins

  The biuret test is used to detect proteins.

  Biuret reagent is sometimes available as a single solution.

  Method

  1. Add 1 cm3 of biuret solution A to the food solution.
  2. Mix the liquids.
  3. Add 1 cm3 of biuret solution B and shake.