Qualitative tests for food

Question 1

Explain the steps of the experiment to test for reducing sugars (6)

Answer 1

1. dissolve glucose in water in test tube.
2. Add an equal amount of benedicts solution.
3. In a 2nd test rube mix equal volumes of water and benedicts solution. This is the control
4. Heat both test tubes in a boiling water bath
5. If reducing sugar is present, the solution turns brick red
6. If it isn’t present the solution remains blue

Question 2

What colour is bendict’s solution and what colour does it turn
In the presence of what does this colour change happen?

Answer 2

Blue to brick red when reducing sugar is present

Question 3

Explain the test for starch (4)

Answer 3

1. Add a few drops of iodine solution to some starch dissolved in water.
2. Add a few drops of iodine solution to some water. This acts as the control.
3. If starch is present the colour turns from red-yellow to blue-black or purple.
4. If it is absent the solution stays red-yellow

Question 4

What colour does iodine change from and to?

In the presence of what does this happen?

Answer 4

red-yellow to blue-black or purple in the presence of starch

Question 5

Explain the test for lipids (6)

Answer 5

1. label a piece of brown paper as lipid.
2. Rub a small piece of butter or cooking oil on the paper.
3. Repeat process using a few drops of water on a piece of paper labeled water. This acts as a control.
4. Leave the 2 pieces of paper over a radiator to dry
5. Lipid produces a permanent stain or translucent spot on the paper
6. The water stain dries out

Question 6

Explain the test for protein (6)

Answer 6

1. Dissolve a smaple of soluble protein (egg white or milk) in water.
2. Add sodium hydroxide (colourless) until the solution clears.
3. Then add a few drops of dilute copper sulfate (blue).
4. As a control, add sodium hydroxide and copper sulfate ( or biuret solution) to water.
5. The appearance of a purple-violet colour shows that proteins are present.
6. If protein isn’t present the colour remains blue.

Question 7

What is a negative result and positive result of a protein test using biuret solution?

Answer 7

A negative result is blue and a positive result is purple-violet