Food Tests

What reagent used to detect the presence of starch

Iodine

What are the steps to carry out the Iodine test

1. The food sample was ground into small pieces using a pestle and mortar

2. The ground food sample was placed into a test tube, and distilled water was added and the test tube was shaken gently to create a food suspension

3. A few drops of iodine solution were added to the test tube

What colour change would happen if starch was present

BROWN (colour of iodine) → BLUE-BLACK

What reagent is used to detect the presence of reducing sugars (monosaccharides)

Benedict’s Solution

Procedure for Benedict’s test

1. The food sample was ground into small pieces using a pestle and mortar

2. The ground food sample was then placed into a test tube, and distilled water was added and the test tube was shaken gently to create a food suspension

3. A few drops of Benedict’s solution were added to the test tube

4. The test tube was placed in a water bath and was heated

In the presence of reducing sugars, what colour change would occur

BLUE (colour of Benedicts) to:

• Brick red (high content of reducing sugars)

• Orange (moderate content)

• Green-yellow (traces/low content)

What reagent is used to detect the presence of proteins

Biuret

How can you make biuret

Equal volume (drop wise) of Copper Sulphate and Sodium Hydroxide

Procedure for Biuret Test

1. The food sample was ground into small pieces using a pestle and mortar

2. The ground food sample was then placed into a test tube and distilled water was added, and the test tube was shaken gently to create a food suspension.

3. Add biuret solution and shake gently

What colour change would be noted for a positive buiret test

BLUE (colour of biuret) → LILAC

What test is used to detect the presence of lipids

Emulsion/ Grease spot test

Procedure for Emulsion Test

1. The food sample was into small pieces using a pestle and mortar

2. The ground food sample was placed into a test tube, and distilled water was added and the test tube was shaken gently to create a food suspension.

3. Add some ethanol to the test tube

4. In another test tube add equal volume of distilled water and the liquid part from the first test tube (do not include any of the solid)

5. Shake the test tube very well

What will a positive test for lipids (emulsion test) show

The solution will have a milky appearance/ milky white precipitate

What is the other test for lipids

Grease spot test

Why is this test different to all the others

It can only be used for solid substances

Procedure of grease spot test

1. Rub some of the food onto a filter paper

What is a positive test for the grease spot test

If a translucent mark appears on the filter paper, lipids are present

Sometimes the colour of a food can interfere with its colour change. What can be done to prevent this?

The food suspension can be filtered

Why?

It removes as much of the solid food as possible to help more clearly distinguish the colour changes without the colour of the food interfering with the test.