PAG 02: Food Tests

Question 1

Aim

Answer 1

Investigate the presence of starch, reducing sugars, lipids and protein using various food tests.

Question 2

Equipment

Answer 2

Equipment
● Iodine solution (0.01 mol/dm3​ )​
● Spotting tile
● Sugar paper
● Ethanol
● Test tube
● Boiling tube
● Water
● Benedict’s reagent
● Beaker
● Boiling water bath
● Biuret reagent

Question 3

Test for starch?

Answer 3

Iodine

Question 4

The Iodine test for starch Equipment

Answer 4

The Iodine test for starch Equipment
● Food sample
● A test tube
● Iodine solution 
● Pipettes.

Question 5

The Iodine test for starch Method

Answer 5

Method
1. Put some of the food sample into a test tube.
2. Add a few drops of iodine solution to the food sample using a pipette.
3. If starch is present, the solution turns from brown to blue-black. Note any colour change in a
table of results.

Question 6

Test for reducing sugars?

Answer 6

Benedict’s test

Question 7

The Benedict’s test for reducing sugars - Equipment

Answer 7

Equipment
● Food sample
● A test tube
● Benedict’s solution
● Hot water bath
● Thermometer
● Pipettes

Question 8

The Benedict’s test for reducing sugars - method

Answer 8

Method

1. Add an equal volume or excess of Benedict’s solution to the food sample in a test tube.
2. Place in a hot water bath for a few minutes.
3. If reducing sugar is present, a brick red precipitate is formed. If reducing sugar is absent, the solution remains blue. Note any colour change in a table of results.

Question 9

Test for lipids?

Answer 9

Emulsion test (ethanol)

Question 10

Test for lipids - Equipment

Answer 10

Test for lipids - Equipment
● Food sample
● Test tube 
● Ethanol
● Distilled water

Question 11

Test for lipids main method

Answer 11

Method A

1. Add a few cm3​ ​ of ethanol to the food sample.
2. Pour this mixture into a test tube of equal volumes of distilled water.
3. If lipids are present, a white emulsion is formed on the surface of the mixture.
4. This is called the emulsion test.

Question 12

Test for lipids strange method B

Answer 12

Method B

1. Rub or place a drop of the food sample onto a piece of sugar paper and allow time for it to dry.
2. View the paper against a light source. If a translucent spot remains, lipid is present.

Question 13

Test for protein?

Answer 13

Biuret’s reagent - Method B (sodium hydroxide and copper (II) sulphate)

Question 14

Test for protein - Equipment

Answer 14

Equipment
● A test tube
● A 10cm3​​ measuring cylinder
● Biuret solution

Question 15

Test for protein - Method

Answer 15

Method
1. Add a few drops of Biuret’s reagent (sodium hydroxide and copper (II) sulphate) to the food
sample in a test tube.
2. Shake the solution to mix and wait for a few minutes.
3. If protein is present, the solution turns from blue to purple.

Question 16

Sources of error

Answer 16

Colour change of Benedict’s test and Biuret test may be subtle and difficult to judge if the concentration of the tested molecule is low.

Question 17

Safety precautions

Answer 17

Tie hair back and wear safety goggles when performing the Benedict’s test using a Bunsen burner and hot water bath.

Handle Biuret solution with care as it contains copper sulphate (poisonous) and sodium hydroxide (corrosive). Wash immediately if it comes into contact with skin and wipe away any spills to surfaces.

Keep ethanol solution away from flames as ethanol is highly flammable.