Food tests

Question 1

In a food test, how do you prepare the food sample

Answer 1

1) Get a piece of food and break it up using a pestle and mortar.
2) Transfer the ground up food to a beaker and add some distilled water.
3) Stir the mixture with a glass rod, so that the chemicals in the food dissolve in the water.
4) Filter the solution using a funnel lined with filter paper to remove suspended food particles.

Question 2

Give examples of foods that contain starch

Answer 2

Pasta, rice, potatoes

Question 3

How do you test for starch

Answer 3

1) Prepare a food sample and transfer 2cm^3 of the sample to a test tube.
2) Add a few drops of iodine solution to the sample and gently shake the test tube to mix the contents.
Observe the colour change
3) If starch is present then the colour of the iodine solution will change from orangey-brown to blue-black

Question 4

Name the two types of sugars

Answer 4

Non-reducing sugars
Reducing sugars

Question 5

Which type of sugar does Benedict’s test for

Answer 5

The benedict’s test can be used to test for reducing sugars in foods.

Question 6

Give an example of a reducing sugar

Answer 6

Glucose is a type of reducing sugar

Question 7

Give an example of a non-reducing sugar

Answer 7

Sucrose is a type of non-reducing sugar

Question 8

Give examples of foods that contain sugars

Answer 8

Biscuits, cereal and bread are examples of foods that contain sugar

Question 9

Give an example of food containing reducing sugars

Answer 9

Bread is an example of food containing reducing sugars

Question 10

How do you test for sugars

Answer 10

1) Prepare the food sample and transfer 2cm^3 of the sample to a test tube.
2) Add 2cm^3 of Benedict’s reagent to the sample.
3) Prepare a water bath so that it’s set to 75°C
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 blue to green, yellow or brick-red - it depends on how much sugar is in the food. (green - small amount of sugar is present, yellow - more sugar is present, brick red - a lot of sugar is present)

Question 11

Describe the test for proteins

Answer 11

1) Prepare a food sample and transfer 2cm^3 of the sample to a test tube
2) Add 2cm^3 of biuret reagent to the sample and gently shake the test tube to mix the contents

3) If protein is present, the solution will change from blue to purple.

Question 12

Describe the test for lipids

Answer 12

1) Prepare a sample of food, but do not filter the solution. This is because lipid molecules can stick to filter paper.
2) Transfer 2cm^3 of the sample to a test tube
3) Add 2cm^3 of ethanol and gently shake the test tube to mix the contents. Allow the mixture to settle. 4)Add 4cm^3 of water to another test tube.
5) Very gently, pour the test tube containing ethanol and the food sample into the test tube containing water

6) If lipids are present, then a white cloudy emulsion forms.

SAFETY - Ethanol is highly flammable, so no naked flames should be present.

Question 13

What type of sugars will the Benedict’s test only work for

Answer 13

The Benedict’s test only works for reducing sugars. It does not work for non-reducing sugars.

Question 14

State hazards and safety precautions needed when testing for starch

Answer 14

Safety
Wear safety goggles - (hazard) iodine solution is an irritant
Avoid contact with skin and eyes

Question 15

State hazards and safety precautions needed when testing for proteins

Answer 15

Safety
Wear safety goggles
(Hazard) as Biuret solution is corrosive
Avoid contact with skin and eyes

Question 16

State hazards and safety precautions needed when testing for lipids

Answer 16

Wear safety goggles
Hazard- Ethanol is highly flammable, so no naked flames should be present.