(todo) Core Practicals

Question 1

How do you prepare a solid food sample if testing for reducing sugars, starch or proteins?

Answer 1

1. Get a piece of your 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. Give the mixture a good stir with a glass rod.
4. Allow the mixture to settle out and then use a pipette to transfer it.

Question 2

How do you test a food sample for reducing sugars?

Answer 2

The Benedict’s test:
1. Transfer 5cm3 of a food sample to a test tube.
2. Prepare a water bath at 75°C.
3. Add some Benedict’s reagent (which is blue) to the test tube (about 10 drops) using a pipette.
4. Place the test tube in the water bath using a test tube holder and leave it for 5 minutes.
5. During this time, if the food sample contains reducing sugars, a green, yellow or brick-red precipitate will form - further with increasing concentration.

Question 3

How do you test a food sample for starch?

Answer 3

The iodine test:
1. Transfer 5cm3 of a food sample to a test tube.
2. Add a few drops of iodine solution (iodine dissolved in potassium iodide solution) and gently shake the tube. If the food sample contains starch the colour of the solution will change from orange-brown to blue-black.

Question 4

How do you test a food sample for proteins?

Answer 4

The biuret test:
1. Transfer 2cm3 of a food sample to a test tube.
2. Add 2cm3 of potassium hyroxide solution to make the solution alkaline.
3. Add a few drops of copper (II) sulfate solution (which is bright blue).
4. If the food sample contains protein, the solution will change from blue to pink or purple.

Question 5

How do you test a food sample for lipids?

Answer 5

An emulsion test:
1. Get a piece of your food and break it up using a pestle and mortar.
2. Transfer some of the food sample into a test tube.
3. Add 2cm3 of ethanol to the test tube.
4. Shake the test tube well for about 1 minute until the food sample dissolves.
5. Pour the solution into a test tube containing 2cm3 of distilled water.
6. If the food sample contains lipids, they will precipitate out of the liquid and show up as a milky emulsion - more noticeable with more lipid.