Biological tests

Question 1

Describe how to prepare a solid food sample for biochemical food testing.

Answer 1

Break up the food using a pestle and mortar.
Transfer to a test tube and add distilled water.
Mix the food with the water by stirring with a glass rod.
Filter the mixture using a funnel and filter paper, collecting the solution.
Proceed with the food tests.

Question 2

Describe how to test a liquid sample for the presence of a reducing sugar and a non-reducing sugar and explain why this works.

Answer 2

If a reducing sugar is present in a solution, adding Benedict’s reagent and heating will form an insoluble red precipitate. Non-reducing sugars do not change the colour of the solution, which is blue, and so we have to break the sugar down to monosaccharides by hydrolysis to prove they’re non-reducing.

Question 3

What are some healthy and safety precautions you should consider when using Benedicts reagent ?

Answer 3

Wearing goggles and washing your hands if you get any benedicts on it.

Question 4

Describe how to test a liquid sample for the presence of a lipid and explain why this method works

Answer 4

• To do this, you take the sample and mix it with equal volumes of ethanol and water followed by shaking. A cloudy white emulsion will form if lipids are present. If lipids are absent, no emulsion will form.

Question 5

Describe how to test a liquid sample for the presence of starch.

Question 5

Describe how to test a liquid sample for the presence of a lipid and explain why this method works

Answer 5

To test for the presence of proteins, the biuret test should be carried out. To do this, a few drops of biuret reagent should be added. A blue to purple colour change will be observed if a protein is present. The higher the concentration of protein, the darker the purple colourisation will occur.

Question 6

Describe how to use a colorimeter to measure the absorbance of a sample.

Answer 6

1. Switch the colorimeter on and leave to stabilise for 5 minutes.
2. Select the red filter (for Benedict’s – use a complementary colour to starting
   solution) on the colorimeter. If using Benedict’s, centrifuge the solution or
   allow to sit to precipitate out the copper solid.
3. Set the colorimeter to zero using a cuvette ¾ filled with distilled water.
4. Ensure the cuvette is placed into the colorimeter so the light passes through the clear
   sides.
5. Make sure the slides are clean and there are no bubbles in the solution.
6. Using a pipette, fill the cuvette ¾ with the sample.
7. Place in the colorimeter and read the absorbance of light.
8. Less light is absorbed by the solution in a paler solution