Biochemical Tests

Question 1

Test for reducing sugars:

Answer 1

1. Prepare the food sample by grinding it and mixing it with distilled water (filter the solid material)
2. Add 5cm3 of food sample to test tube
3. Add 5cm3 of Benedict’s reagent to the test tube
4. Place the test tube in a thermostatic water-bath at 80 degrees for 3 minutes
5. If reducing sugar is present a green, yellow, orange or red precipitate will present

Question 2

Test for non-reducing sugars:

Answer 2

1. Start with a fresh test sample. Add 5cm3 of prepared food sample to a test tube
2. Heat the food with a sample of hydrochloric acid
3. Neutralise by adding sodium hydroxide
4. Add 5cm3 of Benedict’s reagent to test tube
5. Heat test tube in a thermostatic water bath at 80 degrees for 3 minutes
6. Look for the colour change indicating reducing sugars, if they are now present it is a non-reducing sugar
7. A dark precipitate will form if both are present
8. If there is still no colour change no sugars are present

Question 3

Test for starch

Answer 3

1. Take a food sample and add it to a test tube
2. Add a drop of iodine to the sample
3. If starch is present it will immediately turn blue-black

Question 4

Biuret test for proteins

Answer 4

1. Crush up sample and add distilled water
2. Add drops of sodium hydroxide then copper(ii) sulfate to each sample
3. Purple for positive result, light blue for negative

Question 5

Test for lipids:

Answer 5

Ethanol emulsion test:
Add a sample to ethanol
Shake sample
Lipids dissolve in ethanol
If you pour ethanol into water, mixture will turn milky if lips are present