31 biochemical testing

Question 1

What does Benedict’s test, test for?

Answer 1

reducing sugars (glucose, galactose, fructose, lactose, maltose)

Question 2

Explain the Benedict’s test

Answer 2

1. Add Benedict’s solution to the sample and heat to 95 degrees Celsius (blue)
2. A colour change from blue to brick red precipitate indicates the presence of reducing sugar.
3. If the sample remains blue, no reducing sugar is present.

Question 3

How do you test for starch?

Answer 3

1. Add two drops of aqueous iodine solution to the solution
   - Dark-blue colour indicates the presence of starch
   - Brownish-orange indicates the absence of starch

Question 4

How do you test for non-reducing sugars?

Answer 4

1. Mix the sample with HCl to hydrolyse the glycosidic bonds
2. Neutralise the solution by adding NaHCO3
3. repeat Benedict’s test; if positive, non-reducing sugar is present.

Question 5

Describe the Emulsion test for lipids

Answer 5

• Mix the sample with ethanol
• Mix the sample with water then shake
• White emulsion layer formed = lipid present

Question 6

Describe the test for proteins (biuret test)

Answer 6

Add equal volumes of the sample and biuret reagent and mix gently, if the protein is present then a colour changes from blue to purple.

Question 7

A colorimeter could be used to determine the concentration of glucose against absorbance. What type of results could the Benedict’s test be?

Answer 7

Semi quantitative.