Biochemical Test

Question 1

How do you test for reducing sugars?

Answer 1

• add 2cm^3 of the food sample in liquid form into test tube
• add 2cm^3 of benedict reagent into test tube
• heat in water bath for 5 minutes

Question 2

What is a positive results when testing reducing sugars?

Answer 2

when the solution turns brick red

Question 3

Why does Benedict solutions turn red when a sample contains sugar?

Answer 3

• Benedict soltution is an alkaline soltuion of copper(II) sulfer
• when heated the sugar donates an electron to the Benedict solution
• this reaction creates copper(I) oxide which is a red precipitate

Question 4

How do you test a non-reducing sugars?

Answer 4

• do reducing sugar test to show that it’s negative
• add 2cm^3 of the food sample and diluted hydrochloric acid into a test tube
• place test tube into warm water bath for 5 minutes
• add sodium hydrocarbonate
• then redo reducing sugar test which will give a positive result

Question 5

Why do you add hydrochloric acid to the sample?

of non-reducing sugar test

Answer 5

it will hydrolyse the reducing sugars (which are disaccharide and polysaccharides) into monosaccharides

Question 6

Why do you add sodium hydrocarbonate to the solution?

of non-reducing sugar test

Answer 6

to neutralise the test tube after adding hydrochloric acid as Benedict Solution doesn’t work in acidic condtions

Question 7

How do you check that the solution is neutralised after adding sodium hydrocarbonate?

in the non-reducing sugar test

Answer 7

• using pH paper
• it will turn green

Question 8

How do you test for starch?

Answer 8

add iodine to sample and if it changes from orange to blue/black starch is present

Question 9

How do you test for lipids?

Answer 9

• put 2cm^3 of the sample in a clean test tube
• add 5cm^3 of ethanol
• shake to dissolve lipids
• add 5cm^3 of water
• then repeat with a control group as water as the sample

Question 10

What will you see if a lipid test is positive?

Answer 10

a white, cloudy solution

Question 11

How do you test for protein?

Answer 11

• add an equal amount of the sample and sodium hydroxide at room temperature
• add diluted copper(II) sulfate soltuion
• gently mix

Question 12

What happens when the test for protein is positive

Answer 12

the solution changes from blue to purple

Question 13

How does the Biuret Test work?

Answer 13

it tests for the peptide bonds within the protein