Biological Molecules 5 - Biological Food Tests

Question 1

Describe the food test for (soluble) proteins in 5 points

Answer 1

biuret test

few drops of biuret reagent added to protein soln.

shake gently

soln. turns from light blue to purple/lilac

no colour change observed if no protein

Question 2

What is the food test for reducing sugars? give 4 points

Answer 2

add to test tube your test sample and benedict’s reagent

boil in a water bath

positive result: light blue soln. turns brick red

negative result: no colour change observed

Question 3

What is the food test for non-reducing sugars like sucrose?

give 5 points

Answer 3

add HCl to test sample and boil

any glycosidic bonds will be hydrolysed

add hydrogen carbonate powder to neutralise

add benedict’s reagent and boil

same results as reducing sugar can be shown but pos. result means sucrose is present

Question 4

Describe the food test for starch.

Answer 4

iodine soln. turns from orange-brown to blue-black in presence of starch

qualitative test

Question 5

Describe food test for lipids in 4 points

Answer 5

mix test sample with equal vol. of ethanol to dissolve lipids

add equal vol. of H2O

shake gently

white and cloudy soln. forms