TOPIC 1 Practicals

Question 1

How do you test for the presence of a reducing sugar?

Answer 1

• add food sample to test tube (if not in liquid form, grind it up in water)
• add equal volume of Benedict’s reagent
• heat mixture in water bath for 5 minutes

Question 2

Positive result for a reducing sugar using Benedict’s Reagent

Answer 2

orange-brown or brick red solution

Question 3

How do you test for a non-reducing sugar?

Answer 3

1. negative reducing sugar result
2. heat wit HCl
3. cool and neutralised with sodium bicarbonate
4. add Benedict’s reagent
5. heat at 85 degrees in water bath
   colour change: BLUE —> BRICK RED

Question 4

Test for starch

Answer 4

• add sample to test tube (or 2 drops into a depression on spotting tile)
• add 2 drops of iodine (potassium iodide solution) solution and shake/ stir
  positive result: yellow —> blue-black

Question 5

Test for lipids

Answer 5

• add sample to test tube with ethanol
• shake thoroughly to dissolve any lipid in sample
• add water and shake gently
• milky-white emulsion indicates presence of lipid
  as a control, repeat procedure using water and final solution should remain clear

Question 6

Test for proteins

Answer 6

• add sample to test tube
• add equal volume of sodium hydroxide solution (at room temp)
• add few drops of very dilute copper (II) sulfate solution
• mix gently
• purple coloration = presence of peptide bonds, hence protein present
  (no protein = solution remains blue)