Food tests

Question 1

Reducing Sugars

Answer 1

1. Add a small amount of Benedicts solution to sample and heat/boil
2. Positive test = Solution turns from blue, (to green, orange) to brick red
   (the higher the concentration of sugar the darker the red)

Question 2

Starch

Answer 2

1. Add a few drops of iodine to sample and shake
2. Positive= Soluion turns from orange to blue/black

Question 3

Proteins

Answer 3

1. Add Biuret solution to sample
2. Positive= Solution turns from blue to purple

Question 4

Lipids

Answer 4

Use a dry/grease free test tube:
1. Dissolve the sample in ethanol
2. Add distilled water + shake
3. Positive test= white emulsion forms (liquid turns from clear to cloudy white)

Question 5

Non-reducing sugars

Answer 5

• E.g Sucrose
• Complete Benedicts test, if negative then:
  1. Add a small amount of dilute acid to sample
  2. Boil/heat solution, and allow to cool
  3. Once cooled, add sodium hydrogen carbonate (NaOH) to neutralise the solution
  4. Add Benedict’s solution and heat
  5. If the test is positive, the solution turns from blue, to green, yellow to orange to brick red

Question 6

What’s the difference between reducing sugars and non-reducing sugars?

Answer 6

• Reducing Sugars- Reduces the copper sulphate (blue) to copper oxide (brick red)
• Non-reducing Sugars- Due to a glycosidic bond, it cannot reduce copper sulphate to copper oxide, unless acid hydrolysis is performed with an alkali