Food Tests (Practical)

This lesson covers:

  1. How to prepare food samples for testing 
  2. The Benedict's test for sugars
  3. The Iodine test for starch
  4. The Biuret test for proteins 
  5. The Sudan III & emulsion tests for lipids 
  6. The DCPIP test for vitamin C
Diagram showing the molecular structure of glucose.

Which test is used to detect the presence of reducing sugars (i.e. small sugars like glucose)?

Bartholomew's test

Benedict's test

Biuret test

Bronchial test

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Does the Benedict's test require heating the sample in a water bath? 

Yes

No

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Photos of Benedict's test

Test tubes showing Benedict's solution for reducing sugars with colours indicating no sugar, low concentration, medium concentration, and high concentration.

Benedict's solution itself in blue, but if reducing sugars are present then it will change colour depending on the concentration of reducing sugar.


Green indicates a low concentration of reducing sugar, orange indicates a medium concentration and brick-red indicates a high concentration.

Testing for vitamin C

Diagram showing the DCPIP test for vitamin C with steps including adding food sample to DCPIP solution and observing colour change for negative and positive results.

To find out if a sample contains vitamin C, you must carry out the DCPIP test:


  1. Place 1 cm3 of DCPIP into a test tube. DCPIP is blue when dissolved in water.
  2. Add a few of drops of a solution containing your food sample and shake.
  3. If vitamin C is not present, the solution will remain blue. 
  4. If vitamin C is present, DCPIP is reduced and the solution will turn colourless. 

What does iodine test for?

Lipids

Glycogen

Proteins 

Starch

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Iodine solution is used to test for the presence of starch. If starch is present, which colour change takes place?

Orange to blue-black

Orange to Pink

Blue to brick red

Blue to lilac / purple

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Illustration of a protein structure with various coloured shapes representing amino acids.

To test for proteins we can use solution.

biuret

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Biuret reagent can be used to test for the presence of proteins. If protein is present, what colour change takes place?

Orange to blue-black

Blue to lilac / purple

Orange to Pink

Blue to brick red

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Which test is used to test for the presence of lipids?

Benedict's test

Iodine test

Emulsion test

Biuret test

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Emulsion test for lipids 


cloudy / water / emulsion / acid / pink / ethanol


  1. Place your food sample in a test tube.
  2. Add 2cm3 of .
  3. Shake.
  4. Add 2cm3 of distilled .
  5. If a emulsion forms then lipid was present.
ethanol
water
cloudy

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A student added a few drops of DCPIP to a sample of freshly squeezed orange juice.

What colour do you expect the solution to become?

blue

yellow

green

colourless

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