Biological molecules- Tests

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Biological molecules- Tests

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Benedict's test- Test for reducing sugars

  • All monosaccharides and some disaccharides are reducing sugars
  • Reduction is a chemical reaction involving the gain of electrons or hydrogen
  • This means that a reducing sugar can donate electrons to another chemical
  • Benedict's reagent is an alkaline solution of copper sulfate
  • When a reducing sugar is heated with Benedict's reagent it forms an insoluble red precipitate of copper oxide

The process

1. Add food sample and grind it up if it's not already in liquid form

2. Add an equal volume of Benedict's reagent

3. Heat the mixture

4. If a reducing sugar is present, an orange-brown colour is formed

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Test for non-reducing sugars

  • Some disaccharides are reducing sugars
  • Other disaccharides are known as non-reducing sugars because they don't change the colour of Benedict's reagent when they're heated with it
  • To detect a non-reducing sugar it must be hydrolysed into its monosaccharide components

The process
1. Grind the food sample up if it's not already in liquid form
2. Add the sample to Benedict's reagent and filter
3. Heat the mixture- If it doesn't change colour a reducing sugar is not present
4. Add the food sample to dilute HCL and heat it- This will hydrolyse any disaccharide into its monosaccharide components
5. Add hydrogencarbonate solution in order to neutralise the HCL- Benedict's reagent doesn't work in acidic conditions
6. Re-test the resulting solution by heating with Benedict's reagent
7. If a non-reducing sugar was present in the original sample, the benedict's reagent will turn orange-brown (This is due to the reducing sugars that were produced from the hydrolysis of the reducing sugar)

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Test for starch

  • Starch is easily detected by its ability to change the colour of iodine in potassium iodide solution from yellow to blue-black
  • The test is carried out at room temperature

The process

1. Place the sample in a test tube

2. Add 2 drops of iodine solution and shake

3. The presence of starch is indicated by a blue-black coloration

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Test for lipids- Emulsion test

The process

1. Add the food sample to ethanol

2. Shake the tube thoroughly to dissolve any lipid in the sample

3. Add water and shake gently

4. A cloudy-whte colour indicates the presence of a lipid

5. As a control, repeat the procedure using water instead of the sample- The final solution should remain clear

  • The cloudy-white colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion- Light passing through this emulsion is refracted as it passes from oil droplets to water droplets making it appear cloudy
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The biuret test- Test for proteins

The process

1. Place a sample of solution into a test tube and add an equal volume of biuret reagent at room temperature

2. Add a few drops of dilute copper sulfate solution and mix

3. A purple coloration indicates the presence of peptide bonds and hence a protein- If no protein is present, the solution remains blue

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