The aim of the experiments was to test food samples to see which food group the sample belongs to. I also did tests to determine what type of carbohydrate an unknown carbohydrate was.

Benedict’s Test for non-reducing sugars

• Benedict’s reagent
• Non-reducing sugar sample e.g. sucrose
• Test tubes
• Water bath or heating apparatus
• Pipettes
• Hydrochloric Acid
• Alkali powder (sodium hydrogen carbonate)

Starch Test

• Starch Sample
• Iodine
• Tray
• Pipette

Emulsion Test

• Ethanol
• Lipid Sample
• Water
• Test Tubes

Methods

The Biuret test

1. Put about 2cm3 of the test sample into a test tube
2. Put an roughly equal amount of biuret’s reagent into the same test tube using a pipette
3. Give the test solution a gentle shake
4. Observe changes

The Emulsion Test

1. Put about 2cm3 of the lipid sample into a test tube
2. Add 4cm3 of ethanol into the test tube using a pipette
3. Decant the lipid sample with ethanol into a test tube of water
4. Observe changes

Benedict’s test for reducing sugars

1. Put about 2cm3 of the sample into a test tube with an equal amount of benedict’s reagent
2. Put the test tube in a test tube rack in a bath of warm water
3. Wait until the changes have taken place
4. Remove test tube from the water bath

Test for non-reducing sugars

1. Add a small amount of hydrochloric acid to a small amount of a sample of a non-reducing sugar solution
2. Wait for a few minutes for hydrolysis to take place
3. Neutralise the hydrochloric acid by adding a small amount of sodium hydrogen carbonate to the solution.
4. When the fizzing has ceased, add some benedict’s reagent and heat in the water bath
5. Also put a test tube of the sample of the non-reducing sugar which has not been hydrolysed by the hydrochloric acid and sodium hydrogen carbonate.
6. Observe and record any changes

Iodine Test for Starch

1. Put a few drops of the starch solution into a spotting tray
2. Add a few drops of iodine to each sample of starch by using a pipette
3. Observe and record any changes

Results

The unknown carbohydrate is a non-reducing sugar.

Scientific Reasoning for the results

         Benedict’s reagent changes from a bluish colour to a brick red colour in the presence of a reducing sugar. This is because the reducing sugars reduce the Cu2+ ions to Cu+ ions, which are present in benedict’s reagent (benedict’s reagent is copper (II) sulphate dissolved in water). Heat is used to speed up the reduction of the reduction of the copper ions. In the emulsion test, ethanol is used to hydrolyse the lipid molecules because lipids cannot dissolve in water. Once the lipids have been hydrolysed, the mixture of ethanol and the lipids are decanted into a test tube of water. A white precipitate is formed on top of the water if lipids are present. This is because lipids are hydrophobic and are trying to get as far away from the water as possible. When testing for non-reducing sugars, the glycosidic bonds must be hydrolysed by hydrochloric acid before performing the benedict’s test for reducing sugars. In the biuret test for proteins, the sodium hydroxide in the biuret reagent breaks the peptide bonds linking the amino acids, this allows the copper to form a coloured substance with parts of the amino acids which shows the presence of proteins.

Evaluation of the tests

        All the tests produced result as expected, despite having trouble with testing for non-reducing sugars in the unknown carbohydrate category. This was due to an excess of hydrochloric acid in the sample and the fact that the benedict’s reagent had been added before the acid had been neutralised by the sodium hydrogen carbonate.