4.2 - Food Tests

Testing for starch with iodine solution

• Iodine solution is used to test a sample for starch.
• Add a few drops of iodine solution to the sample.

Reading the result

• Starch present - The iodine solution changes colour from yellow-brown to dark blue-black.
• No starch present - The solution stays yellow-brown.

The result is shown by the change in colour, so a positive starch test is described as a change from yellow-brown to blue-black - not simply as "blue-black".

Testing for reducing sugars with Benedict's solution

• Benedict's solution, also called Benedict's reagent, tests a sample for reducing sugars - the commonest of which is glucose.
• Benedict's solution starts off blue.

Carrying out the test

1. Add Benedict's solution to the sample.
2. Heat the sample in a hot water bath.
3. If a reducing sugar is present, a coloured precipitate forms (solid particles suspended in the solution) and the colour changes from the original blue.

Reading the result

• The colour runs through a sequence: blue → green → yellow → orange → brick-red.
• The more reducing sugar there is, the further along the sequence the colour goes, so the test is partially quantitative:
  • Brick-red - A high concentration of reducing sugar.
  • Green or yellow - A lower concentration.
• If no reducing sugar is present, the solution stays blue.
• This is the only one of the common food tests that needs heating.

Testing for protein with biuret reagent

• The biuret test is used to test a sample for protein.
• The biuret reagent is blue.

Carrying out the test

The test can be done in one of two forms:

1. Two-solution form - Add a few drops of an alkali (such as potassium hydroxide or sodium hydroxide) to make the sample alkaline, then add a little copper(II) sulfate solution (bright blue).
2. Ready-mixed form - Add a single ready-mixed biuret solution to the sample.

Either way, the mixture is then gently shaken.

Reading the result

• Protein present - The solution changes from blue to purple (also described as lilac or violet).
• No protein present - The solution stays blue.

The colours can be pale, so the result is read carefully against the original blue.

Testing for lipids (ethanol emulsion test)

• The ethanol emulsion test checks a sample for lipids (fats and oils).
• An emulsion is tiny droplets of one liquid suspended in another.

Method

1. Shake the test substance with ethanol for about a minute, so any lipid dissolves into the ethanol. Ethanol is colourless.
2. Add water to the ethanol solution.

Result

• Lipids present - The lipids come out of solution in the water, showing up as a cloudy white emulsion. The more lipid there is, the more obvious the milky cloudiness.
• No lipid present - The mixture stays colourless.

Food tests summary

• Each food test uses a specific reagent that changes colour when the food substance is present.
• The colour change from the starting colour to a new colour is the positive result that confirms the substance is there.

The five food tests