2.6 - 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 (Sudan III test)

• The Sudan III stain test checks a sample for lipids (fats and oils).
• Sudan III is a dye that stains lipids.

Method

1. Add a few drops of Sudan III stain solution to the food sample.
2. Gently shake the tube.

Results

• Positive - The mixture separates into two layers, and the top layer takes up the dye and turns bright red.
• Negative - No separate red layer forms.

Another accepted lipid test

• Lipids can also be tested for with the ethanol emulsion test.
• Shake the sample with ethanol, then add water. A cloudy white emulsion shows that lipids are present.

Food tests summary

Each food test uses a specific reagent that changes colour when the food substance is present. The four common tests are summarised below.