Food Tests.

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  • Created by: TBird
  • Created on: 02-12-12 09:39

Test for starch

1. Place a little starch powder on a spotting tile.

2.A drop of dilute iodine solution is added to the starch, forming a very dark blue, or 'blue black' colour.

3. Keep some starch powder back to act as a control or comparison of the starch which has had dilute iodine solution added to it.

Starch is insoluble, but this test will work on a solid sample of food such as a potato.

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Test for Glucose.

Glucose is called a reducing sugar. This is because the test for glucose involves chemicaly reducing an alkaline solution of copper sulphate to copper (I) oxide.

The test for glucose is called the Benedict's test. Simply add a few drops of Benedict's solution, which contains alkaline copper sulphate, to some glucose.

Add enough to turn the mixture blue.

Prepare a water bath by half filling a beaker with water and heating it on a tripod and gauze.

Heat the mixture in the boiling water bath.

The clear blue solution will gradually change colour, forming a cloudy orange or brick red precipitate of copper (I) oxide

All 'single' sugars such as fructose are reducing sugars, as well as some 'double' sugars such as the milk sugar, lactose.

However ordinary table sugar (sucrose) is not. If sucrose is boiled with Benedict's solution it remains a clear blue colour.

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Test for proteins.

The test for protein is called the Biuret test. 

It is done by firstly mixing some protein such as powdered egg white to some water.

Then, dilute potassium hydroxide solution is added to the mixture. 

Finally, a small amount of copper sulphate solution is added to the mixture.

A mauve colour develops.

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Test for lipids.

Fats and oils are insoluble in water, but will dissolve in ethanol

The test is done by dissolving a lipid in ethanol. The mixture is then poured into cold water. 

A white cloudy layer forms on the top of the water. This is caused by the etanol dissolving in the water and leaving the lipid behind as a suspension of tiny dropleys.

Called an emulsion.

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