PAG 02: Food Tests Flashcards
Aim
Investigate the presence of starch, reducing sugars, lipids and protein using various food tests.
Equipment
Equipment ● Iodine solution (0.01 mol/dm3 ) ● Spotting tile ● Sugar paper ● Ethanol ● Test tube ● Boiling tube ● Water ● Benedict’s reagent ● Beaker ● Boiling water bath ● Biuret reagent
Test for starch?
Iodine
The Iodine test for starch Equipment
The Iodine test for starch Equipment ● Food sample ● A test tube ● Iodine solution ● Pipettes.
The Iodine test for starch Method
Method
1. Put some of the food sample into a test tube.
2. Add a few drops of iodine solution to the food sample using a pipette.
3. If starch is present, the solution turns from brown to blue-black. Note any colour change in a
table of results.
Test for reducing sugars?
Benedict’s test
The Benedict’s test for reducing sugars - Equipment
Equipment ● Food sample ● A test tube ● Benedict’s solution ● Hot water bath ● Thermometer ● Pipettes
The Benedict’s test for reducing sugars - method
Method
- Add an equal volume or excess of Benedict’s solution to the food sample in a test tube.
- Place in a hot water bath for a few minutes.
- If reducing sugar is present, a brick red precipitate is formed. If reducing sugar is absent, the solution remains blue. Note any colour change in a table of results.
Test for lipids?
Emulsion test (ethanol)
Test for lipids - Equipment
Test for lipids - Equipment ● Food sample ● Test tube ● Ethanol ● Distilled water
Test for lipids main method
Method A
- Add a few cm3 of ethanol to the food sample.
- Pour this mixture into a test tube of equal volumes of distilled water.
- If lipids are present, a white emulsion is formed on the surface of the mixture.
- This is called the emulsion test.
Test for lipids strange method B
Method B
- Rub or place a drop of the food sample onto a piece of sugar paper and allow time for it to dry.
- View the paper against a light source. If a translucent spot remains, lipid is present.
Test for protein?
Biuret’s reagent - Method B (sodium hydroxide and copper (II) sulphate)
Test for protein - Equipment
Equipment
● A test tube
● A 10cm3 measuring cylinder
● Biuret solution
Test for protein - Method
Method
1. Add a few drops of Biuret’s reagent (sodium hydroxide and copper (II) sulphate) to the food
sample in a test tube.
2. Shake the solution to mix and wait for a few minutes.
3. If protein is present, the solution turns from blue to purple.
Sources of error
Colour change of Benedict’s test and Biuret test may be subtle and difficult to judge if the concentration of the tested molecule is low.
Safety precautions
Tie hair back and wear safety goggles when performing the Benedict’s test using a Bunsen burner and hot water bath.
Handle Biuret solution with care as it contains copper sulphate (poisonous) and sodium hydroxide (corrosive). Wash immediately if it comes into contact with skin and wipe away any spills to surfaces.
Keep ethanol solution away from flames as ethanol is highly flammable.
