Required practical - food tests
Qualitative tests for foods
There are several Results that are not expressed as numbers or numerical data, eg they can be expressed as present or absent, or as colours. tests for food chemicals. These can be used to detect the presence of food chemicals, but not how much is present (which would be a Results that can be expressed using numerical values. analysis).
Test for sugars
Benedict's test is used to detect A simple carbohydrate that is sweet to the taste..
Method
Result
Sugars classed as Sugars that act as reducing agents. Glucose, fructose, lactose and maltose are reducing sugars. Sucrose is a non-reducing sugar. will react with Benedict's solution on heating for a few minutes. Glucose is an example of a reducing sugar.
Reducing sugars give a red-brown A suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction. with Benedict's solution. The precipitate takes a while to settle in the tube – you're more likely to see simply a red or brown colour. If there's not much glucose present, the final colour may be green or yellow, or orange if there's a little more.
Hazards
- Wear safety goggles.
- Benedict's solution is an irritant.
- Avoid contact with skin and eyes.
Test for starch
Add iodine solution to the food being tested.
Foods containing starch will turn a blue-black colour.
The iodine test can also be used with a microscope to stain starch grains in plant cells.
Hazards
- Wear safety goggles.
- Iodine solution is an irritant.
- Avoid contact with skin and eyes.
Test for proteins
The biuret test is used to detect Organic compound made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair..
Biuret reagent is sometimes available as a single Mixture formed by a solute and a solvent..
Method
- Add 1 cm3 of biuret solution A to the food solution.
- Mix the liquids.
- Add 1 cm3 of biuret solution B and shake OR add 1 cm3 of biuret solution B carefully down the side of the test tube so as to form two layers.
Results
Hazards
- Wear safety goggles.
- Biuret solution A is corrosive.
- Biuret solution B is an irritant.
- Avoid contact with skin and eyes.
Test for fats
The Sudan III test
The Sudan III test is one test used to test for Fat or oils, composed of fatty acids and glycerol..
Method
Results
If lipid is present in the sample, it stains red.
Hazards
- Wear safety goggles.
- Sudan III solution is flammable as it is dissolved in alcohol.
- Avoid contact with skin and eyes.
The emulsion test
The emulsion test is an alternative test for lipids.
Hazards
- Wear safety goggles.
- Ethanol is flammable.
- Avoid contact with skin and eyes.
