Tests for Proteins
The biuret and millon are ways of testing for the presence of protein in food.
Biuret test: this test is basically used for detecting the presence of peptide bonds in foods.
- obtain small quantity of egg white or ground meat.
- place in a test tube and mix with a few drops of water.
- add about 1cm³ of dilute caustic soda (NaOH), mix well.
- add drop by drop 1% copper sulphate solution. Shake thoroughly after each drop.
- a purple or violet colour indicates the presence of protein.
• place 1-3g of egg-white or other ground meat in a test tube.
• add about 4 ml of water of Millon's reagent.
• observe a white precipitate that will be formed.
• place the test tube in a beaker that is half-filled with water.
• heat the water gently.
The precipitate will turn red, indicating the presence of protein.
Effects of dry heat on protein:
1. Obtain a small piece of beef.
2. Trim off any excess fat.
3. Wash and allow to drain.
4. Place in a pin.
5. Place on the centre of a preheated oven.
6. Turn meat over half way through cooking.
When it is cooked it would be observed that it has become smaller. This shows that protein shrinks when heated. Dry heat hardens the outside of the meat and seals on the juices.