L8. Protein Identification

Introduction:

The biuret reaction can be used to assess the concentration of proteins because peptide bonds occur with the same frequency for amino acid peptide. The intensity of the colour is directly proportional to the protein concentration.

The solution to be tested in treated with a strong base followed by a few drops of copper (II) sulphate. If the solution turns purple, protein is present. Only peptides with a chain of a least 3 amino acids (two peptide bonds) give a significant measureble colour shift with these reagents.

Material:

- 250 mL beaker
- 6 test tubes
- test tube rack
- pipet
- mortar
- glass marking pen
- gloves
- goggles
- milk
- rice milk
- egg
- yogurt
- potato
- distilled water
- NaOH 20%
- 10 drops of CuSO4

Procedure: 

First of all we are going to dilute the protein:

1- Add 100 mL of distillied water to each 250 mL beaker. Label them with M (milk),  R (rice nilk), EW (egg white), EY (yolk), Y (yogurt) and P (potato)
2- Separate the egg white and the yolk in another beaker.
3- Smash the potato in a mortar and add some amount of the smashed patato tho the P beaker.

Prepare the samples:

1- Add 10 mL of a dispersion of each food (M, R, EY, EW and Y) to the indicate beaker. Calculate the final concentration. All the groups will use the same dispersion from the beakers.
2-  Prepare 6 test tubes (clean and dry) and label M, R, EW, EY, Y and P. Add 2mL of every food dilution of each beaker.  1 pipet for dissolution.
3- Add 2ml of 20% NaOH dissolution.
4- Shake gently and add 5 drops of CuSO4 in each tube. Allow the mixture to stand for 5 minutes.
5- Note any colour change. Remember that proteins will turn solution pink or purple.
6- Compare the test tubes.

Questions:


1- Which food has proteins?
Egg white, yogurt, potato, milk

2- Wich food has more proteins? Why?
Eggwhite because it's animal food and it's rich in proteins

3- Do you find any difference between rice milk and cow milk? Rice milk doesn't have proteins however cow milk does as i had said before.

4- Is there any difference among milk and yogurt? why?
Yes, yogurt has more protein than milk because the protein is more concentrated than in the milk, it has less water.