Proteins are complex nitrogenous organic compounds of high molecular mass. These help in the growth and maintenance of living body and in transmission of genetic information. Proteins are made up of α-amino acids as the basic building blocks. In all, there are 20 different amino acids required by the body. Some of these are called essential amino acids as they cannot be synthesized by the organisms. These must form a part of our food.

Tests for Proteins

1. Biuret Test

When a sample of protein is treated with a solution of copper sulphate in alkaline medium, a violet colour appears. The intensity of the colour also indicates the relative amount of protein present in different samples.

2. Ninhydrin Test

When a few drops of a 0.1% alcoholic solution of ninhydrin, are added to a very dilute solution /suspension of protein and the contents are boiled for 1-2 minute, a red-purple colour appears.

3. Xanthoprotein Test

When an aqueous solution of protein is treated with a few drops of nitric acid and contents are allowed to stand, a deep yellow colour appears.

4. Million’s Test

A few drops of milk are warmed with mercuric nitrate in presence of dil. sulphuric acid and cooled. One drop of sodium nitrate solution is added and the mixture is again warmed. A red colour appears showing the presence of proteins in milk.

How To Perform Experiment

i) Biuret Test

Take one mL of milk or any other protein containing sample in a test tube. Make it alkaline by adding a few drops (4-5) of NaOH solution Then add 4-5 drops of dil solution of CuSO4.

ii) Ninhydrin Test

Take about 1 mL of 5% solution of egg white in a test tube. Add to it 4-5 drops of 0.1% ninhydrin solution. Boil the contents for about one minute and cool it.

iii) Xanthoprotein Test

Take 2-3 grams of gjutenin of wheat in a test tube. Add 2-3 mL of conc. HNO3 to it. Shake the contents and keep aside.

iv) Milton’s Test

Take about 1 mL of milk in a test tube. Add 1-2 drops of mercuric nitrate solution and one drop of di H2SO4. Boil and cool the contents and then add 1 drop of NaNO3 solution. Warm the mixture again and cool it.