Casein is a globular protein found in milk that exists as a calcium salt. It precipitates out of solution when milk is acidified to a pH below 4.6, which is casein's isoelectric point. To isolate casein from milk, the milk is heated and acidified with acetic acid, causing the casein to coagulate out of solution. The casein precipitate is then collected via suction filtration and washed with an ether-ethanol solution before being dried.

1. Isolation of Casein from milk
Milk consist of three protine,one of them is casein. It is a globular protine. The phosphate groups
are attached to the hydroxyl groups of some of the amino acid side chains. Casein exists in milk
as the calcium salt, calcium caseinat. Calcium caseinate has an isoelectric point of pH 4.6.
Therefore, casein is insoluble in solutions of pH less than 4.6.The pH of milk is about 6.6,casein
has charge at this pH and is solubilized as a salt. If we change the PH of milk 6.6 to 4.6 which is
I.E point of casein, the neutral protein precipitates, with the calcium ions remaining in solution.
Casein, the protein predominantly found in milk, is present in cow’s milk to the extent of 3.0-
3.5% and in human milk to the extent of 0.3-0.6%. It gets precipitated by acidifying the milk to a
pH of 4.6-4.7. Casein is used to coat the paper for making books and magazines and also used in
textile industry for fixing colors.
Procedure:
Measure 50 mL milk by a graduated cylinder and put it in a 100 mL beaker. Heat the solution to
55°C, measure the temperature with thermometer.
Prepare 20 mL, 10 % acetic acid solution. Add drop wise the 10% acetic acid solution while
stirring
Collect the casein with suction filtration. Prepare 20 mL1:1 ethyl ether - ethanol solution and
then add this mixture to the casein precipitate.
Weigh the filter paper and collect the casein by suction filtration. Keep the casein and allow it to
dry
Discussion:
Casein is present 3.5% in the cow milk so the theoretical value is 3.5g /100ml (3.5%) but in our
experiment milk (tarang) contains 1.13%.
Reference:
Lab manual in biochemistry by Arti Nigam.