Why and how do milk and water get separated when lime juice is added?

Milk contains an average of 13 per cent solids and 87 per cent water. Milk solids include fat, proteins, carbohydrates, minerals and vitamins.

In milk, fat is in emulsion form, proteins in colloidal form and carbohydrates and minerals in true solution. The milk protein, casein, is present in milk as calcium caseinate in combination with calcium.

These calcium caseinate particles are in equilibrium with milk serum in which they are suspended. Thus any change in ionic concentration of milk serum will disturb the colloidal stability of calcium caseinate particles.

When lime juice is added to milk, the citric acid present in lime reduces the pH value and changes the ion concentration of milk serum which alters the equilibrium. When pH value becomes 4.7 which is the iso-electric point, the protein (calcium caseinate) gets precipitated and separated from milk solution leaving the watery portion.

This precipitated protein mass is called channa/cakka and the watery portion which contains dissolved carbohydrates and water soluble vitamins is called whey.

Source: The Hindu