Why does orange juice cause milk to curdle

Why does orange juice cause milk to curdle  

We have already seen how milk is an emulsion comprising oil as a dispersion medium in a water-based dispersion medium. Milk fats also form colloids. The aqueous component of milk contains many vitamins, especially the salts of calcium, which baby mammals need to produce strong teeth and bones. 

Milk is an unusual colloid in comprising oil particles suspended in water. Adding, say, olive or sunflower oil to water will not produce a stable colloid. Two layers will re-form rapidly even after vigorous shaking, with the oil floating above the water. Milk is stable because it contains an emulsifier, i.e. a compound to promote the formation of a colloidal emulsion. 

We now look more closely at the structure of casein. It is a long molecule with different ends one end is polar and the other is nonpolar. In milk, the polar group (ending with a phosphate group) is positioned to face the polar water, and the non-polar end faces the oil. In effect, each particle of oil has a double coating the inner layer is the non-polar end of the casein emulsifier, and the outer layer is a sheath of polar phosphate groups. 

Each colloid particle is surrounded with an electric double layer. 

---