Casein is precipitated from milk at

Casein is precipitated from milk at pH 4.6.
Casein is precipitated from skim milk by acidifying it to produce acid casein, or the milk is treated with rennet to produce rennet casein. The precipitated casein curd is separated from the whey, washed and dried.
For casein, the IEP is approximately 4.6 and it is the pH value at which acid casein is precipitated. In milk, which has a pH of about 6.6, the casein micelles have a net negative charge and are quite stable.