The isoelectric point ($\text{pI}$) is the specific $\text{pH}$ at which a protein carries no net electrical charge. For milk proteins, particularly caseins, minimal solubility and maximum aggregation (coagulation) occur at their isoelectric point.
Milk coagulation is influenced by $\text{pH}$ changes.
Therefore, the isoelectric point of milk, which results in maximum coagulant activity (or minimum solubility leading to precipitation/coagulation), is 4.6.