In mastitic milk, the permeability of mammary epithelial tight junctions increases, allowing blood sodium and chloride ions to pour into the milk, elevating chloride concentration from normal ($0.08\text{ to }0.12\%$) to $\gt 0.14\%$. The Mohr titration method using standard silver nitrate ($\text{AgNO}_3$) and potassium chromate ($\text{K}_2\text{CrO}_4$) indicator is the classic chemical screening test for mastitic milk.

Option analysis:

• Detection of mastitic milk: Correct. Silver nitrate and potassium chromate are used in chloride estimation for diagnosing mastitis.
• Differentiating faecal and non faecal coliforms: Tested via IMViC tests and elevated temperature incubation ($44.5^\circ\text{C}$ in EC broth).
• Assessing activity of starter cultures: Tested via horner/acid development test using standard sodium hydroxide titration.
• Differentiating Gram positive and Gram negative bacteria: Accomplished using crystal violet, iodine, alcohol, and safranin.