Question

​​​Milk contains lactic acid but it is a insulator Why?

Answer 1

Actually milk is not an insulator.
Please remember, For any solution to conduct, you need ions or free electrons.
Milk is a good conductor of electricity because it contains water and lactic acids and other salts.

Electrical conductivity of milk is mainly due to its soluble salt fraction. Lactose does not conduct current, and fat decreases conductivity. The contribution of proteins and peptides is of minor importance.
In addition, pH decrease causes hydrogenation of monohydrogen phosphate ions to dihydrogen phosphate ions, which have lower molar conductivity. Thus, addition of lactic acid to a phosphate buffer solution (pH 6.5) decreases conductivity. However, fermentation of lactose to lactic acid sharply increases conductivity; also, acidification of milk changes equilibria of buffer systems and solubilizes casein-bound calcium and phosphorus salts. This phenomenon increases conductivity sharply, as shown by the acidification of diafiltered milk that is free from soluble salt fractions.
The relationships among such equilibria increase the electrical conductivity of milk by acidification, which is why milk is a suitable substrate for the automatic monitoring of lactic acid bacteria growth by conductimetric methods.