Pasteurization is a process most commonly done to milk, where something is mildly heated to extend its shelflife by destroying some bacteria. The process was invented by Louis Pasteur.
This sounds like pastor. The character makes a joke, by intentionally mistaking a pastor as someone who pasteurizes milk.
Haven't watched the series, but since the lawyer says "Padre" when addressing the priest, I'm assuming at least one of them speaks Spanish.
In Spanish (maybe in other languages as well), "pastor" means shepherd, and shepherds take care of milk-producing animals, so "go boil some milk" could be understood as "mind your own ...