פירוש על עבודה זרה 69:3

The Talmud asks why R. Joshua did not respond that the reason the cheese is only prohibited in eating and not in benefit is that the rennet is no longer visible. This is like the murias (fish brine) into which wine was put. It is prohibited but only in consumption because the wine is not visible.
The answer is an important distinction in halakhah—the rennet is an active agent, it is what makes the cheese curdle. Therefore, even though one cannot see it, it is not nullified.