If we're talking about medieval beer production, it certainly started with boiling the water.

In absence of temperature measurement, they would time how long the water had been off the boil before it would be used.

They also commonly believed that the boil was integral to the carbonation in the final product. That the amount of time spent boiling the water correlated to more bubbles in the beer, and would often do things like boil the water until it was half gone.