The boil is the stage of the brewing process during which wort is boiled in the brew kettle and hops could be added. When hops are added to the boil, hop resin/alpha and beta acid isomerization occurs, which imparts bittering and hop aroma in the finished beer. A typically boil time lasts between 60 and 90 minutes. The longer the hops’ isomerization in the boiling wort, the greater the potential for bittering that exists. In addition to hop isomerization, the boil also sterilizes the wort, denaturing the enzymes that were active in the mash. The boil is also responsible for the hot break, which removes several unwanted compounds that can cause both unwanted flavors in chill haze.
Below is a photo of a boil kettle a few minutes after the boil was achieved.