The boil is such a standard part of brew day that many recipes only mention it implicitly in indicating the intervals at which hops are to be added. Grists that use a good deal of Pilsner malt might call for a 90- to 120-minute boil, but that’s about as specific as it usually gets. But the boil is an essential life stage through which every beer must pass on its way from grain to glass. Here’s why it deserves your attention.
Boiling stops enzymatic activity.
All of those mash enzymes that pry apart proteins and snip starches have a temperature above which they denature and become nothing more than chemical soup. The mash-out does a pretty good job of stopping enzymes in their tracks, but boiling hammers the last nail into the coffin, ensuring a fixed wort composition going into the fermentor.
Boiling sanitizes wort.
Lactobacillus bacteria just love malt. If you don’t believe me, just leave your spent grain in the mash tun after you brew and come back for a sniff a couple of days later. That’s what your wort would start to smell like if you didn’t first boil it. Boiling eliminates populations of unwanted microbes, thus creating a blank canvas for your chosen Saccharomyces strain to do its thing.