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.
Boiling brings out bitterness.
The alpha acids so prized in hops are of little use without a good boil. Boiling those acids isomerizes them, which is the chemist’s way of saying that they retain their molecular composition but change chemical structure, kind of like giving a few twists to a Rubik’s cube: You still have all of the same elements, but the structure has changed. Isomerization is the change that brings out the bitterness in alpha acids.
Boiling drives off unwanted volatile compounds.
The malting process creates a compound called s-methyl-methionine (SMM), a precursor to the dreaded dimethyl sulfide (DMS), which lends a corn-like flavor to certain beers, particularly lagers that rely on large amounts of Pilsner malt. A good, long, rolling boil drives off some of this SMM, reducing the opportunity for DMS to rear its ugly head in the finished beer.
Boiling coagulates proteins.
A nice boil encourages wort proteins to combine with tannins and form clumps. As these compounds grow in mass, they eventually reach a size at which they precipitate out of solution. Such precipitation is essential to making clear beer. This actually happens twice, first during the so-called “hot break,” when brown scum forms on top of the wort just before it starts to boil. The second, or “cold break,” occurs during the rapid chilling of hot wort right after the boil, when it takes on a miso soup appearance.
Keep in mind that with the right technique and some high alpha-acid hops, it’s entirely possible to brew an extract-based beer with only a short boil of 15 minutes or so. In such cases, however, the manufacturer has already boiled and concentrated a basic wort into liquid or dry malt extract, obviating many of the above needs for an extended boil. A brief boil is nonetheless required for purposes of sanitation and alpha-acid isomerization.