The boil — bitterness, sterilisation, and DMS

What the boil does, why 60 minutes (or 90 for Pilsner malt), and how to manage evaporation and DMS.

What the boil accomplishes

A vigorous wort boil does five things simultaneously:

Isomerises hop alpha acids, making them bitter
Sterilises the wort (the only true sterilisation step on brew day)
Coagulates proteins (the “hot break”), which clarifies the finished beer
Drives off DMS — a sweet-corn off-flavour from Continental Pilsner malts
Concentrates the wort through evaporation

Why 60 minutes for most beers

The 60-minute boil is a compromise between:

Long enough to hit good hop utilisation (most isomerisation in first 20 min, decay after 45)
Long enough to drive off significant DMS
Short enough to not over-darken the wort or waste energy

Why 90 minutes for Pilsner-malt-heavy beers

Pilsner malt is lower-kilned than pale ale malt, retaining more of an SMM (S-methylmethionine) precursor. SMM thermally degrades into DMS during the boil. A 60-minute boil isn’t long enough to drive off DMS from heavy Pilsner-malt grists; 90 minutes is.

Boil-off rate

A vigorous open boil evaporates 8-12% of starting volume per hour. So:

Starting wort	After 60 min vigorous boil	After 90 min
23 L	21 L	20 L
26 L	24 L	23 L

Account for boil-off when planning your pre-boil volume.

Hop additions

See hop usage for the full breakdown. In brief:

60-min bittering — for the IBU
15-min — flavour
5-min and flame-out — aroma
Whirlpool / hop stand at 70-85 °C — aroma without isomerisation

Lid policy

Lid off during the boil. A covered boil traps DMS volatiles. This matters most with Pilsner malt; less so with pale ale base malts.
Lid on while cooling. Once below 80 °C, you want to keep airborne contaminants out.