Fermentation
Fermentation is where wort becomes beer. After the chilled wort from The Boil is pitched with Yeast, the yeast consumes sugar and produces alcohol, carbon dioxide, and a host of flavor compounds. For an IPA, fermentation is also where Biotransformation reshapes hop character.
#What the Yeast Does
- Lag phase (0–12 h) — yeast absorbs oxygen and nutrients, reproduces.
- Active fermentation (1–4 days) — vigorous CO2, foam (krausen), rapid gravity drop.
- Attenuation (3–7 days) — fermentation slows as sugar runs out.
- Conditioning — yeast cleans up rough compounds like diacetyl.
#Pitching Rate and Yeast Health
Healthy yeast in sufficient quantity ferments cleanly and fully. Under-pitching causes stress, off-flavors, and stalled fermentation.
| Beer | Approx. pitch rate |
|---|---|
| Standard IPA (~1.060) | 0.75 million cells / mL / °P |
| Double IPA (~1.075+) | 1.0+ million cells / mL / °P |
A Double IPA needs a larger pitch — often a yeast starter — because of its higher gravity. See Double IPA Recipe.
#Temperature Control
Fermentation temperature is critical. Too warm produces harsh fusel alcohols and excess esters; too cold can stall it.
- Clean American ale yeast — 18–20 °C
- Hazy "juicy" strains — 19–22 °C, slightly warm for ester expression
- English ale yeast — 18–20 °C with characterful esters
Fermentation is exothermic — the beer self-heats several degrees above ambient during the peak. Plan cooling accordingly. See IPA Yeast Strains.
#Oxygen: Friend Then Foe
Yeast needs oxygen at pitching to build healthy cell walls — aerate the chilled wort well. After that, oxygen is the enemy: post-fermentation pickup causes Hop Fade and Oxidation. Never aerate fermented beer.
#Fermentation and Dry Hopping
Modern practice often adds dry hops during active fermentation (around day 2–4) so the active yeast drives Biotransformation, converting hop precursors into vivid new aromas. This intersects with Hop Creep and Refermentation — dry-hop enzymes can restart attenuation. See Dry Hopping.
Take two gravity readings 2–3 days apart. Stable readings mean fermentation is complete and the beer is safe to dry hop, package, and carbonate.