Brewers make wort; yeast makes beer. This single-celled fungus consumes sugar and releases alcohol, carbon dioxide and a whole palette of flavour compounds. Choose a different strain and the same wort can become a fruity English bitter, a clove-scented Hefeweizen or a razor-clean lager.
What Happens During Fermentation
Fermentation unfolds in rough phases:
- Lag phase (a few hours to a day) — Yeast absorbs oxygen and nutrients, building healthy cell walls before getting to work. Little visible activity.
- Growth / active phase (1–5 days) — Yeast multiplies and ferments vigorously. A foamy head called krausen forms, the airlock bubbles, and the temperature rises on its own.
- Conditioning phase (days to weeks) — Activity slows. Yeast cleans up by-products such as diacetyl and acetaldehyde, then settles out, or flocculates, to the bottom.
The basic reaction: glucose becomes roughly equal parts ethanol and carbon dioxide by molecule count, plus heat.
Ale Yeast vs Lager Yeast
Ale Yeast (Saccharomyces cerevisiae)
Ale yeast ferments warm, typically 15–24 °C. Traditionally it rises to the top of the fermenter, earning the name “top-fermenting.” Warm fermentation encourages esters (fruity notes like pear, banana or apple) and phenols (spicy, clove-like notes in some strains).
Lager Yeast (Saccharomyces pastorianus)
Lager yeast is a hybrid that likely arose from S. cerevisiae and a cold-tolerant wild species, S. eubayanus. It works best at 7–13 °C and tends to settle at the bottom (“bottom-fermenting”). Cooler temperatures mean fewer esters and a cleaner, crisper beer — but fermentation takes longer and is followed by weeks of cold storage, or lagering (from the German word for “to store”).
| Trait | Ale Yeast | Lager Yeast |
|---|---|---|
| Species | S. cerevisiae | S. pastorianus |
| Fermentation temp | 15–24 °C | 7–13 °C |
| Speed | Fast (days) | Slower (1–3 weeks + lagering) |
| Flavour | Fruity, sometimes spicy | Clean, crisp, malt-forward |
| Traditional behaviour | Top-fermenting | Bottom-fermenting |
| Example styles | Pale ale, stout, IPA, Hefeweizen | Pilsner, Helles, Märzen, Bock |
The In-Betweens
- Kölsch and Altbier use ale yeast fermented cool for a lager-like finish.
- California Common uses lager yeast at warmer temperatures.
- Kveik, a family of Norwegian farmhouse yeasts, ferments cleanly at astonishing temperatures of 30 °C and above.
Wild Yeast and Bacteria
Not every beer relies on tidy lab strains:
- Brettanomyces adds barnyard, leathery and tropical-fruit funk
- Lactobacillus produces lactic acid for tart styles like Berliner Weisse and Gose
- Pediococcus creates deeper sourness in lambic-style beers
Belgian lambic producers traditionally let wild microbes inoculate wort in open cooling vessels called coolships — a practice known as spontaneous fermentation.
Key Fermentation Terms
- Attenuation — The percentage of sugar the yeast consumes. High attenuation means a drier beer.
- Flocculation — How readily yeast clumps and drops out. High-flocculating strains clear fast.
- Pitch rate — How much yeast you add. Too little can cause stress and off-flavours.
- Original and Final Gravity (OG/FG) — Density readings before and after fermentation, used to calculate alcohol.
A handy estimate: ABV ≈ (OG − FG) × 131.25. So a beer going from 1.050 to 1.010 lands around 5.25%.
Temperature Control Is Everything
Most off-flavours in homebrew trace back to temperature. Fermentation generates heat, so the beer can run several degrees warmer than the room. Too warm and you get harsh fusel alcohols and excessive esters; too cold and yeast may stall.
Simple fixes include:
- A cool basement or closet
- A water bath with frozen bottles swapped daily
- A dedicated fridge with an external temperature controller
Keeping Yeast Happy
- Sanitise everything that touches cooled wort — wild microbes love sugar as much as your yeast does
- Aerate the wort before pitching (not after fermentation starts, when oxygen causes staleness)
- Pitch enough healthy yeast — make a starter for big beers or lagers
- Be patient — let the yeast finish and clean up before packaging
Safety Note
Fermentation produces pressure. Never seal an actively fermenting vessel without an airlock or blow-off tube, and use proper equipment rated for any pressure you intend to hold. And while fermenting beer at home is legal in many countries, distilling it is not in most — never try to distill at home.
Treat your yeast well and it will reward you with cleaner, tastier beer.