Yeast Fermentation: The Biology Behind Beer
Yeast turn the sugars in wort into alcohol and carbon dioxide, plus flavour compounds. Here are the phases of fermentation, what yeast need, and how temperature shapes the beer.
In this post
The short answer: brewer’s yeast (Saccharomyces) eat the sugars in wort, mainly maltose, and turn them into ethanol and carbon dioxide. For each glucose unit, that’s C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. Along the way they make small amounts of flavour compounds such as esters, phenols and fusel alcohols. A fermentation runs in phases, and yeast need oxygen, nutrients and the right temperature to do it cleanly.
| Phase | Typical timing (ale) | What happens |
|---|---|---|
| Lag | 3–15 hours | Yeast take up oxygen and nutrients and build healthy cell membranes; little visible activity |
| Growth and active fermentation | Days 1–5 | Yeast multiply and ferment hard: krausen, CO₂, heat, most of the flavour compounds |
| Slowdown | Days 4–10 | Sugar runs low; yeast begin to settle |
| Conditioning | Days to weeks | Yeast reabsorb byproducts such as diacetyl and acetaldehyde, then drop clear |
Lagers go through the same phases, only slower, at 48–55°F (9–13°C).
What yeast are
Yeast are single-celled fungi, about 5–10 micrometres across. Beer uses two main species:
- Saccharomyces cerevisiae, ale yeast. It’s the same species bakers use, in strains selected over centuries for brewing.
- Saccharomyces pastorianus, lager yeast. It’s a natural hybrid of S. cerevisiae and the cold-tolerant S. eubayanus, which is why it keeps working in cold wort.
Yeast multiply by budding: a daughter cell grows out of the mother and splits off. They can live with or without oxygen, but in sugary wort they ferment either way. More on that in is fermentation aerobic or anaerobic?
What happens inside the cell
- Sugar goes in, in order. Yeast take up the simple sugars first: glucose, fructose, and sucrose, which they split outside the cell. Then maltose, the biggest share of wort sugar, and maltotriose last. Longer chains (dextrins) stay behind and give the beer body.
- Glycolysis. Each glucose is split into two molecules of pyruvate. The cell gains 2 ATP, its energy currency.
- Fermentation. Pyruvate gives off CO₂ and becomes acetaldehyde, which the cell turns into ethanol. That last step recycles the cell’s energy carriers so glycolysis can keep running.
- Ethanol and CO₂ leave the cell. The CO₂ bubbles out through the airlock, or stays dissolved if you ferment under pressure.
Two ATP per glucose is a poor return (respiring with oxygen gives about 15 times more), so yeast run a lot of sugar through to live. That’s what brewers want. By weight, the products split almost evenly: 180 g of glucose becomes 92 g of ethanol and 88 g of CO₂. A 5 gal (19 L) batch at 5% ABV makes about 750 g of alcohol and 700 g of CO₂, several hundred litres of gas. The reaction also releases heat, which is why a fermenter warms up at its peak.
What yeast need to ferment cleanly
| Need | Why | Typical target |
|---|---|---|
| Enough cells | Too few means a long lag, more esters and more diacetyl | 0.75 million cells per mL per °Plato for ales, 1.5 for lagers |
| Oxygen, at pitching only | Yeast use it to build sterols and fatty acids for their cell membranes | 8–10 ppm for ales, 10–12 ppm for lagers and strong beers |
| Nutrients | Amino acids, vitamins and minerals such as zinc for growth | All-malt wort usually has enough; add yeast nutrient for big or sugar-heavy worts |
| Right temperature | Sets the speed and the flavour | Most clean ale strains 62–68°F (17–20°C); lager strains 48–55°F (9–13°C) |
In real numbers, a 5 gal (19 L) batch of 1.050 ale needs about 175 billion cells; the same wort as a lager needs about 350 billion. That’s why lagers and big beers usually get a yeast starter. For the oxygen side, see how to aerate wort.
Where the flavour comes from
Most fermentation flavour forms while the yeast are growing, in the first two or three days. That’s the window where temperature control matters most.
| Compound | Tastes or smells like | You get more when |
|---|---|---|
| Esters | Banana, pear, apple; solvent when very high | Fermenting warm, underpitching, high gravity, low oxygen; some strains by design |
| Fusel alcohols | Hot, solvent-like | Fermenting warm early on; stressed yeast |
| Phenols | Clove, spice | Strains bred for it (hefeweizen, Belgian, saison); otherwise wild yeast |
| Diacetyl | Butter, butterscotch | Underpitching; crashing before the yeast clean up |
| Acetaldehyde | Green apple | Taking the beer off the yeast too early |
| Sulphur compounds | Rotten egg, struck match | Lager strains, stressed yeast; fades with time |
Warmer fermentations run faster and make more esters and fusels; colder ones are cleaner and slower, and yeast that get too cold can drop out before they finish. The usual plan is to pitch at the low end of the strain’s range, hold it through the first three days, then let it rise a few degrees to finish. Hold the beer, not the room: a jacketed fermenter with a temperature control kit does it automatically.
How fermentation ends
As the sugar runs out, yeast take back up the diacetyl and acetaldehyde they made earlier, then clump together (flocculate) and settle. How much of the sugar they eat is the attenuation: most strains reach about 65–85% apparent attenuation, depending on the strain and how fermentable the wort is.
It’s done when the gravity holds steady for 2–3 days. Give it a diacetyl rest before you cold crash. Healthy yeast from the bottom can be harvested and reused. Beer left warm on dead yeast for months can pick up meaty, rubbery flavours from autolysis.
Frequently asked questions
Is yeast fermentation aerobic or anaerobic?
Anaerobic. Yeast ferment sugar without using oxygen. They only use oxygen at the start, to build cell membranes, which is why you aerate chilled wort and then keep air out.
How long does fermentation take?
A normal ale ferments hard for 3–5 days and is usually finished in 1–2 weeks. Lagers take 2–3 weeks, then weeks of cold lagering. Big beers take longer.
Why isn’t my fermenter bubbling?
It may still be in the lag phase, or CO₂ is escaping through a lid or fitting. Take a gravity reading: if it’s dropping, the yeast are working. See troubleshooting fermentation problems.
What happens to the yeast after fermentation?
Most go dormant and settle to the bottom. They’re still alive, which is how bottle-conditioned beer carbonates and why you can reuse them.
Does fermentation carbonate the beer?
Only a little: with an airlock, most of the CO₂ escapes. Carbonate by bottling with priming sugar, force-carbonating in a keg, or closing a spunding valve near the end of fermentation. See pressure fermentation.
Related guides
Ale vs lager fermentation, dry vs liquid yeast, oxygen and oxidation, fermenting big beers and making a yeast starter.