Fermentation basics
How yeast turns wort into beer, what temperature and pitch rate do, and what to watch for during primary fermentation.
Last updated 10 April 2026 · 8 min read
What’s happening in the fermenter
Yeast metabolises wort sugars into ethanol, carbon dioxide and a long list of flavour-active byproducts. The yeast strain, the pitch rate, and — above all — the fermentation temperature determine how much of each byproduct shows up in the finished beer.
Pitch rate
A starter pitch that’s too small forces the yeast to over-budget on cell-growth metabolism, which produces more esters, fusel alcohols and diacetyl. As a rough rule of thumb:
| Beer strength | Target pitch (billion cells per litre) |
|---|---|
| Low-gravity ale (OG ≤ 1.050) | 0.75 |
| Standard ale (OG 1.050–1.070) | 1.0 |
| High-gravity ale or lager | 1.5+ |
Temperature
Most ale yeasts ferment cleanest at the lower end of their published range. S-04 will work at anything from 15 to 24 °C, but at 24 °C you’ll get pronounced fruity esters; at 17 °C you’ll get a much cleaner beer. Pitch cool, then let the temperature rise slightly during peak fermentation to keep the yeast active without producing off-flavours.
When fermentation is done
Use gravity, not time. Take a hydrometer or refractometer reading on three consecutive days. If it doesn’t move, fermentation is complete.