Krausen and trub and yeast cake explained for new brewers

Three words every brewer uses

Open any homebrew forum and you will read about krausen, trub and yeast cake. They describe what you see at different stages of fermentation. Once you know what they are, they become useful signals rather than worrying mysteries.

Krausen: the foam on top

Krausen is the thick, rocky foam that forms on top of fermenting beer. It is a mix of yeast, proteins, hop resins and CO2 bubbles pushing up from below. It usually appears within 12 to 48 hours of pitching, rises to a peak called high krausen and then falls back as fermentation slows.

Its colour changes as it goes. Early krausen is creamy white. As it matures, brown and green-tinged patches appear from hop resins and proteins. That is normal. When it falls, it often leaves a brown ring on the fermenter wall above the beer line.

In warm Indian rooms, krausen rises faster and higher than recipe books suggest. A 20 litre batch at 24 °C can push foam through the airlock in a day. Leave at least 20% headspace, or fit a blow-off tube into a jar of sanitiser for the first few days.

Some yeasts produce very little krausen. Lager strains and some clean ale strains are modest. A thin krausen does not mean fermentation failed. Check gravity instead. Our guide on how long beer takes to ferment explains what to measure.

Trub: the sludge from the kettle

Trub (pronounced "troob") is sediment made of proteins, hop debris and fats. It forms in two stages:

• Hot break forms during the boil, as proteins clump together into flakes.
• Cold break forms when the wort cools quickly, as more proteins and polyphenols drop out.

Some trub always ends up in the fermenter. A moderate amount does no harm and may even give the yeast useful nutrients. Large amounts can add a little harshness and make the yeast cake messy. Leaving the thickest sludge behind in the kettle, by whirlpooling or letting it settle before transfer, is good practice.

Yeast cake: the layer at the bottom

As fermentation finishes, yeast cells flocculate, meaning they clump together and settle. They form a creamy, beige layer at the bottom of the fermenter, on top of the trub. That is the yeast cake.

It tells you fermentation is winding down, but not that it is finished. The only reliable way to know is to measure gravity on two or three consecutive days. If the reading does not change, the beer is done.

How long can beer sit on the yeast?

Leaving beer on the yeast cake for a week or two after fermentation finishes is usually good for it. The yeast cleans up by-products such as diacetyl and acetaldehyde. Most homebrew ales are happy on the cake for two to three weeks in total.

Much longer, especially in warm conditions, raises the risk of autolysis. That is when dead yeast cells break down and release meaty, savoury or rubbery flavours. At home, at sensible temperatures, it takes weeks to months to become noticeable. In an Indian summer without temperature control, do not leave beer sitting on yeast longer than you need to.

Reusing the yeast cake

The yeast cake is full of healthy yeast. Many brewers reuse it for the next batch, which saves money and often gives a lively start. A few rules:

• Use yeast from a clean, healthy fermentation only.
• Use a portion, not the whole cake. Pitching onto the entire cake usually over-pitches.
• Go from lighter to darker beers and from lower to higher gravity, not the other way round.
• Store harvested yeast cold and use it within a couple of weeks.

Our guide to yeast harvesting and reuse covers the method in detail.

What to worry about

Most things you see in a fermenter are normal. Things that deserve a closer look include fuzzy, raised spots on the surface (mould), a thin white skin with a wrinkled or web-like texture after fermentation (pellicle, often from wild yeast or bacteria) or a strong vinegar or rotten smell. If you see any of these, taste a small sample before you decide.

Understanding what yeast is doing at each stage lets you plan fermentation rather than wait and hope. The yeast growth phases guide explains the biology behind it.

Common questions

Is brown krausen a problem?

No. Brown and green-tinged patches come from hop resins and proteins and are completely normal.

Should I stir the krausen back into the beer?

No. Leave it alone. It falls on its own when fermentation slows. Some of the bitter hop resins stick to the fermenter wall, which is fine.

Can I bottle as soon as the yeast cake forms?

Not without checking. Measure gravity on two or three consecutive days. If it is stable, fermentation is complete.