If you can see foam on top of your wort, fermentation is happening. The foam may look dramatic — sometimes climbing up the inside of the fermenter, sometimes spilling out into the airlock — but it's a sign your yeast is doing exactly what it should. The thicker the foam, the healthier the fermentation.
"There's brown foam on top of my beer and I'm worried." This is one of the most common first-time-brewer concerns, and the answer is almost always "that's krausen, it's meant to be there, and it's actually good news." This article walks through what krausen is, what it looks like at each stage of fermentation, and the rare cases where what you're seeing isn't krausen at all.
What krausen actually is
Krausen (pronounced "kroy-zen") is the foam layer that forms on top of fermenting wort when yeast is actively producing CO2. The foam is a mixture of:
- CO2 bubbles rising from the wort and getting trapped at the surface
- Yeast cells riding the bubbles up to the surface
- Hop oils and resins that float to the top during agitation
- Wort proteins that stabilise the foam and give it structure
The result is a thick, off-white to brown foamy layer that can be anywhere from 1cm to 8cm thick depending on the stage of fermentation. The colour ranges from creamy white in the early hours to a darker brown-orange at peak fermentation as hop compounds and yeast colour it.
The four stages of krausen
What you see in the fermenter changes predictably over the first 7-10 days. If you have a translucent fermenter, you can watch this progression. Each stage looks different.
Stage 1: Lag phase (Day 0-1)
Immediately after pitching yeast, nothing visible happens. The wort surface looks like wort — clear, possibly with some debris from the hops or yeast sachet, but no foam. The yeast is rehydrating, taking up oxygen, and multiplying. This phase lasts 12-36 hours.
Stage 2: Krausen formation (Day 1-2)
A thin, foamy layer starts to appear on the wort surface. Initially patchy — small islands of foam in the middle and around the edges — then expanding to cover the whole surface. Colour is mostly white at this stage, sometimes pale yellow. Bubbles rising from below are visible through the foam.
Stage 3: Peak krausen (Day 2-4)
This is the dramatic stage. The foam thickens to 3-8cm, forms cauliflower-like rocky peaks, and develops a brown or rusty-coloured layer on the very top. This brown tint comes from hop oils and is normal — not a sign of contamination. The foam may climb the sides of the fermenter, and in particularly active ferments, push up into the airlock or even blow off through the lid.
If you see this happening, it's a sign of a very healthy fermentation, not a problem. You may need to clean and resanitise the airlock if foam reaches it.
Stage 4: Krausen falling (Day 4-7)
The foam starts to collapse. As CO2 production slows, the bubbles supporting the foam pop and the krausen layer drops back into the beer. What remains is a brown ring on the inside of the fermenter (called "krausen residue") and a thinning surface foam. The wort below starts to clear as yeast settles.
Stage 5: Fermentation slowing (Day 7+)
By day 7-10, surface foam is largely gone. The wort surface looks clear again, sometimes with a thin haze. A pale tan layer of yeast is visible at the bottom of the fermenter (this is normal sediment). Activity has slowed dramatically. Fermentation is approaching completion.
Healthy krausen is rocky, off-white with a brown top layer, and dramatic. The drama is the feature, not the problem.
What healthy krausen actually looks like
At its peak, healthy krausen has these characteristics:
- Colour: Off-white to pale tan on the main body. Brown to rusty-orange layer on the very top.
- Texture: Thick, rocky, cauliflower-like. Not smooth or flat. The bumpy texture comes from constant bubble formation pushing the foam upward.
- Thickness: 2-8cm at peak, depending on yeast strain and gravity. Wheat beers and Hazy Pale Ales typically produce thicker krausen than clean American Pale Ales.
- Behaviour: Continuously moving. Bubbles can be seen forming and bursting at the surface. The whole layer slowly rises and falls.
- Smell: Sweet, malty, yeasty — smells like the wort did when you pitched, plus a bit of bread dough.
What's NOT a problem (common false alarms)
The rusty brown coating at the very top of the krausen looks alarming to first-timers but is just hop oils and resins floating on the foam surface. Completely normal. Pale Ales and IPAs with heavier hopping show this more prominently.
Normal — do nothing
If fermentation is vigorous, krausen can push up into the airlock and even out through it. This is sometimes called a "blowoff." It looks dramatic but isn't damaging. Clean and resanitise the airlock, or temporarily replace it with a length of tubing leading into a container of sanitiser solution.
Normal — just clean up
Krausen often falls in patches rather than uniformly. You might see partial foam coverage with some areas clear. This is normal — CO2 production isn't uniform across the surface.
Normal — do nothing
After krausen falls, a brown ring is left at the highest point the foam reached. This is "krausen residue" — dried hop compounds and yeast. Doesn't affect the beer at all. Comes off easily during cleaning.
Normal — clean off later
What WOULD be concerning
Genuinely worrying signs are rare and look different from healthy krausen.
After day 7-10, when krausen has fallen, a fresh thin film forming on the surface is different from regular krausen. Smooth rather than rocky, persistent rather than active. This can indicate wild yeast or bacterial contamination.
Possible contamination
A "pellicle" — wrinkled, leathery, sometimes bubbly — is a sign of wild yeast contamination. Distinct from rocky krausen because of the leathery texture.
Confirmed contamination
Green, black, or blue patches on the surface or fermenter walls are mould, not krausen. Mould has a distinct fuzzy appearance with colour, unlike the uniform off-white to brown of healthy fermentation.
Mould — dispose of batch
For more on contamination signs and what to do about them, see our is my homebrew contaminated guide.
If you can't see inside your fermenter
Many homebrew fermenters are opaque plastic — you can't see the krausen directly. In that case, you can't observe stages, but other signals tell you fermentation is progressing:
- Airlock activity — bubbling indicates active fermentation, though absence of bubbling doesn't mean fermentation isn't happening (see our not bubbling guide).
- Lid lifted slightly — some opaque fermenters have lids that visibly bulge during peak fermentation.
- Krausen on the lid underside — when you eventually open the fermenter, the underside of the lid often has dried krausen residue, confirming foam reached that level.
- Hydrometer readings — the only definitive measure. Two readings 48 hours apart that show gravity dropping confirm active fermentation regardless of what's happening visually.
The hydrometer is more reliable than any visual signal for confirming fermentation health. The visual stages are useful for understanding the process and for diagnosis when something looks unusual, but they're not strictly necessary for a successful brew.
For broader fermentation troubleshooting, see why is my homebrew not bubbling and how to fix a stuck fermentation. For the full brewing process, see how to brew from a fresh wort kit.