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RennetNormal: a smooth, moist, pale skin on the surface, translucent or milky patches in the new layer, bubbles underneath, brown or almost black strings hanging into the tea, and a smell of apple and vinegar. Mould: dry, fuzzy, round spots sitting on top of the skin, often green, blue, black or white and hairy, with a musty smell like a damp cellar. If it really is mould, everything goes: tea and SCOBY. Start again with a clean jar and a fresh culture.
Why a SCOBY looks so strange
A SCOBY is a living skin of bacteria and yeasts, held together by a mat of cellulose that the bacteria make themselves. It is not smooth like a plastic disc, and it is not the same thickness everywhere. Where yeasts gather, you see brown patches. Where gas rises, bubbles stay trapped under the skin. And a new layer starts as a thin, see-through film that slowly turns milky and firm.
To an untrained eye all of this looks suspicious, and yet it is exactly what should happen. How a SCOBY is put together is explained in detail in What is a kombucha SCOBY?
Normal: all of this belongs
| What you see | What it is |
|---|---|
| A thin, see-through film on the surface | The new SCOBY forming, often from day 3 or 4 |
| Milky white, slightly dull patches in the skin | Young culture thickening; they feel moist and smooth |
| Brown or almost black strings under the SCOBY | Strands of yeast, a sign of a lively culture |
| Brown specks and edges in the layer | Yeast caught up in the skin |
| Bubbles under or inside the skin | Gas that cannot escape |
| Holes, bumps, a lopsided or wrinkled layer | Simply the shape of this layer, often because the jar was moved |
| Sediment at the bottom of the jar | Yeast that has settled; leave it or pour it away |
| A SCOBY that sinks or hangs sideways | No matter, the new layer forms at the top |
The colour can give you a fright too. With black tea, an old SCOBY is often brown to dark brown; with green tea, it stays more of a cream colour. A dark layer is simply old, not spoiled. If your jar smells fresh, of vinegar and apple, you are almost always fine.
Day by day: what to expect
Knowing how a normal batch develops makes it much less alarming.
| When | What you usually see |
|---|---|
| Days 1 and 2 | Clear, dark tea. The SCOBY sinks or floats and nothing seems to happen. |
| Days 3 and 4 | A greasy-looking sheen on the surface, sometimes small islands that join up. |
| Days 5 to 7 | A thin, see-through skin across the whole surface, with bubbles beneath. The smell turns sharper. |
| Days 7 to 10 | The skin is milky and a few millimetres thick. Brown strings hang down and the tea is paler. |
In a cool kitchen everything moves a few days later. If after a week you see no skin at all, but no mould either, the jar is probably too cold. Move it somewhere warmer and give it time.
Mould: what it looks like
Mould is rare in kombucha, but it happens. It always grows on top, in contact with the air, never in the tea itself. The signs:
- Dry and fuzzy. It looks like the mould on a slice of brown bread forgotten in the bread bin: a downy tuft, not a wet skin.
- Round and raised. It starts as small round spots sitting on top of the skin and slowly spreads.
- Coloured. Green, blue-green, black, sometimes white and hairy.
- Musty. It smells of a damp cellar rather than of vinegar.
Still not sure? Shine a torch across the surface from the side. Mould stands up with tiny hairs. A young SCOBY lies flat and glistens. And if you are still in doubt, send us a photo through the contact page.
And that white, wrinkly film?
Now and then a thin, dull white film appears, slightly wrinkled or powdery, lying flat on the surface. That is usually kahm yeast, a wild yeast from the air that turns up mainly when the tea is not acidic enough or the jar is too cold. It is not mould: it is flat, not fuzzy, and not coloured. It does give a flat, slightly musty taste.
Lift the film off gently with a clean spoon and pour this batch off a little earlier. For the next one, use a full cup of strong starter liquid and a warmer spot. If it keeps coming back, or you are honestly unsure, start again. Better one batch too many in the sink than a doubtful jar in the fridge.
What to do about real mould
1. Throw it all out
Pour away the tea and throw out the SCOBY. Mould goes deeper than you can see, and scraping it off does not help.
2. Clean the jar well
Wash the jar, cloth and spoons in hot water with washing-up liquid, rinse them very thoroughly and let them dry.
3. Start again
Take a SCOBY from your SCOBY hotel if it looks sound and clean, or begin with a fresh kombucha SCOBY with starter liquid.
4. Deal with the cause
Check the table below for what went wrong, so it does not happen again.
Why it went wrong, and how to prevent it
| Cause | Why it matters | What to do |
|---|---|---|
| Too little or too weak starter liquid | The tea is not acidic enough in the first days and is left unprotected | Always a full cup (about 250 ml) of finished, sour kombucha per litre |
| A cold spot | The culture works slowly and mould gets a head start | 20 to 25°C; in winter, a warmer room |
| Tea still warm when the SCOBY went in | The SCOBY is damaged and starts badly | Let it cool completely |
| Fruit or herbs in the first jar | Pieces sticking out of the liquid go mouldy quickly | Flavour only in the second fermentation |
| Fruit flies | They bring spores with them | A close-woven cloth or kitchen paper with a tight elastic band |
| A damp room | Spores thrive where the air stays humid | Keep the jar away from a steamy kitchen window or a damp utility room |
The best protection is acidity. A batch that is acidic enough from the start gives mould almost no chance. That is why we always send a SCOBY with its own starter liquid, and why you keep a cup back from every batch. The full basic recipe is in How to make kombucha.
In the bottle too
In swing-top bottles a small, jelly-like disc sometimes grows at the top, or brown threads float about. That is normal: the culture carries on working in the bottle. Strain it out as you pour. Fruit that sticks up out of the liquid into the air can go mouldy, though, so push it under or cut it smaller. More about bottling is in Kombucha: the second fermentation.
Frequently asked questions
My SCOBY has brown patches. Is that mould?
Can I just scoop off a spot of mould?
The new layer has holes in it. Is that right?
How likely is mould?
Can I rinse a mouldy SCOBY in vinegar and keep it?
Had to start again? A fresh kombucha SCOBY (£16) arrives in good form with its own cup of starter for a safe start. More about kombucha is on the kombucha page.