Mould is the most feared outcome in kombucha brewing, and justifiably so — a mouldy batch can’t be salvaged. But the vast majority of strange-looking things in or on kombucha are completely normal: yeast strands, brown patches, new SCOBY layers, and cloudiness are all part of a healthy, active ferment. The skill is learning to tell them apart confidently.
The core rule: wet vs fuzzy
The single most useful test is texture, not colour.
If you can’t decide from a distance, get closer — touch it gently with a clean spoon if you’re unsure. Fuzzy is unmistakable up close.
Normal things often mistaken for mould
Most “is this mould?” panics are one of these harmless features.
What actual mould looks like
Mould almost always appears on the top surface — never underwater, since it needs oxygen — and reads as clearly out of place.
- Fuzzy growth with a dry, powdery, or hairy texture
- Coloured — most often green, black, blue, or grey (white is possible and trickier)
- Discrete spots or patches sitting on the surface rather than integrated into it
- Sometimes an off-smell distinct from the normal tangy kombucha aroma
White mould is hardest to call because a new SCOBY layer can also look white. Fall back on the fuzzy test: smooth and wet is a new layer; dry, fuzzy, or powdery is mould.
If you see mould: what to do
There is only one right answer — discard the entire batch. Mould grows hyphae that reach below the visible surface, and some moulds produce mycotoxins the acidity won’t destroy.
- Do NOT scrape off the mould and keep fermenting
- Do NOT boil the SCOBY to "sterilise" it
- Do NOT use the liquid from underneath
- Do NOT save any starter liquid from this batch
Afterwards, wash the jar thoroughly with hot water and soap and rinse well. Start fresh with a spare SCOBY from a hotel if you have one, using a high starter-liquid percentage (around 20%) for a strongly acidic start; otherwise you’ll need a new culture.
How to prevent mould
Mould is almost always caused by too little acidity at the start. A brew that begins acidic enough is hostile to spores from day one.
- 1Use enough starter liquid At least 10% by volume, and ideally 20% for the first few batches or when your starter liquid is from a young batch.
- 2Sanitise all equipment Soap and hot water is usually enough; a no-rinse sanitiser like Star San adds extra protection.
- 3Use a secure breathable cover A tight-weave cloth under a rubber band keeps out spores, insects, and debris while still letting the SCOBY breathe.
- 4Brew away from mould sources Keep the vessel away from overripe fruit, damp areas, and spaces with active mould issues.
Make sure your starter-liquid ratio is high enough to acidify the brew and keep mould out.