Mould in kombucha: how to recognise it (and what isn't mould)

You open the jar, look at the surface, and see something strange. A white clump. A dark patch. Something that wasn’t there yesterday.

The instinctive fear is mould. And that fear is justified - mould is the only problem in kombucha that can’t be fixed. You throw it away.

But 90% of the time, what looks like mould isn’t.

The list of things that look alarming but are harmless

New SCOBY forming

In the first 24–72 hours, a thin translucent, whitish layer begins to form on the liquid’s surface, sometimes with bubbles trapped inside. It can look gelatinous, opaque, or “patchy”. This is the new baby SCOBY - proof that fermentation has started correctly.

Characteristics: smooth or slightly granular surface, never fuzzy or furry. Smells like kombucha, not mould.

Yeast deposits

Brown, grey, or dark beige threads floating in the liquid or settled on the bottom and SCOBY. More common in advanced batches or when using a lot of black tea. Completely harmless - they’re the fermentation yeasts.

Brown or dark spots on the SCOBY

Often mistaken for mould. They’re yeast deposits or tea pigments that have fixed themselves to the SCOBY’s surface. They’re not smooth or fuzzy - they’re part of the SCOBY’s solid structure.

Bubbles on the surface

Sign of active fermentation. Normal.

Cloudy liquid

Normal, especially at higher temperatures or in advanced fermentation stages. Not a problem.

The characteristics that leave no doubt

Mould in kombucha has precise features that distinguish it from anything normal:

It’s fuzzy. It has a furry, velvety texture. It’s not smooth, not gelatinous - it’s clearly three-dimensional and soft (even though you shouldn’t touch it).

It has unmistakable colour. Green, black, pink, orange. Not light brown, not beige - these are clear colours that don’t belong to any normal element of fermentation.

It sits on the liquid’s surface. Mould is aerobic: it grows where there’s air. You will never find mould underwater or submerged in liquid. If you see something strange underwater, it’s not mould - it’s yeast.

It has a distinct smell. Not vinegar, not fermented kombucha: it smells of mould, musty, stale.

No middle ground: throw it all away

If you’ve identified real mould - fuzzy, coloured, on the surface - there is only one answer: throw away everything in the jar, SCOBY included.

There’s no way to “save” the SCOBY by removing the mouldy surface. Mould spores are already in the liquid and the SCOBY, even if you can’t see them. Continuing to use it risks future contamination.

Don’t do these things:

• Don’t remove just the mouldy part and use the rest
• Don’t boil the SCOBY to “sterilise” it - it kills it
• Don’t taste the liquid to check “if it tastes mouldy” - it’s not worth it
• Don’t put the SCOBY in different liquid hoping it will “clean itself”

What to do instead:

1. Empty and wash the jar with warm water and mild soap. Rinse thoroughly.
2. Optionally, sanitise with diluted white vinegar and rinse.
3. Start over with a new SCOBY from a reliable source.

The most common causes

Mould in kombucha is relatively rare when basic conditions are respected. When it does form, it’s almost always for one of these reasons:

Insufficient starter liquid. This is the most common cause. Starter liquid immediately lowers the initial batch pH to around 4.0–4.5, creating an acidic environment that inhibits mould during the first critical hours. With less than 100ml per litre, the batch is vulnerable.

Unclean SCOBY or jar. Contaminated equipment before the batch. Always use well-washed jars - not necessarily sterilised, but free of soap residue or other liquids.

Temperature too low. Below 18°C, fermentation is very slow, the pH stays high for a long time, and the vulnerability window extends. Not a direct cause of mould, but increases the risk.

Inadequate cover. The jar should be covered with breathable cloth (fabric, not airtight plastic). An airtight lid blocks oxygen and creates conditions that favour certain types of mould. Too porous a cloth lets in insects or environmental spores.

Weakened SCOBY. A malnourished or poorly stored SCOBY produces fewer organic acids and protects the batch less.

Five rules that reduce the risk to almost zero

1. Use enough starter liquid. At least 100ml per litre. This is the single most important factor in mould prevention.

2. Make sure the tea is completely cooled. Adding SCOBY and starter liquid to still-warm tea (above 30°C) damages the microorganisms and weakens the batch’s protection.

3. Cover with the right fabric. A dense cotton cloth, paper coffee filter, or fermentation cloth. Fixed with an elastic band around the jar’s neck.

4. Keep the jar away from other fermented products. Sourdough, kefir, fermented vegetables: you don’t need to isolate them completely, but avoid keeping them directly adjacent in the same confined space.

5. Check the first 48–72 hours. Mould forms in the initial window, when pH is still high. A daily glance in the first few days is enough to spot it in time.

FAQ

I saw a soft white spot on the SCOBY - is it mould?

If it’s on the liquid’s surface and is fuzzy with visible furriness, yes. If it’s on the solid SCOBY surface and has a compact (non-fuzzy) texture, it’s almost certainly a yeast deposit or part of the new SCOBY forming.

My kombucha has a strange smell - could it be mould?

Mould has a specific and unmistakable smell, very different from the normal acidity of kombucha. If the smell is vinegar, acidic, slightly fruity or wine-like - it’s normal fermentation. If the smell is stale, unpleasantly earthy, or cheese-like - there might be a problem. Check the surface carefully.

Can I make kombucha in a plastic container?

Better not. Plastic can retain smells and bacteria from previous uses, and scratches easily, creating micro-environments. Glass is always the best choice.

I found mould once - do I need to change my whole setup?

Not necessarily. Restart with a clean SCOBY and jar, use more starter liquid than usual in the first batch, and check fermentation conditions (temperature, cover). Mould is an isolated event in most cases.

Read also: How to choose and store a SCOBY → · Complete guide to kombucha fermentation →