A contaminated strain shows one thing: a dry, fuzzy patch on the surface, blue, green or black. In that case discard the liquid and the SCOBY together, wash the jar and start again. Everything else, including brown strings, holes, dark staining, a sinking disc and a batch that is simply slow, is not contamination. A weak but clean culture recovers in two to three batches with extra starter liquid and a room at 22 to 24 C.
Almost every message we receive about a contaminated strain describes something harmless. That is understandable: a jar of fermenting tea grows things that look alarming. This page separates the four signs that genuinely matter from the six that do not, and gives the two protocols that follow.
One sentence covers the whole diagnosis: mould is dry, fuzzy and sits on top of the liquid. Anything wet, smooth, stringy or suspended is part of a working culture. A contaminated strain does not look cloudy or slimy, it looks like the mould you would find on forgotten bread, in a dry patch above the surface.
Colour is the second test. Blue, green and black belong to mould. Cream, beige, tan and brown all belong to a healthy culture, including the darker patches caused by tea tannins staining the cellulose.
This is the decision the whole page turns on. A contaminated strain is discarded completely, liquid and disc together, because what is visible on the surface is not the full extent of what is in the jar. A weak but clean culture is kept, given better conditions and left alone for two or three cycles.
Do not taste a batch with visible mould, and do not rinse the SCOBY to reuse it. This is the single situation in home fermentation where no salvage attempt is worth making.
| Batch | What to change | What to expect |
|---|---|---|
| 1 | 150 ml starter liquid, room at 24 C | still slow, possibly sweet at day 10 |
| 2 | same again, nothing else | a thin new layer appears |
| 3 | same again | sours on schedule |
| 4 | back to normal quantities | usual rhythm |
| 5 | remove the spare layer | start the hotel |
Nothing on that list is an intervention. A culture that has been neglected, kept too cold or started without enough starter liquid is not a contaminated strain, it is an underfed one, and the fix is food and warmth over several cycles.
| Cause | What happens | Prevention |
|---|---|---|
| No starter liquid | pH stays high for days, mould gets its window | 100 to 150 ml of finished kombucha, always |
| Tea still hot | above 30 C the culture is damaged at once | cool to hand warm first |
| Tight lid | no air in the first phase, condensation on the surface | cloth and a rubber band |
| Flavoured tea with oils | weakens the culture over several batches | plain black or green tea |
| Too little sugar | the culture lives on its reserves | 70 to 90 g per litre |
| Fruit in the first phase | sugars and skins bring competing organisms | flavour in the second fermentation only |
| Uncovered jar | insects reach the surface | always cover with cloth |
The first two account for most genuine cases. Starter liquid is the cheapest insurance in home fermentation: it drops the pH within minutes and closes the only window in which a contaminated strain can develop.
A second jar, usually called a SCOBY hotel, turns a lost batch from a setback into an inconvenience. It costs one batch of sweet tea and five minutes.
General household food hygiene guidance is published by the World Health Organization, and it applies to fermented drinks as it does to anything else in a kitchen.
Understanding what a kombucha culture is makes the difference between panic and a correct diagnosis. The disc is not an organism, it is a structure that organisms build. Three groups live in it, and each produces something visible.
| Group | What it does | What you see |
|---|---|---|
| Acetic acid bacteria | build the cellulose disc, produce acid | the new pale layer, the sour smell |
| Yeasts | produce gas, aromas and traces of alcohol | brown strings, bubbles, jelly blobs |
| Lactic acid bacteria | contribute acidity | nothing visible |
| Airborne mould spores | grow only above the liquid, on a dry surface | dry fuzzy patches, wrong colours |
The last row explains the whole diagnostic rule. Mould needs air and a dry surface, which is why a contaminated strain always appears ON the surface and never suspended in the liquid. Anything floating, sinking or hanging belongs to the three rows above it.
Run through these five questions. In our experience they resolve almost every case without needing a photograph or a second opinion.
Four correct answers and a no to the first question mean the culture is healthy. That is the entire diagnosis, and it takes under a minute once you know the order.
Starting again with the same habits produces the same result. These are the six adjustments worth making, in order of how often they turn out to be the cause.
| Change | Why | How much difference |
|---|---|---|
| Add 150 ml starter liquid, always | drops the pH within minutes | the largest single factor |
| Cool the tea to hand warm | above 30 C the culture is damaged | large |
| Cloth instead of a lid | air in, condensation out | moderate |
| Plain tea, no oils | flavouring oils weaken the culture | moderate over several batches |
| 70 to 90 g sugar per litre | enough food for a strong fermentation | moderate |
| Flavour only in the second phase | fruit in the first phase brings competitors | small but real |
If you only change one thing, change the starter liquid. Most reports of a contaminated strain come from a first batch started without any, usually because the instructions that came with the culture did not insist on it.
There is one more case worth separating out, because it looks like failure and is not. A jar left three or four weeks is not a contaminated strain, it is vinegar, and vinegar is useful.
| Use | How | Keeps for |
|---|---|---|
| Starter liquid | 150 ml into the next batch | months in the hotel |
| Salad vinegar | with oil, salt and herbs | months in a bottle |
| Marinade | 2 to 4 hours for poultry | use fresh |
| Cleaning surfaces | diluted, for worktops | weeks |
Nothing needs to be poured away because it went too far. The only kombucha that is genuinely lost is a contaminated strain with visible mould, and that is rarer than the internet suggests.
| Appearance | Meaning | Action |
|---|---|---|
| Brown strings hanging or settled | yeast, a sign of activity | strain when bottling |
| Holes through the disc | a layer formed around a bubble | none |
| Dark brown or near black patches | tea tannins staining cellulose | none |
| A jelly like blob underneath | yeast colony, common in warm weather | none |
| Disc at the bottom of the jar | gas content, not vitality | watch for the new layer |
| Thin, translucent new layer | a young SCOBY | none, it thickens each batch |
All six are normal. Any of them can be mistaken for a contaminated strain by someone seeing them for the first time, and each of them has caused someone to throw away a perfectly good culture.
Live kombucha SCOBY
Shipped with its own starter liquid, ready for a first batch the day it arrives.
See the SCOBYThe full symptom list: sick kombucha. The method from the start: how to make kombucha.
One sign only: a dry, fuzzy patch sitting on top of the surface, blue, green or black. Brown strings, bumps, holes, dark staining and a sinking SCOBY are all normal. A contaminated strain looks like mould on bread, not like something slimy in liquid.
No. If there is visible mould, discard the liquid and the SCOBY together. What you can see on the surface is not the whole of what is present in the jar. Cutting away a patch is the one rescue attempt that is never worth making.
Almost certainly not. Dark patches, holes, ridges and an uneven shape are cosmetic. A contaminated strain is identified by dry fuzz and by colour, not by looking unattractive.
Three things account for nearly all cases: too little starter liquid, tea that was still hot when the culture went in, and a jar closed with a tight lid instead of a cloth. All three are avoidable.
Give it 150 ml of starter liquid, a room at 22 to 24 C and two or three normal batches. No intervention beyond that. A weak culture recovers on its own if the conditions are right.
No, it is a young one. Each batch adds a layer, so a disc that is only a millimetre thick will thicken over three or four cycles.
No. Very sour kombucha makes excellent vinegar, but that is a different situation. A contaminated strain with visible mould is discarded, not repurposed.
Keep a second jar, the SCOBY hotel: sweet tea with the spare layers and plenty of starter liquid. One bad batch then costs you a week, not the culture.
