Kombucha troubleshooting: 12 common problems and how to fix them

The quick diagnosis

The rest of this guide takes each of those in turn. If you only remember one thing: acidity is what protects a batch, and starter liquid is where acidity comes from. Most problems start with too little of it.

1. No new SCOBY is forming

A new pellicle usually appears as a translucent film within three to five days and thickens over the next week. If there is nothing after ten days, the culture is working slowly or not at all. The usual causes are a room below 18 °C, too little starter, or a culture that arrived dehydrated, refrigerated, or exhausted. Move the jar somewhere warmer, add a cup of strong, sour starter, and give it another week. Taste it too: a batch can be fermenting fine with only a thin film, and taste tells you more than the disc does. If after three weeks there is no film, no sourness, and no yeast activity at all, the culture is not viable; what a SCOBY is explains how to grow or source a new one.

2. It is still sweet after two weeks

Sweetness that will not fade is the slow version of the same problem. Cold is the first suspect: at 16 to 18 °C a batch can need three weeks, and at that pace the surface sits unprotected for a long time. The second is starter: a splash is not enough, and weak starter from a sweet batch does not acidify anything. The third is a tired culture. Warm the jar, add starter from a hotel, and check the pH if you have strips: above 4.2 after a week means it needs help. How long to ferment kombucha has the temperature table.

3. It went sour too fast

The opposite complaint, and usually a summer one. Above 27 °C a batch can be sharp by day four. A very yeast-heavy culture does the same at any temperature. Neither is a safety problem, just a scheduling one: taste from day 3, bottle on the sweet side of balanced, move the jar to a cooler room, or brew a slightly larger batch, which takes longer. An over-sour batch is excellent starter and hotel liquid, and diluted with juice it is still a drink; only very acidic brews, drunk in quantity, are worth avoiding.

4. The bottles are flat

Carbonation needs three things in the bottle: sugar, yeast, and a seal. Even very sour kombucha may retain sugar; if carbonation is weak, add a measured amount at bottling next time. Yeast is present in the liquid even without visible strands, but very fine filtering can remove much of it. Worn gaskets and reused caps leak; replace them. Below 20 °C the yeasts are sluggish; move the bottles somewhere warmer and give them another day or two. And every time you open a bottle to check, you let the pressure out, so use one plastic test bottle instead. Second fermentation goes through this in detail.

5. The bottles are too fizzy, or one burst

Too much sugar, too warm, or too long, in that order of likelihood. Fruit with a lot of sugar or wild yeast, such as ginger, grapes and mango, ferments faster than you expect. Put the whole batch in the fridge immediately, open the bottles cold over the sink, and halve the sugar next time. Use only pressure-rated bottles and keep fermenting bottles in a box or cupboard. If a bottle has burst, treat the rest as dangerous until they are cold.

6. It smells wrong

Kombucha is supposed to smell like vinegar, sharper the longer it goes, and a warm or yeast-heavy batch can add a bready, slightly sulphurous note that fades after bottling. Those are normal. What is not normal is a musty, mouldy, or genuinely rotten smell, or the smell of something dead. Look at the surface: mold or larvae explain it and mean the batch goes. If the surface is clean, the batch is probably stressed rather than spoiled: give the next one more starter, more airflow, and a steadier temperature, and skim any yeast film. When smell and appearance both say “off”, do not taste it to decide.

7. Brown strings, sediment, and cloudiness

The brown, stringy, sometimes slimy strands hanging from the SCOBY or drifting in the jar are yeast, and they are a sign of a working culture. Sediment at the bottom is the same yeast settling, and cloudiness is yeast in suspension. None of it is a problem. If the batch is getting more yeasty every round, sour and fizzy and bready, the culture is drifting yeast-heavy: rinse the SCOBY, leave the sediment behind when you set aside starter, and use hotel liquid, which is bacteria-rich, to rebalance.

8. The SCOBY sank, has holes, or grew sideways

The disc floats or sinks depending on trapped gas, not on health. It can hang vertically, split, grow bumps, bubbles, ridges and holes, and attach to the old one or not. A new layer forms on the surface regardless, and the sunken one keeps working below. The only appearance that matters is dry, fuzzy, round spots on top; kombucha mold has the side-by-side comparison.

9. A white film or spots on the surface

Three things look white on kombucha. A translucent, wet, glossy film is the new pellicle: leave it. A flat, wrinkled, chalky film across the entire surface is kahm yeast: harmless, but a sign of imbalance, usually too warm or too little starter; skim it and correct the next batch. Round, dry, fuzzy, raised spots are mold: discard the batch, the SCOBY, and any layers that shared the jar, wash everything, and restart from a hotel with plenty of strong starter. If you cannot tell a film from mold, wait 24 hours: mold grows visibly fuzzier, films do not.

10. Fruit flies, or tiny larvae on the surface

Fruit flies love vinegar, and a loose or coarse cover lets them in. Flies on the outside of the cloth are a warning; flies inside the jar, or small white larvae on the pellicle, mean the batch and the SCOBY go in the bin. It is not worth rescuing. Use a tightly woven cloth or a coffee filter with a rubber band, keep the jar away from the fruit bowl, and keep a hotel so a lost batch is a setback rather than a restart.

11. The SCOBY has dark brown or black patches

Flat, dark patches inside or under a thick, old pellicle are yeast deposits and tea stains, and they are normal, especially on black tea. A layer that has gone uniformly dark and leathery is simply old: retire it and brew with the newest, palest layer. Raised, dry, fuzzy black spots on top are mold, and the texture is the difference. When in doubt, the 24-hour test applies.

12. It tastes bitter, astringent, or like nothing

Bitterness and a mouth-drying astringency come from the tea, not the ferment: too much tea, too long a steep, or water that was boiling when it hit green tea. Use about 5 g per liter, steep 10 to 15 minutes for black and less for green, and let green tea water cool for a minute first. A watery, characterless batch is the reverse: too little tea, or too much starter diluting it. The best tea for kombucha covers amounts and steeping.

When to throw a batch away

• Dry, fuzzy, round spots of any colour on the surface: mold.
• Larvae, or flies inside the jar.
• A rotten, musty, or mouldy smell together with anything strange on the surface.
• A batch that has not acidified after several days, especially if it remains sweet after weeks: discard it rather than taste or bottle it.
• Anything brewed in a container you now suspect was not food safe, such as a ceramic pot with an unknown glaze.

Everything else on this page is recoverable. And the recovery is nearly always the same three moves: more strong starter, a steadier temperature between 20 and 26 °C, and a fresh layer from a hotel. How to make kombucha lists the first-batch mistakes that cause most of these symptoms in the first place.

Frequently asked questions