Kombucha Troubleshooting: 15 Common Problems & Fixes

Most kombucha problems happen during F1. The good news is that nearly all of them are fixable with small adjustments to your process.

1. Mold on the Surface

True mold appears as fuzzy, dry, circular patches on the surface of the liquid — typically white, green, blue, or black. It looks exactly like bread mold. Mold only grows on the surface where it has access to air. If you see it, discard the entire batch and the SCOBY. Mold cannot be scraped off or salvaged. Common causes include insufficient starter tea (use at least 1 cup per gallon), a starting pH above 4.5, cold temperatures below 18°C (65°F), or contaminated equipment. Prevent mold by always using enough acidic starter tea to bring the initial pH to 4.0–4.5.

2. No SCOBY Growth

A new SCOBY pellicle should begin forming on the surface within 3–7 days. If nothing appears after 10 days, check the temperature — below 20°C (68°F), growth stalls significantly. Also confirm you used real tea (Camellia sinensis), not herbal tea, and plain sugar rather than a sugar substitute. Artificial sweeteners give the culture nothing to consume. Move the jar to a warmer location (22–28°C / 72–82°F) and avoid disturbing it. Even slight movement restarts pellicle formation.

3. Vinegar Smell or Taste

If your kombucha smells strongly of vinegar, it has over-fermented. This happens when F1 runs too long, temperatures are too high (above 30°C / 86°F), or the starter tea ratio is very high. The brew is still safe — it just has a pH well below 2.5 and most of the sugar has been converted to acetic acid. You can use over-fermented kombucha as starter tea for the next batch, as a cleaning vinegar, or as a salad dressing base. To prevent it, start tasting your brew at day 5–7 and bottle when it reaches your preferred balance.

4. Slow Fermentation

If your kombucha still tastes like sweet tea after 10+ days, the fermentation is sluggish. The most common cause is low temperature. Use a heating mat or brew belt to keep the jar at 24–27°C (75–80°F). Other causes include weak starter tea (use strong, well-fermented kombucha as starter), chlorinated water (switch to filtered water), or an old, inactive SCOBY. Check pH — if it has dropped from 4.5 to around 3.5–4.0 over 10 days, fermentation is happening, just slowly.

5. Flat Kombucha (No Carbonation)

The number one complaint from new brewers. Flat kombucha after F2 usually means one of three things: the bottles are not truly airtight, there was not enough sugar or yeast in the bottle, or the F2 duration was too short. Use swing-top (Grolsch-style) bottles with intact rubber gaskets. Add at least 1 teaspoon of sugar or 1–2 tablespoons of fruit per 500 ml bottle. Ferment at room temperature for 3–4 days (not in the fridge). Check our Carbonation Guide for exact sugar amounts per bottle size.

6. Explosive Bottles (Over-Carbonation)

Over-carbonation is the opposite problem — and it can be dangerous. Bottles can shatter if pressure builds too high. This happens when too much sugar is added during F2, when F1 kombucha was still very sweet (high residual sugar), or when F2 runs too long at warm temperatures. Always burp your bottles once a day during F2 by briefly opening the cap to release excess pressure. Limit F2 to 2–4 days and refrigerate promptly. Avoid adding more than 1 tablespoon of sugar per 500 ml bottle.

7. Off Flavors (Sulfur, Cheese, Feet)

Unpleasant flavors usually point to stressed yeast or bacterial imbalance. Sulfur smells (rotten eggs) come from certain yeast strains under stress — this often resolves by using less fruit during F2 or ensuring adequate ventilation during F1. Cheesy or feet-like smells may indicate contaminated equipment. Sanitize everything with white vinegar (not soap) and make sure your cloth cover is tightly woven enough to block fruit flies, which carry acetobacter and wild yeasts.

8. Yeast Chunks in Bottles

Brown, stringy, or blobby yeast sediment in your F2 bottles is harmless — it is a natural byproduct of fermentation. However, it is unappealing to drink. To reduce it, strain your kombucha through a fine-mesh sieve or cheesecloth when transferring from F1 to bottles. You can also cold-crash your bottles by refrigerating for 24 hours before drinking, which settles yeast to the bottom.

9. Thin or Weak SCOBY

A healthy SCOBY pellicle should be at least 3–6 mm (1/8 to 1/4 inch) thick after a full F1 cycle. If your SCOBY is paper-thin or patchy, the culture may be struggling. Common causes include insufficient nutrients (use real tea, not herbal), too little sugar, low temperatures, or disturbances during fermentation. Each time you move the jar, the pellicle formation restarts. Leave the jar completely undisturbed for the full fermentation period and ensure temperatures stay between 22–28°C (72–82°F). The pellicle itself is not essential for brewing — it is the liquid starter tea that contains the active culture — but a healthy pellicle is a good indicator of a thriving brew.

10. Dark Spots or Discoloration

Brown or tan spots and patches on your SCOBY are almost always embedded yeast colonies. They are completely normal and harmless. Yeast strands often attach to the underside of the pellicle and can look alarming. The key distinction: yeast is wet, smooth, and submerged or on the underside, while mold is dry, fuzzy, and on top. If the spots are dark brown or black and wet, they are yeast. If they are fuzzy and on the air-exposed surface, that is mold — discard everything.

11. Sinking SCOBY

A SCOBY that sinks to the bottom or floats sideways is perfectly healthy. The position of the mother SCOBY has zero effect on fermentation quality. A brand-new pellicle will always form at the surface of the liquid regardless of where the old SCOBY sits. Some SCOBYs float, some sink, and some hover in the middle. Do not try to reposition it — just leave it alone. After several brews, the mother SCOBY may become waterlogged and always sink. This is fine. You can trim old layers and keep the newer, lighter ones if your SCOBY hotel is getting crowded.

Learning to tell the difference between mold and kahm yeast is the single most important troubleshooting skill for any kombucha brewer.

12. True Mold

Mold is fuzzy, dry, and grows in circular patches on the surface of the liquid or on the SCOBY pellicle. It is typically white, green, blue-green, or black. Under a magnifying glass, you can see tiny hair-like filaments. Mold needs air, so it only grows on the very top surface. If you see mold, there is no safe way to salvage the batch — discard all the liquid and the SCOBY. Clean your jar with hot water and white vinegar before your next brew. Mold usually appears because the starting pH was too high (above 4.5), the environment was too cold, or there was not enough starter tea.

13. Kahm Yeast

Kahm yeast looks like a thin, wrinkled, white film on the surface — often described as looking like a brain or crumpled tissue paper. It is flat and smooth, not fuzzy. Kahm yeast is not dangerous, but it can produce off flavors that taste musty or yeasty. You can skim it off and continue brewing, though the flavor may be affected. Kahm yeast tends to appear when temperatures fluctuate, when the brew is too sweet, or when there is not enough acidity. Increasing the amount of starter tea and maintaining consistent temperatures helps prevent it.

Quick identification guide:

• Fuzzy, raised, dry, circular patches → Mold → Discard everything
• Flat, smooth or wrinkled, wet film → Kahm yeast → Skim and continue
• White, smooth, rubbery layer → New SCOBY → Perfectly normal
• Brown strands hanging below surface → Yeast colonies → Perfectly normal

Sometimes the best course of action is to toss the batch and begin fresh. Here are the clear signals that you should start over:

14. Confirmed Mold

As described above, any fuzzy mold means the entire batch — liquid and SCOBY — must go. Do not try to scrape off mold and continue. Mold roots penetrate deep into the pellicle and the liquid may contain mycotoxins. Acquire a new SCOBY and starter tea, and thoroughly clean all equipment with hot water and distilled white vinegar before your next brew.

15. Persistent Off Flavors After Multiple Batches

If your kombucha consistently tastes wrong despite adjusting temperature, sugar, tea type, and starter tea amount, the culture itself may have an imbalanced microbial population. This can happen when a SCOBY is very old or has been stressed repeatedly. Starting fresh with a new SCOBY from a reputable source resets the microbial balance. Save some of your old kombucha as backup starter tea, but brew with a new pellicle.

Before starting over, double-check these fundamentals:

• Water is filtered or dechlorinated (chlorine kills beneficial bacteria)
• Tea is real Camellia sinensis (black, green, white, or oolong)
• Sugar is plain white cane sugar (at least 1 cup per gallon)
• Starter tea is at least 1 cup per gallon and has a pH below 3.5
• Temperature is consistently 22–28°C (72–82°F)
• Equipment is clean but not sanitized with antibacterial products

If all of these are correct and you still have problems, read our Complete Beginner's Guide and SCOBY Care Guide for a thorough process review. Use our pH Guide to monitor acidity throughout the process.