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Troubleshooting · Updated on 9 October 2026

Kombucha problems: mould, no fizz, too sour and exploding bottles

The real risks of homemade kombucha and how to handle them: mould, a weak starter, over-carbonated bottles, alcohol and who should not drink it.

The methodRecord the batch

Homemade kombucha has a few real risks: mould, a starter that is not acidic enough, pressure in capped bottles and alcohol that rises with longer fermentation. Each has a simple control. 34 Some people are advised not to drink homemade kombucha at all: people with weakened immune systems or prone to acidosis, and pregnant or breastfeeding people. 1

Fuzzy spots on the SCOBY

Discard the batch. Do not try to rescue it.

Likely causes

  • Mould growing on the surface, more likely when the starter is too weak to acidify the tea quickly 31

What to do

  • Discard the kombucha and the SCOBY and wash the vessel thoroughly 2
  • Next time check that the starter reads pH 4.0 or lower and use the full amount 34

Slow fermentation, no tang

Likely causes

  • Too little starter, or starter not acidic enough 4
  • A cool room; cool temperatures slow fermentation 4

What to do

  • Reserve 20% of the liquid as starter and check it is at pH 4.0 or lower 43
  • Keep the vessel at 20–25 °C, away from vents 4

Too sour or vinegary

Likely causes

  • Fermented too long or too warm 4
  • A half batch, which tested as excessively acidic 4

What to do

  • Taste from about a week on and bottle earlier 5
  • Brew the full tested batch rather than half 4

Still too sweet

Likely causes

  • Bottled too early, or a cool room 4

What to do

  • Give it more days, up to 14 in a cool room, tasting as you go 5

Bottles over-fizzy or bursting

Likely causes

  • Capped bottles left too long at room temperature 53

What to do

  • Limit capped flavouring to 1–3 days at room temperature, then refrigerate 5
  • Open the caps once a day to release carbon dioxide 6
  • Open chilled bottles carefully, pointing away from faces

Tastes of alcohol

Likely causes

  • Longer fermentation produces more alcohol 4

What to do

  • Shorten the fermentation and keep flavoured bottles cold
  • At 0.5% alcohol or more it is legally an alcoholic beverage 7

Change one thing at a time

When a batch disappoints, compare it with the method, change one variable, and note the result in a Batch Card. Two changes at once hide which one helped.

Sources

  1. University of Georgia Extension: What’s the word on homemade kombucha? (C 1312)Do not use glazed earthenware; discard tea and SCOBY with dry, fuzzy mould of any colour; people with weakened immune systems or susceptibility to acidosis, and pregnant or breastfeeding people, are advised against homemade kombucha. Accessed 2026-10-09.
  2. Colorado State University Extension: Understanding and making kombuchaFuzzy blue, grey, green, brown or black mould: discard SCOBY and kombucha and wash the vessel; second fermentation cautions. Accessed 2026-10-09.
  3. National Center for Home Food Preservation: Kombucha teaTested recipe: 1 gallon water, black and green tea, 1 cup sugar, SCOBY and starter liquid of pH 4.0 or lower; steep at 160–170 °F (71–77 °C), cool below 95 °F (35 °C); ferment 7–10 days at 68–77 °F (20–25 °C) under a cloth; refrigerate; airtight storage can lead to explosion. Accessed 2026-10-09.
  4. NCHFP / University of Georgia: Crafting kombucha at home, FAQUse a vessel of at least 1.5 gallons; half batches are not recommended and gave excessive acidity; reserve 20% of the liquid and the SCOBY; signs of fermentation; tested batches below 0.016% alcohol, more with longer fermentation; refrigerate and use within 30 days, flavoured within 2 days; do not store a SCOBY airtight. Accessed 2026-10-09.
  5. UC ANR Master Food Preservers: KombuchaFerment 7–14 days at 64–79 °F (18–26 °C), tasting after a week; for flavouring add 10–20% juice or fruit, cap 1–3 days at room temperature, then refrigerate; longer capped at room temperature risks pressure and explosion. Accessed 2026-10-09.
  6. Utah State University Extension: Homemade kombuchaOpen flavoured, capped bottles once a day to release carbon dioxide pressure. Accessed 2026-10-09.
  7. TTB: KombuchaKombucha at 0.5% alcohol by volume or more at any time during production or after bottling is an alcohol beverage. Accessed 2026-10-09.

Numbers on this page come from these sources. Where sources give a range, we quote the range rather than a single figure.

A new, independently written guide at the address previously used for “The Dark Side to Kombucha”.

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