Short answer
Flat kombucha usually has one of four causes: a leaky seal, too little sugar, a cold room, or not enough time. Check the bottle first, then the sugar, then the temperature, then the timing. Fizz needs a sealed bottle, sugar for the yeast and a warm enough room.
How fizz forms
In the second ferment, yeast eats the sugar in the bottle and makes carbon dioxide. If the gas cannot escape, it dissolves into the liquid. That dissolved gas is the fizz. If the gas leaks out, or if there is not enough sugar left to make it, the drink stays flat.
Check the bottle first
- The seal: swing-top bottles need a thick, undamaged rubber gasket, and the lid must lock down firmly.
- The bottle type: use bottles designed for carbonation. Mason jars with a loose or two-piece lid, or decorative bottles, may not hold the gas.
- The filling: fill close to the neck, leaving about 2.5 cm (1 inch) of headspace. Too much space leaves less gas in the drink.
If a bottle leaks, its batch will be flat. Use a new bottle for the next batch.
Sugar and fuel
Fruit, juice and a little sugar give the yeast the fuel it needs. A splash of fruit juice, a spoon of fruit purée or a small pinch of sugar in each bottle is often enough to bring a flat batch back to life. Use fresh first-ferment kombucha, which still has yeast and sugar. Plain kombucha with no added sugar carbonates much more slowly.
Temperature
Yeast slows right down in a cold room. Below about 21 °C (70 °F), fermentation can be very slow. Aim for a room around 21 to 29 °C (70 to 85 °F). A heat mat or a warmer shelf can help. Keep the bottles out of direct sun, which can overheat them.
Time and burping
Second fermentation usually takes one to five days, and sometimes longer in a cool room. Flat bottles often need more time, not less.
Burping matters, but do not vent every bit of pressure away each day. If you open the bottle and release all the gas, the drink cannot build fizz. Instead, check the firmness of the bottle by feel, or squeeze a plastic gauge bottle. Vent only if the bottle is very hard, slowly and over a sink. Then close it again.
Stir before bottling
Yeast settles at the bottom of the first-ferment jar. If you pour without stirring, some bottles get plenty of yeast and others get very little. Give the brew a gentle stir before you bottle, so each bottle gets a fair share.
Chill before opening
Cold liquid holds carbon dioxide better. Chill the bottle for at least 12 hours before opening, which also helps the gas stay in solution when you pour.
Checklist
- Is the seal tight, with an undamaged gasket?
- Is there enough sugar or fruit in the bottle?
- Is the room at least 21 °C (70 °F)?
- Has the bottle had at least three days?
- Did you stir the brew before bottling?
- Are you venting too often?
Work through the list in order, and change one thing at a time in the next batch.
When it’s too fizzy
If your bottles gush, the same causes can be at work in reverse. Chill them sooner, burp more often, and use less sugary fruit. Bottles that are very full or very sugary can also gush, so open them over a sink and keep them away from children. For a full guide to flavouring and bottle safety, see how to flavour kombucha.
Related fixes
For apple and basil, the fizz should come from the apple. If it stays flat, see the Pink Lady apple and basil kombucha recipe. For berry batches, the fruit usually supplies enough sugar. For a fruit-first approach, see what is the best fruit to add to kombucha.