If kombucha stays flat after the second fermentation, the cause is almost always one of three things: you bottled it while the pH was still too high, the bottle stayed too cold, or the seal wasn’t actually holding pressure. Daily burping doesn’t create gas - if anything, it lets some escape early. Check pH before bottling, not just taste.
You followed the recipe. Fruit in the bottle, an airtight seal, the right number of days. You open the first bottle expecting resistance, a little hiss, maybe a splash.
Nothing. It opens like any cold tea would.
The problem is almost never the second fermentation itself - it’s something that happened before, or around it.
01 — The three main causes
Why kombucha stays flat: what it actually takes to build up gas
Fizz in the second fermentation isn’t magic: it’s CO₂ produced by yeast that, sealed inside an airtight bottle, has nowhere to go except dissolve into the liquid.
For that to happen you need three things together: yeast that’s still active, sugar left to consume, and a seal that actually holds pressure. Miss even one of the three and the gas either doesn’t form, or forms too little to notice.
Almost every case of a flat bottle comes down to one of these three points.
02 — Bottled too early
Flat kombucha from bottling too early: the problem of a pH that’s still too high
If the kombucha had a pH above 3.5 when you transferred it, the first fermentation hadn’t produced enough acid yet - and that usually also means the yeast wasn’t at peak activity.
Young kombucha is sweet, low in acid, and can taste “ready” if you don’t yet have the experience to spot the right stage. But that sweetness hides a problem: if the pH hasn’t dropped enough, fermentation hasn’t reached the pace it needs to produce gas quickly in the bottle.
The most reliable way to avoid this is checking pH before bottling, not just taste. Between 2.5 and 3.5 you’re in the right window.
pH at bottling time is the key factor for a successful second fermentation. GetBolla tells you exactly when the kombucha is in the right range - so you don’t bottle too early without knowing it. Discover GetBolla →
03 — Second fermentation temperature
How second fermentation temperature causes flat kombucha: below 20°C yeast slows down
Even with perfectly ready kombucha, if the bottle sits somewhere cold, carbonation takes far longer than expected.
| Temperature | Typical days to carbonate |
|---|---|
| Below 18°C | 5 – 7 days |
| 18 – 21°C | 3 – 5 days |
| 21 – 24°C | 2 – 3 days |
| Above 24°C | 1 – 2 days - check pressure daily |
If you’ve moved the bottles somewhere cooler than usual - a cellar, a garage, a colder month - that’s the first thing to check before assuming something more serious is wrong.
04 — Bottles that don’t hold pressure
Bottles that don’t hold pressure: not every seal is equal
A Grolsch-style bottle with a worn gasket, a screw cap that doesn’t close all the way, or a jar with a regular lid not designed for pressure: in all these cases the gas that forms escapes instead of dissolving into the liquid.
Check the rubber gasket on flip-top bottles: if it’s hardened or cracked, replace it. With screw caps, make sure the thread is intact and closes with real resistance, not loosely.
05 — Not enough residual sugar
How much fruit you actually need for the second fermentation of kombucha
During the first fermentation, yeast has already consumed most of the original sugar. What’s left, plus whatever you add with fruit or juice in the bottle, is the fuel for the second round of fizz.
As a starting rule: 10-20% of the bottle’s volume in juice, or 15-25% in fresh fruit (exact figures are in the second fermentation guide). Fruit like mango or grapes gives more available sugar than sharper fruit like berries.
For finer control, you can measure the kombucha’s Brix before bottling, to know exactly how much fermentable sugar is left in the liquid - but for most batches, adjusting the amount of fruit batch by batch is enough.
06 — Pre-bottling checklist
How to avoid flat kombucha: five checks before sealing the bottle
- pH between 2.5 and 3.5. Not just taste - measure it.
- Fruit or juice in the right proportion. 10-20% juice, 15-25% fresh fruit.
- A bottle rated for pressure, with an intact seal.
- Room temperature above 20°C for the days of the second fermentation.
- Daily burping to check pressure, not to create it: every opening lets some gas out, so one check a day is enough.
If you’ve ticked all five and the bottle is still flat after 4 days at room temperature, suspicion shifts to the SCOBY: a sluggish SCOBY, perhaps just out of the fridge, produces kombucha that’s less ready for carbonation even when the pH looks normal.
07 — FAQ
Frequently asked questions about flat kombucha
How long should I wait before saying a kombucha bottle hasn’t carbonated?
At 21-24°C, 4 days is usually enough to tell whether it’s heading the right way. If there’s no resistance at all after 4 days, check your starting pH, the amount of fruit and room temperature.
Can I add more sugar to an already sealed kombucha bottle?
Yes, with a teaspoon of sugar or fruit juice, but reseal properly and check the pressure daily - it can climb faster than expected.
In the second fermentation of kombucha, does more fruit always mean more fizz?
Only up to a point. Beyond a certain amount, the gas produced doesn’t increase much, but the risk of overpressure does. Better to increase gradually batch by batch than double the dose in one go.
Why does bottled kombucha always fizz well in summer and never in winter?
Because room temperature changes, and yeast is very sensitive to cold. In winter, without somewhere above 20°C, carbonation simply takes longer - plan for it instead of assuming something’s broken.
Read also: Second fermentation: complete carbonation guide →