In short
Second fermentation means transferring finished kombucha to sealed, pressure-rated bottles with a measured amount of fruit, juice or sugar. Live yeast can ferment those sugars and produce carbon dioxide, which dissolves into the drink under pressure. CSU Extension suggests one to three days at room temperature before refrigeration, but carbonation speed varies by batch and temperature. Check the bottles throughout this stage and refrigerate them once carbonation develops.
For a 500 ml bottle, 30 to 60 ml of juice or 1 tablespoon of crushed fruit gives you a practical starting point. These amounts don’t guarantee a particular pressure or alcohol level because sugar content and culture activity vary. Leave 2 to 3 cm of headspace, keep every bottle cold before opening, and point it away from faces and breakable objects.
How the second fermentation works
A cloth-covered first fermentation lets carbon dioxide escape. In a sealed bottle, gas produced by live yeast stays inside and some dissolves into the kombucha. When you open a fully chilled bottle, the pressure drops and the dissolved gas forms bubbles. Fruit, juice or sugar can supply more fermentable sugar, but unflavored kombucha may also carbonate if it still contains residual sugar. The bottle has to withstand that pressure.
Sealing the bottle also limits oxygen. Acetic acid bacteria use oxygen when converting ethanol into acetic acid, while yeast can continue fermenting available sugar in the closed bottle. The result can include more carbon dioxide and alcohol; it won’t follow the same schedule in every bottle. Sugar content, culture activity, temperature and time all affect the change. Keep notes for each batch, but don’t treat flavor or fizz as an alcohol measurement.
When is kombucha ready to bottle?
Bottle after the first fermentation has reached the acidity you want and passed the checks in your brewing recipe. A slightly sweet taste can leave sugar for the bottle stage, but taste doesn’t measure how much fermentable sugar remains. Very sour kombucha can still contain residual sugar, and adding fruit or juice introduces more. How long to ferment kombucha explains the first-fermentation checks.
Before flavoring, reserve the SCOBY and the measured starter liquid required for your next batch. Keep this starter unflavored so fruit, herbs and their residues don’t carry into the brewing culture. Bottle only the portion you plan to drink.
The right bottles
- Use glass swing-top bottles that the manufacturer rates for carbonated drinks. Check the glass, closure and gasket before every fill; retire a bottle with chips, cracks or a damaged seal.
- Reuse commercial kombucha bottles only when the manufacturer or supplier confirms they suit repeated pressure use and the closures still seal. Beer bottles require new crown caps and a capper; never reuse removed crown caps.
- Skip decorative or square bottles, thin glass, ordinary jars and cork closures. A container that holds still liquid isn’t necessarily built for pressure.
- A PET soda bottle can show pressure building in that bottle as it firms up. It doesn’t prove that nearby glass bottles have the same pressure, so inspect and refrigerate the whole batch on its own schedule.
Leave 2 to 3 cm of headspace and fill bottles consistently. Label each bottle with its flavor, bottling date and added sugar or juice; those notes make the next batch easier to adjust. Headspace affects carbonation, but it doesn’t prevent excessive pressure.
Wash bottles and removable parts thoroughly, rinse them, and follow the bottle maker’s cleaning or sanitizing instructions. Acidity doesn’t excuse dirty equipment. Inspect gaskets, caps and glass under good light before filling, and avoid thermal shock from pouring hot liquid into cool glass.
Flavoring: how much to add
Flavor additions are starting points, not pressure controls. Juice and fruit differ in sugar content, while herbs and zest may add flavor with little fermentable sugar. Measure what goes into each 500 ml bottle and record the product or fruit used. If you change both the ingredient and the amount, you won’t know which change affected carbonation.
| Flavor | Per 500 ml bottle | What to expect |
|---|---|---|
| Fresh ginger, grated | 1 teaspoon | Strong flavor; carbonation depends on the batch and any sugars present. |
| Fruit juice | 30–60 ml | Sugar varies by juice. Record the brand or nutrition label as well as the volume. |
| Fresh or frozen fruit, mashed | 1–2 tablespoons | Adds variable sugar and pulp. Leave enough room and chill fully before opening. |
| Dried fruit | 1 teaspoon, chopped | Sugar concentration varies; chopped pieces can swell and obstruct the bottle neck. |
| Herbs (mint, basil, rosemary) | A few leaves | Adds little sugar on its own. Pair only with a measured sugar source if you want more carbonation. |
| Citrus juice or zest | 1–2 teaspoons | Adds acidity and aroma. Zest oils and juice sugar vary, so start small and keep notes. |
| Plain sugar | ½–1 teaspoon | Easy to weigh and repeat, but pressure still depends on residual sugar, yeast activity and temperature. |
Add clean flavoring to the bottle, pour in kombucha, leave the planned headspace and seal. For more even flavor, mix one measured addition into a known volume of kombucha before filling several bottles, stirring gently so the liquid stays consistent. Avoid an extra uncovered day of fruit infusion; it adds handling without building carbonation. Kombucha flavors gives combinations you can scale and record.
A few combinations to start with
- Ginger and lemon: a teaspoon of grated ginger and a teaspoon of lemon juice.
- Raspberry and mint: a tablespoon of mashed raspberries and three or four mint leaves.
- Mango and lime: a tablespoon of mashed mango and half a teaspoon of lime juice.
- Apple and cinnamon: 40 ml of apple juice and a small piece of cinnamon stick.
- Blueberry and lavender: a tablespoon of crushed blueberries and a small pinch of dried lavender, no more.
How long to leave it
CSU Extension gives one to three days at room temperature as a home-brewing starting point before refrigeration. That is a range for checking, not a guaranteed finish time. Warmth, residual sugar, added fruit and culture activity can shorten or lengthen it. Inspect bottles at least daily for leaks, deformation or rapid pressure buildup. A firm PET test bottle reports only its own pressure, and opening a glass bottle releases the carbonation you are trying to keep. Refrigerate the batch when carbonation develops or sooner if pressure concerns you.
Refrigerate bottles until the liquid is fully cold before opening. Cold kombucha retains more dissolved carbon dioxide and usually foams less, but chilling doesn’t remove pressure or stop fermentation permanently. Keep live kombucha refrigerated and open it carefully. If storage continues for weeks, check the bottles and follow the storage guidance rather than assuming the pressure is unchanged.
Why is my kombucha flat?
- Not enough fermentable sugar reached the bottle. Review the measured addition and the first fermentation rather than adding an unmeasured pinch next time.
- Little active yeast reached the bottle. Yeast also lives in the liquid, so visible brown strands aren’t required; very fine filtration can remove part of the live culture.
- The closure leaks. Replace worn gaskets or caps and retire bottles whose sealing surfaces are chipped.
- The bottle stayed cool. Lower temperatures slow fermentation; record the actual room temperature before moving the batch.
- The batch needs more time. Extend the next check by a short interval while continuing to watch for pressure, rather than assuming a fixed number of extra days.
- The headspace varied or the bottle was opened repeatedly. Each opening releases pressure, and inconsistent fill levels make bottles harder to compare.
- The sweetener contained little or no fermentable sugar. Artificial sweeteners may add sweetness without supplying yeast with sugar.
Change one variable in the next batch: verify the seal, measure the sugar source, retain some live culture in the liquid, or adjust the temperature. Adding both sugar and yeast sediment to a flat bottle can create an unpredictable pressure jump. If you reopen a bottle to make an addition, use clean equipment and treat its new sealing time as a separate trial.
Too much fizz, or an exploding bottle
- Too much fermentable sugar may have entered the bottle from fruit, juice, residual tea sugar or a direct addition. Measure each source next time.
- Warm storage or a longer sealed stage allowed more fermentation. Record the temperature and shorten the next check interval.
- Different fruits carry different amounts of sugar and pulp. A previous ginger, grape or mango schedule doesn’t predict the pressure from a new ingredient or quantity.
- A chilled bottle warmed up again during transport or storage. Live yeast can become more active as the drink warms, so return it to refrigeration promptly.
If a PET bottle is very hard or a closure is leaking, refrigerate the whole batch without shaking it and let the bottles become fully cold. Keep people away from damaged or bulging containers. Open an intact cold bottle over a sink, pointed away from your face, and stop if the glass or closure looks compromised. Routine burping releases carbonation and requires repeated handling of pressurized bottles; adjust measured additions, temperature or time in the next batch instead.
A note on alcohol
Fermentation in a sealed bottle can increase alcohol as well as pressure. US TTB rules apply if kombucha reaches 0.5 percent alcohol by volume at any point, including after bottling. A British Columbia study reported 684 ethanol measurements with a mean of 0.77 percent and a maximum of 3.62 percent among its commercial samples. Those figures don’t predict a home batch. Sugar, time and temperature can affect alcohol, but only a suitable test establishes the percentage.
Keep measured additions modest, limit warm storage and refrigerate live bottles promptly. These choices may limit further fermentation; they don’t make home-brewed kombucha alcohol-free. For children, pregnancy, driving, medicines or any need to avoid alcohol, don’t rely on flavor, fizz or a recipe estimate. Does kombucha have alcohol? explains testing and label thresholds.
Frequently asked questions
Do I have to do a second fermentation?
No. Kombucha from the first fermentation is a finished drink once it has passed the recipe’s checks. Refrigerate it in a suitable container if you don’t want a sealed bottle fermentation. It may contain some carbon dioxide and alcohol already; skipping F2 avoids an added carbonation stage rather than removing either one.
Should I burp the bottles?
Don’t use routine burping as the main pressure-control method. Opening a bottle releases carbonation and makes the pressure harder to compare, while handling pressurized glass adds risk. Measure additions, use pressure-rated bottles, check the batch and refrigerate it when carbonation develops. If a bottle appears damaged, leaking or overpressurized, cool it and keep people away.
Can I add the fruit during the first fermentation?
Keep the first fermentation plain so you can reserve clean, unflavored starter liquid for the next batch. Add fruit after removing that starter. Raw fruit also adds variable sugar and microbes, which makes the sealed stage harder to predict; wash produce and use clean utensils.
How long does bottled kombucha keep in the fridge?
No single number suits every raw bottle. Continued fermentation can change acidity, alcohol and pressure even under refrigeration, and added fruit can deteriorate. Keep bottles cold, inspect them, and discard any drink with mold, a damaged container or an unexpected spoiled odor. Follow the storage guidance for the ingredients you added.
Why is there a blob in my bottle?
A small cellulose pellicle can form because live culture remains in raw kombucha. Its presence alone doesn’t confirm that the drink is safe or spoiled. Check the bottle, smell and exposed surface for other signs; discard it if you find mold or an unusual rotten odor.
Can I carbonate with CO₂ instead?
Yes. A keg and CO₂ system can carbonate chilled kombucha without supplying more sugar for bottle fermentation. Alcohol already present remains, and live kombucha still needs suitable cold storage. Follow the equipment maker’s pressure limits, cleaning steps and serving instructions.
Sources
The guidance above draws on these references. Health and safety points are conservative on purpose: home brewing has real variability, and no article can check your batch for you.
These guides were drafted with AI assistance and reviewed by a hobby home kombucha brewer.
- Alcohol and Tobacco Tax and Trade Bureau (TTB): Kombucha information and resources
- BC Centre for Disease Control: Safety of Fermented Foods, section 3.11 Kombucha and Jun (2024)
- Ethanol concentration of kombucha teas in British Columbia, Canada (2021). Journal of Food Protection
- Pennsylvania Department of Agriculture: Kombucha brewing and bottling guidelines
- Utah State University Extension: Kombucha