Flavoring & Carbonation

Flavoring and Carbonating Kombucha: A Complete Guide

The second fermentation is where flavour and fizz happen. How to add fruit, herbs and spices without over-sweetening, how much priming sugar to use, and how to get consistent carbonation you can trust.

August 19, 2026 13 min read By Nadia Okonjo

Flavoring and Carbonating Kombucha: A Complete Guide — original illustration

What this article covers

The second fermentation is where flavour and fizz happen. How to add fruit, herbs and spices without over-sweetening, how much priming sugar to use, and how to get consistent carbonation you can trust.

Topics covered: Second Fermentation · Flavoring · Carbonation · Priming Sugar · Bottling

The second fermentation is where kombucha becomes interesting. In the first fermentation the culture turns sweet tea into something tart and drinkable; in the second, you add flavour and let the brew carbonate in a sealed bottle. It is also the stage with the only genuinely dangerous failure mode in home kombucha — an over-carbonated bottle that fails.

This guide covers how to flavour, how to carbonate deliberately rather than hopefully, and how to keep the pressure inside the range where a bottle is a bottle rather than a hazard.

Fruit, ginger and herbs laid out for a second fermentation.
Fruit, ginger and herbs laid out for a second fermentation.

What changes in the second fermentation

Two things happen at once when you seal flavoured kombucha in a bottle.

  • The remaining sugar ferments. Any sugar left in the brew, plus the sugar you add with the flavouring, is consumed by the still-living yeast. The carbon dioxide that produces has nowhere to go, so it dissolves into the liquid. That is carbonation.
  • The flavour infuses. Fruit, herbs and spices give up their aromatics into the brew over a day or two. Some also add acidity, tannin or bitterness.

Those two things are coupled, and that coupling is the source of almost every carbonation problem. Fruit adds sugar, sugar increases pressure, and pressure is what makes bottles fail. Flavouring and carbonation cannot be planned separately.

Taste before you flavour

The first fermentation determines your base. If it is still noticeably sweet, the second fermentation will produce too much pressure. If it is already sharply vinegary, the flavouring has less sugar left to work with and will carbonate weakly.

Choosing a flavouring

The table below gives rough starting quantities per litre of second fermentation, along with how much sugar each option contributes. The sugar column is the one that matters for pressure.

FlavouringQuantitySugar addedNotes
Fresh ginger10–15 gLowGrate or slice thinly. Sharp and aromatic; a little goes a long way
Berries50–70 gModerateFresh or frozen, crushed lightly. Strain if you want clarity
Citrus zest20–30 gLowUnwaxed fruit; avoid the bitter pith
Citrus juice30–50 mlModerateAdds acidity as well as flavour
Stone fruit60–80 gModerate to highPeach, apricot, plum — chopped. Reduce priming sugar
Tropical fruit70–80 gHighMango, pineapple, passion fruit. Ferments vigorously
Fresh herbs5–8 gVery lowBasil, mint, lemon verbena. Bruise gently
Dried flowers6–10 gVery lowHibiscus, chamomile. Steep in hot water and cool first
Fruit juice80–100 mlHigh100 % juice, not squash. Prime conservatively
Spices1–3 gVery lowCinnamon, cardamom, star anise. Easy to overdo — start small

A few combinations that work reliably and are hard to ruin: ginger and lemon zest; raspberry and mint; peach and hibiscus; blueberry and a scrape of vanilla. A useful principle is to pair something aromatic (herbs, zest, spices) with something fruity (berries, stone fruit) so the result has both depth and lift.

Some things should not go in the bottle

Do not add whole pieces of hard fruit, nuts, seeds or anything with fat or oil. Fat can go rancid and oil can affect carbonation. Avoid honey at this stage if you want predictable results — its sugar composition varies. And never add artificial sweeteners, which the yeast cannot ferment.

Carbonation: the numbers that matter

Carbonation is measured in volumes of CO2 — the volume of gas dissolved in a given volume of liquid at standard conditions. It is a convenient scale because it maps onto how fizzy something feels.

Volumes CO2Comparable toSuitability for kombucha
1.5–1.8Cask aleBarely sparkling — often considered flat
2.0–2.2LagerGentle, soft carbonation. Safest for glass bottles
2.4–2.8Typical ale or soft drinkThe usual kombucha target — lively and clearly sparkling
3.0–3.2Highly carbonated lager, some sodasSparkling. Requires pressure-rated bottles and early chilling
3.5+Soft drinks, champagneNot recommended for bottle-conditioned kombucha

The rule of thumb for priming is that roughly 2 grams of sugar per litre will produce about 1 volume of CO2 when fully fermented in a sealed bottle. That single figure is enough to plan a batch.

Two adjustments matter:

  • Residual carbonation. Kombucha is rarely completely flat before bottling. A batch that is already slightly sparkling holds roughly 0.5 volumes, and a noticeably fizzy one around 1.0. Subtract that from your target.
  • Sugar from flavouring. Convert the flavouring's sugar contribution to volumes the same way and subtract it too. A fruit-heavy second fermentation often needs no priming sugar at all.

Plain kombucha needs priming; fruity kombucha often does not

If you are adding 80 g of berries or 100 ml of juice per litre, that is already a substantial sugar addition. Adding a full priming dose on top is the most common route to an over-carbonated bottle.

A repeatable bottling method

  1. Prepare your bottles. Wash them thoroughly and sanitise them. Swing-top bottles are the easiest choice because the gasket can be checked and replaced.
  2. Decide your carbonation target. Gentle (2.0), lively (2.6) or sparkling (3.2) volumes. Start gentle if you are new to this.
  3. Calculate priming sugar. Use the calculator on this site, or the 2 g/L per volume rule, then subtract residual carbonation and the flavouring's sugar.
  4. Dissolve the sugar. Dissolve priming sugar in a small amount of warm water before adding it to the batch, and stir gently. Dry sugar added to a bottle settles and causes one bottle to over-carbonate while others stay flat.
  5. Add the flavouring. Divide it evenly between bottles rather than adding it to the whole batch, unless you want every bottle to be identical.
  6. Fill with 2–3 cm of headspace. Oxygen in the headspace matters less than in beer, but the headspace itself is needed for the liquid to expand slightly.
  7. Seal and leave at room temperature for one to three days. Warm weather halves this.
  8. Test after one day. Open one bottle over a sink. If it is already lively, refrigerate the whole batch.
  9. Refrigerate. Cold stops the fermentation. Bottles should be chilled within a day or two of reaching your target carbonation.
Bottles filled and sealed, ready for a short second fermentation at room temperature.
Bottles filled and sealed, ready for a short second fermentation at room temperature.

Understanding pressure and bottle failure

A bottle fails when the internal pressure exceeds what the glass can hold. The pressure depends on how much CO2 was produced and how warm the bottle is. Warming a bottle raises the pressure considerably — which is why a batch that seemed fine on the counter can fail once it is somewhere warm.

FactorWhy it raises the risk
Too much priming sugarMore sugar means more CO2 means more pressure
Sugary flavouring plus full primingThe two are additive and easy to double-count
Warm storagePressure rises with temperature; warmth also speeds fermentation
Leaving bottles too longFermentation continues until the sugar runs out
Wrong bottlesDecorative glass, thin glass and juice bottles are not pressure-rated
OverfillingLess headspace means less room for the liquid to expand
Damaged bottlesChips, star cracks and worn gaskets fail well below rated pressure

Bottle bombs are preventable and genuinely dangerous

Over-carbonated glass can fail with enough force to cause serious injury, particularly to eyes and hands. Use bottles designed for carbonated drinks, start with the gentle target, test after one day, chill early, and handle a suspect bottle with a towel and eye protection. If a bottle is visibly bulging or the cap is deforming, chill it and dispose of it carefully rather than opening it.

Troubleshooting carbonation

ProblemLikely causeFix for next batch
No carbonation at allNot enough sugar; too cold; bottles not sealedAdd priming sugar; keep bottles at 20–24 °C; check gaskets
Fizzes but goes flat quicklyLow carbonation overall; large headspaceIncrease priming slightly; fill closer to the top
One bottle gushes, others do notUneven sugar distributionDissolve priming sugar in the batch, not per bottle
Foams over on openingOver-carbonated or stored too warmReduce priming; refrigerate longer before opening
Yeasty, bready flavourToo much sediment carried over; warm second fermentationRack off the sediment before bottling; keep it cooler
Sour and flatFirst fermentation ran too long; little sugar leftBottle earlier; add a little priming sugar
Sediment in the bottleNormal — yeast and fruit particles settlingRack more carefully; pour gently when serving
Chilled bottles ready to open — cold is what stops the fermentation and holds the carbonation.
Chilled bottles ready to open — cold is what stops the fermentation and holds the carbonation.

Serving and storing

Once carbonated, keep bottles refrigerated. Cold storage slows the fermentation to a crawl and holds the carbonation steady; a bottle left at room temperature will continue to build pressure.

Open a chilled bottle carefully. Swing-top bottles hold pressure well and release it when opened normally; if a bottle has been sitting in the cold for a while, the pressure will be lower and the release gentler.

Finished kombucha contains trace alcohol and caffeine. It is not suitable for children, and anyone who is pregnant, breastfeeding, immunocompromised, diabetic or taking medication should speak to a healthcare professional before drinking it.

Nadia Okonjo

Contributing Brewer

Focuses on flavour, carbonation and troubleshooting. Has opened enough over-carbonated bottles to hold strong opinions about priming sugar.

Please read

This article is educational reference material. It is not professional brewing, food-safety, legal or medical advice, and it cannot guarantee a brewing result. Conditions in your kitchen, the health of your culture and the water you use all affect the outcome.

  • Home-brewing rules differ between countries, states, provinces and municipalities, and change over time. Check your own local requirements before you brew, bottle or share what you make.
  • Work with food-grade equipment, keep hot liquids away from children and pets, and follow the manufacturer's instructions for any appliance.
  • Bottles under pressure can fail. Use bottles designed for carbonated drinks, do not over-prime, and refrigerate once carbonation is right.
  • Consult a qualified professional for anything involving gas pressure, electricity, high heat, chemical cleaners or acids.

Finished kombucha contains trace alcohol and caffeine. It is not suitable for children, and anyone who is pregnant, breastfeeding, immunocompromised, diabetic or taking medication should speak to a healthcare professional before drinking it. Kombucha is a food, not a medicine, and it does not treat, cure or prevent any disease. Drink responsibly and never supply alcohol to anyone below the legal drinking age.

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