The production of kombucha has two distinct phases: a first open fermentation where organic acids are generated, and a second fermentation in the bottle where flavour and natural carbonation are added. We explain how it works and how to control it.
Reminder: first fermentation
kombucha ferments thanks to a colony of yeasts and bacteria —the SCOBY— which transforms sugar into organic acids. This first fermentation is aerobic: it takes place in the presence of oxygen, in an open container covered with a breathable cloth.
The acetic acid bacteria in kombucha (Acetobacter, Komagataeibacter) need oxygen to work: they oxidise the ethanol produced by the yeasts and transform it into acetic acid, which is responsible for the characteristic sour taste. This is why the first fermentation is carried out in an open container, covered with a breathable cloth.
Yeasts are facultative anaerobes: they can function both with and without oxygen. If they have oxygen, they respire (consume sugar and produce CO₂ and water). If they do not have oxygen, they ferment (consume sugar and produce ethanol and CO₂). This dual capacity is key to understanding what happens in second fermentation.
In this first stage, yeasts break down sucrose molecules into glucose and fructose (via an enzyme called invertase), because bacteria cannot metabolise sucrose directly. Once the kombucha has reached the desired flavour and acidity, it is time to bottle it.
What is second fermentation
Second fermentation takes place inside the already sealed bottle. When bottling the kombucha, fruit juices, roots, or infusions can be added to provide flavour: ginger, turmeric, lemon, hibiscus, mango… any combination one wishes to explore.
The sugar from the added juices serves as additional fuel for the yeasts present in the liquid. As they are now in a sealed, oxygen-free environment, the yeasts ferment (instead of respiring) and produce alcohol and carbon dioxide. The CO₂ cannot escape the bottle, so it accumulates in the liquid and provides the characteristic natural carbonation of kombucha.
As long as residual dissolved oxygen remains in the liquid, the acetic acid bacteria remain active during the first few hours of second fermentation and oxidise part of the ethanol produced by the yeasts. When the dissolved oxygen is depleted, the acetic acid bacteria are inhibited, and only the yeasts continue working in anaerobiosis. This is why the balance between carbonation, acidity, and final alcohol content depends heavily on the exact moment of bottling and chilling.
In long-fermented kombuchas, lactic acid bacteria (Lactobacillus) may also appear, producing small quantities of lactic acid, contributing to the organoleptic profile and flavour complexity.
Chemistry summarised
- 1st fermentation (aerobic, open container): yeasts respire and produce CO₂ + H₂O. Acetic acid bacteria oxidise ethanol → acetic acid.
- 2nd fermentation (anaerobic, sealed bottle): yeasts ferment and produce ethanol + CO₂. The CO₂ accumulates and creates the bubbles. Ethanol also accumulates, until chilled.
During second fermentation, the alcohol produced by the yeasts can no longer be oxidised to acetic acid because the bottle is sealed and acetic acid bacteria need oxygen to do so. This is why alcohol accumulates. If left for too long, the kombucha's alcohol content increases. Controlling the duration is key.
Time and conditions for second fermentation
Second fermentation usually lasts between 1 and 5 days, depending on:
- Ambient temperature: the warmer it is, the faster the fermentation.
- Amount of sugar added: more sugar means more material to ferment and more CO₂ produced.
- Population of active yeasts at the time of bottling.
It is advisable to keep the kombucha in a place at ambient temperature and dark: sunlight can negatively affect the fermentation process and alter the flavour.
It is also important to use pressure-resistant glass bottles. The generated bubbles increase internal pressure, and an unsuitable bottle can break. Champagne or beer-type bottles with mechanical closures are the safest.
Once the kombucha has reached the desired level of carbonation, it can be refrigerated to stop the process. Cold slows down the activity of yeasts and bacteria, maintaining the flavour balance.
A trick to control second fermentation at home
If you ferment at home, you will not have instrumental control over this second phase. Especially because, without analysis equipment, it is difficult to know the sugar level of the kombucha at the time of bottling. If there is too much sugar, the yeasts will produce CO₂ until the sugar runs out… or until the bottle bursts.
The trick: use a used plastic soft drink bottle as a sample bottle. When bottling, press it and you will see that it is soft. As the days pass and carbonation occurs, the bottle will become harder due to internal pressure.
When you notice that the plastic sample bottle is sufficiently hard, it is time to move all your kombuchas to the fridge to stop second fermentation. This way, you avoid having to open real bottles to test the carbonation level.
Frequently asked questions
What is kombucha second fermentation?
It is the production phase that takes place inside the sealed bottle, once the base kombucha has been bottled. In this phase, the yeasts consume the sugar from the added juices and produce CO₂, generating the natural carbonation of the drink.
How long does second fermentation last?
Between 1 and 5 days, depending on the ambient temperature, the amount of sugar added, and the population of active yeasts. The warmer it is or the more sugar, the faster the fermentation.
What ingredients can be added during second fermentation?
Fruit juices, roots, or infusions: ginger, turmeric, lemon, hibiscus, mango, strawberry, mint, red berries… any ingredient that provides sugars or flavours works.
Why does kombucha have more alcohol after second fermentation?
Because with the bottle sealed and without oxygen, the yeasts switch to fermenting (instead of respiring) and produce more ethanol. If left for too long, the alcohol content can increase. That is why it is advisable to control the duration and refrigerate in time.
Can a bottle explode during second fermentation?
Yes, if a bottle not suitable for pressure is used or if it is left to ferment for too long. That is why it is essential to use resistant glass bottles (champagne or beer type) and to control the carbonation level.
How do I stop second fermentation?
By refrigerating the bottle. Cold greatly slows down the activity of yeasts and bacteria, effectively stopping the fermentation.
Does Mūn Kombucha undergo second fermentation?
Yes. Mūn Kombucha varieties undergo a second fermentation in the bottle where fruit juices, roots, or infusions are added, providing them with flavour and the characteristic natural fizz. All our carbonation is natural: we do not add CO₂.
How long does Mūn ferment before bottling?
Up to 30 days in first fermentation, following the traditional recipe. The longer the first fermentation, the less residual sugar and less alcohol at the end of the process, because the acetic acid bacteria have more time to oxidise the ethanol produced by the yeasts. This is why Mūn kombuchas have between 0.09 g and 1.80 g of sugar per 100 ml.
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Natural fizz without tricks
Mūn kombuchas are bottle-fermented, unpasteurised, with the natural carbonation generated by the process itself. Since 2015.
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