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: the 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 bacteria present in kombucha need oxygen to ferment. Yeasts also do, but with nuances: they can function both with and without oxygen. If they have oxygen, breathe; if not, they ferment.
In this first stage, yeasts break down sucrose molecules into glucose and fructose (using an enzyme called invertase), because bacteria cannot metabolise sucrose directly. Once the kombucha has reached the desired flavour and acidity, it's time to bottle it.
What is second fermentation
The second fermentation takes place inside the already sealed bottle. When bottling 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 closed, oxygen-free environment, the yeasts ferment (instead of breathing) and produce alcohol and carbon dioxide. The CO₂ cannot escape the bottle, so it accumulates in the liquid and provides the natural carbonation characteristic of kombucha.
During the second fermentation, more alcohol is produced than in the first. If left for too long, the kombucha can become excessively alcoholic. Keeping it controlled is key.
Time and conditions for second fermentation
The 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 room 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 bubbles generated increase internal pressure, and an unsuitable bottle can break. Champagne or beer 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 won't have instrumental control over this second phase. Especially because, without analysis equipment, it's difficult to know the sugar level of the kombucha at the time of bottling. If there's 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'll see it's 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's time to move all your kombuchas to the fridge to stop the second fermentation. This way, you avoid having to open real bottles to test the carbonation level.
Frequently asked questions
What is kombucha's second fermentation?
It is the production phase that takes place inside the sealed bottle, once the base kombucha has been bottled. In this phase, yeasts consume the sugar from the added juices and produce CO₂, generating the drink's natural carbonation.
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 aromas works.
Why does kombucha have more alcohol after second fermentation?
Because with the bottle sealed without oxygen, the yeasts switch to fermenting (instead of breathing) and produce more ethanol. If left for too long, the alcoholic content can increase. That's why it's 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's left to ferment for too long. It is essential to use resistant glass bottles (champagne or beer type) and 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 fermentation.
Does Mūn Kombucha undergo second fermentation?
Yes. Mūn Kombucha varieties undergo a second fermentation in the bottle where fruit juices or infusions are added, providing them with flavour and the characteristic natural fizz.
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Natural fizz, no tricks
Mūn kombuchas are bottle-fermented, unpasteurised, with the natural carbonation generated by the process itself. Since 2015.
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