Kombucha second fermentation: fruit, flavours and carbonation

The kombucha from first fermentation is already ready to drink. But it’s flat, barely effervescent, with a direct acidity and a flavour that’s mostly fermented tea.

Second fermentation is the step that transforms it: it adds fizz, softens the acidity, and lets you customise the flavour with fruit, herbs, spices, or other aromas.

It’s not mandatory. But once you do it, it’s hard to go back.

Yeasts keep working, in a sealed bottle

In first fermentation, the jar is covered with breathable cloth. The CO₂ produced by yeasts can escape freely - which is why kombucha is flat or only slightly effervescent, never very bubbly.

In second fermentation, you bottle the kombucha in a sealed airtight bottle with added sugar (from fruit, juice, or plain sugar). The yeasts continue consuming that sugar and produce CO₂ - but this time it has nowhere to go. It dissolves into the liquid under pressure.

When you open the bottle, pressure drops suddenly and CO₂ forms bubbles.

The result is fizzy kombucha - exactly like a carbonated drink, but produced by natural fermentation.

Fruit, juice, spices: options and proportions

Fresh or bottled fruit juice (no preservatives)

The simplest and most predictable option. Juice brings natural sugars, aromas, and colour.

Standard proportion: 10–20% of the bottle’s volume. For a 500ml bottle, 50–100ml of juice.

Juices that work well: fresh ginger (not strictly a juice, see below), mixed berries, mango, peach, pomegranate, apple, pear, citrus. Avoid juices with preservatives - they interfere with fermentation.

Fresh or frozen fruit

More aromatic than juice, with slightly slower fermentation. Fresh fruit also introduces its own natural yeasts, which can make fermentation less predictable.

Proportion: 15–25% of volume (e.g. 75–125g of fruit for 500ml).

Fresh ginger

One of the classic pairings with kombucha. Grated or in thin slices, it adds spiciness and speeds up fermentation (ginger has active natural yeasts). Use 5–15g per 500ml - more and it becomes very spicy.

Spices, herbs and flowers

Don’t add sugars for fermentation, so use them in combination with fruit or sugar. Cinnamon, cardamom, lavender, hibiscus, mint, vanilla: small quantities (1–3g per 500ml), otherwise they can overpower the kombucha.

Plain sugar

If you want effervescence without added flavours, add 4–8g of sugar per litre (about 2–4g for a 500ml bottle). Produces fizzy kombucha with the first fermentation flavour unchanged.

Not all bottles handle the pressure

Second fermentation generates significant pressure inside the bottle. Using unsuitable bottles is risky.

Recommended bottles:

• Grolsch-style bottles (or similar with ceramic and rubber flip-top closure): perfect for kombucha, they handle pressure and open easily for the daily burp.
• Glass bottles with fermentation-specific screw cap: available at homebrewing shops.
• PET bottles for carbonated drinks: not the most attractive, but work excellently and have the advantage of being squeezable - when the bottle becomes hard, carbonation is ready.

Avoid:

• Wine or beer bottles with improperly sealed crown caps
• Standard glass jars with regular non-pressure lids
• Any bottle not designed to withstand the pressure of a carbonated drink

From first fermentation to the bottle

When to start 2F:

When the first fermentation kombucha has reached pH between 2.5 and 3.5. At this point it’s ready to bottle.

Process:

1. Remove the SCOBY and set aside with 100–200ml of kombucha as starter liquid for the next batch.
2. Add chosen flavourings to the bottle (fruit, juice, spices).
3. Pour the kombucha into the bottle, leaving 3–5cm of headspace.
4. Seal airtight.
5. Leave at room temperature (20–24°C) for 2–4 days.
6. Every day, briefly open the bottle to release excess pressure (“burp”). Reseal immediately.
7. When the pressure feels right (bottle is firm, or PET is rigid), refrigerate.
8. In the fridge, fermentation almost completely stops. Kombucha keeps for 2–4 weeks.

Neither flat nor a geyser

The pressure that builds depends on:

• Amount of added sugars
• Fermentation temperature
• Fermentation time
• Yeast activity in the batch

Carbonation in 2–4 days at 22°C with 100ml of juice per 500ml is good for most batches. At higher temperatures or with more sugar, it goes faster.

How to test without opening:

If using PET bottles, squeeze them daily. From soft to firm = carbonation in progress. When very firm, refrigerate.

If using glass bottles, do a daily burp: open briefly and reseal. Feel how much pressure escapes. When the sound is significant, carbonation is good.

Problem: excessive pressure

If the bottle is rock-hard and liquid explodes when you open it: fermentation went too far. Refrigerate immediately and open very slowly over the sink next time.

Problem: no carbonation after 4 days

Possible causes: too little sugar, temperature too low, very acidic kombucha that inhibited the yeasts. Try adding a pinch of sugar and wait another 1–2 days.

Six starting points

FAQ

Should I filter the kombucha before 2F?

Not necessary. Yeast threads don’t harm the result. If you prefer clearer kombucha, filter through a fine strainer before bottling - but not with paper filters, which also remove good yeasts and reduce carbonation.

Can I do 2F directly in the jar?

Yes, if you have an airtight-seal jar. But the pressure generated can be significant - make sure the jar is rated for it.

Is fruit fermented in the bottle safe to eat?

Yes. It’s simply fruit that has fermented in an acidic environment. The taste changes (becomes less sweet, more complex), but it’s safe.

How long can I do 2F?

Maximum 4–5 days at room temperature. After that, acidity keeps rising and pressure can become problematic. If you forget a bottle, open it carefully over the sink.

Can I use honey instead of sugar?

Yes, but honey has mild antimicrobial properties that can slow down yeasts. It works, but carbonation may be less predictable. Use unpasteurised honey and increase the quantity slightly.

Read also: Complete guide to kombucha fermentation → · What tea to use for kombucha →