Kombucha F2 fermentation blog post below, for info on F1 and kombucha brewing, go here:
F2 kombucha is where flavor and carbonation is added to finished F1 kombucha. Pay attention to how mature (acidic) your F1 kombucha is as you may need to dilute it with water to get a potable F2 drink that’s not too vinegar forward. Sweetness and water help round out F1 kombucha into F2 kombucha.
If your F1 kombucha is very acidic, consider proceeding to F2 as if your are making lemonade except instead of acidic lemons you have acidic kombucha.
During F1, your jar is covered with a breathable cloth, so oxygen is constantly available. The yeast and acetic acid bacteria in your SCOBY both have access to air, and any CO2 the yeast produce simply escapes into the room. During F2, you move the finished kombucha into a sealed bottle. That single change removes the oxygen supply and traps every bit of gas the yeast produce from that point forward. Add a new source of sugar (fruit, juice, or a second dose of sweetened tea) and the yeast that are still present in your F1 liquid go to work on it, producing CO2 that has nowhere to go. That trapped gas is what carbonates the bottle.
Flavor and fizz will come from feeding the kombucha yeast a new sugar source inside a sealed container.
What you need per bottle
| Ingredient | Amount |
|---|---|
| Acid-corrected F1 kombucha* | 80-90% of the bottle |
| Fruit, juice, or puree (your flavoring and sugar source)** | 10-20% of bottle volume (see combos below) |
*Matt’s F2 Kombucha Note: Quality F2 kombucha comes from tasting the end-product of your F1 kombucha. If it is too acid then it’ll raise the difficulty of creating a quality F2 kombucha. Taste the F1 kombucha and dilute with water (if necessary) to the desired acidity before making the F2 kombucha base. You should genuinely like the 80-90% ‘Acid-corrected F1 kombucha’ then add your 10-20% flavorings.
**Matt’s F2 Kombucha Note: The 10-20% range is recommended for beginners. Below it, the yeast often don’t have enough new sugar to build meaningful pressure. Above it, you have more potential for over-carbonating and the bottles will need to be burped and properly processed to avoid shattering bottle bombs. Start at the low end until you know how your specific culture and kitchen temperature behave.
Equipment
- Swing-top glass bottles, enough for your batch size
- Fine mesh strainer
- Funnel
- A cool, dark place to store bottles during F2
- A dish towel and a bowl or the sink, for opening bottles safely
Step-by-step F2 process
- Strain your F1 kombucha. This removes any SCOBY fragments or yeast strands so your bottles pour clean.
- Add your flavoring to each bottle first. Fruit puree, juice, or fresh ginger goes in before the liquid, so it’s easier to measure and distribute.
- Fill with F1 kombucha*. Leave about 10-20% of headspace. That headspace matters. It gives the CO2 somewhere to accumulate as gas before it fully pressurizes the liquid. (See Matt’s Note)
- Seal the bottle. The swing-top gasket is what allows pressure to build in the first place.
- Ferment at room temperature, out of direct sunlight, for 1-3 days. Warmer kitchens carbonate faster. This is a much shorter window than F1 because you’re not waiting on pH to drop into a safe range again, that already happened during F1. You’re just waiting on gas.
- Burp the bottles daily. Crack the swing top open over a sink for a second, then reseal. This releases excess pressure and lets you gauge how carbonated the batch is getting.
- Personally, I can burp once every 48 hours in my kitchen, but this comes from experience and understanding my kombucha culture. Be safe with carbonation! Use plastic bottles if necessary to gage pressure.
- Refrigerate once you’ve got the carbonation you want. Cold temperature slows the yeast down dramatically, which locks in your current fizz level instead of letting it keep climbing.
Glass bottles under pressure can shatter. Burp your bottles daily, every day, without skipping. If a bottle feels rock-hard with no give when you press the top, or if it’s been more than 3 days without burping, open it slowly over a sink with a towel over your hand, not against your face or body.
*Matt’s Bottling Note: Usually I’ll measure out 80-90% of the jar with water before starting and I’ll mark the glass bottle with a dry-erase marker to get a visual of where I should be pouring to. Sometimes 80-90% can be hard to tell with the sloped edges of the bottle.
What I personally do: Alternatively, you can also pour all of your Kombucha into a big mixing bowl take it’s weight and multiply by 0.1 or 0.2 to get the 10-20% liquid addition you’ll need then just put this into your flip top bottles for F2 Kombucha. You ‘ll have some leftover liquid you can drink right there.
Three flavor combos to start with
Each of these is scaled for one 16 oz (1/2 liter) bottle. All three sit at the lower end of the 10-20% range, since these are meant as a starting point, not your ceiling. Use the ‘What I personally do’ note above for larger batches.
| Flavor | Add-ins | Notes |
|---|---|---|
| Ginger-lemon | 1 tbsp fresh grated ginger, 1 tsp lemon juice | Ginger tends to carbonate fast. Check daily starting on day 1. |
| Mixed berry | 2 tbsp strawberry or raspberry puree | Puree produces sediment at the bottom of the bottle. This is normal, not spoilage. |
| Mango-ginger | 2 tbsp mango puree, 1 tsp fresh grated ginger | A milder, less acidic combination if you find straight ginger too sharp. |
Matt’s Note: Juice ferments faster than puree, and puree ferments faster than whole fruit chunks. This is because the yeast need direct contact with dissolved sugar. If your batches are carbonating slower than you’d like, that’s often the first thing to pay attention to, not the fermentation time.
Reading your bottle
- Flat after 3 days: Your flavoring likely didn’t provide enough available sugar, or your F1 kombucha was already low on active yeast. Try juice instead of puree, or extend F2 by a day. Open the bottle and oxygenate the yeasts to kick start fermentation. If it’s not sweet then you need to add sugar. Sugar is NOT optional here with this problem as it’s the substrate that yeasts must feed on to function.
- Over carbonated or gushing on open: You went past the point your burping schedule could keep up with. Shorten your F2 window next time and burp twice a day in warm weather. Make sure you’re not doing too much sugar. More sugar means more burping is required.
- Sediment at the bottom: Normal with pureed or pulpy fruit. It’s yeast and fruit solids settling out, not a sign anything went wrong.
This covers the core F2 process and a few combinations to start experimenting with. If you want a larger library of tested flavor ratios, troubleshooting for specific fruits, and guidance on building your own combinations from scratch, that’s exactly what the F2 Flavor Guide is built for. It pairs with the free Fermentation Logbook if you want to track carbonation timing and flavor notes across batches.
A full video walkthrough of this process is coming. This post will get an update with the video once it’s live.
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