What are the two fermentation stages of kombucha production?
In F1, the fermentation sequence follows predictable microbial phases. The first 2–4 days are dominated by yeast activity (primarily Brettanomyces and Zygosaccharomyces) converting sucrose to fructose and glucose, then fermenting glucose to ethanol and CO2. Days 4–10 see acetic acid bacteria (Komagataeibacter, Gluconobacter) oxidising the ethanol to acetic acid and glucose to gluconic acid. Lactic acid bacteria contribute more slowly throughout, adding lactic acid as a secondary acidulant. The SCOBY pellicle grows incrementally as K. xylinus produces cellulose. Target F1 end-point: pH 2.8–3.5, total acidity 0.5–0.8%, residual ethanol 0.2–1.2% ABV depending on original sugar and fermentation duration.
F2 is where carbonation is created and flavour is developed. After F1, the base kombucha is decanted off the SCOBY, flavourings or additional sugar (fruit juice, herbs, additional honey) are added, and the bottles are sealed. Residual yeast in the liquid continues fermenting residual sugars, producing CO2 that has nowhere to escape — it dissolves under pressure, creating natural carbonation. Temperature control during F2 is critical: too warm and fermentation is too rapid (pressure builds too fast, creating explosion risk and overly acidic, harsh kombucha); too cold and carbonation fails to develop adequately. 20–24°C for 2–5 days, then transferred to cold (4°C) storage to arrest further fermentation, is a standard artisan protocol.
Commercial kombucha producers face a regulatory challenge: if F2 is allowed to progress substantially, alcohol content can exceed 0.5% ABV (making it legally an alcoholic beverage requiring different labelling). This is why commercial kombucha is often force-carbonated rather than naturally secondary-fermented — giving up the quality of natural carbonation for regulatory predictability.
| Stage | Duration | Container | Primary objective | Target outcome |
|---|---|---|---|---|
| F1 (Primary) | 7–14 days | Open or loosely covered vessel | Build acidity, develop base flavour | pH 2.8–3.5, TA 0.5–0.8% |
| F2 (Secondary) | 2–5 days | Sealed bottle under pressure | Create natural carbonation, develop flavour complexity | Target carbonation, alcohol < 0.5% |
F1 and F2 management are covered in the zeroproof.one kombucha production guide — including temperature control, sugar addition rates, and how to avoid over-carbonation and explosion risk.