The first fermentation (F1) is the open, aerobic stage where a SCOBY turns sweet tea into tart kombucha over roughly one to several weeks.
The second fermentation (F2) happens after fruit, juice or botanicals go in and the vessel is sealed, so the culture feeds on the new sugar and the carbon dioxide it makes stays in the drink as natural fizz.
Kombucha ferments twice. The first fermentation, F1 in brewers' shorthand, is the open stage where the culture turns sweet tea into something tart. The second, F2, is sealed, and it is where most of the flavor on the label and nearly all of the natural fizz comes from. Knowing which is which explains most of what you taste in a can.
| First fermentation (F1) | Second fermentation (F2) | |
|---|---|---|
| Vessel | Open to air, under a cloth or in a vented tank | Sealed bottle, keg or tank |
| What feeds the culture | The sugar in the sweet tea | Sugar in the added fruit, juice or honey |
| Typical length | About 7 to 14 days in published reviews, up to 2 to 4 weeks in large tanks | Two days to several weeks |
| What it adds | Acidity, tea character, a new cellulose mat | Flavor and carbonation |
| What to taste for | Balance of sweet and sour | Bright fruit and fine bubbles |
What happens in the first fermentation
F1 starts the moment the culture goes into cooled sweet tea. The Kombucha Code of Practice from Kombucha Brewers International asks for at least 10 percent finished kombucha as starter, which drops the pH quickly and protects the batch. Then the vessel sits open to the air, covered against fruit flies. Yeast splits the sugar, and acetic acid bacteria turn what the yeast makes into acids, acetic acid above all. On the surface the bacteria spin a fresh cellulose mat, the pellicle, which is why a jar of kombucha seems to grow a lid.
How long F1 runs is a matter of taste. The Code describes the drink moving from pleasantly fruity and tangy after a few days to vinegary after a long ferment, and Oregon State researchers note that producers set their end point by the sweet and acid balance they want. Baba's Brew ran seven days when PA Eats visited. Brew Dr. reports around two to four weeks in its larger tanks. Temperature, vessel size and how much culture went in all move the clock.
What happens in the second fermentation
At the end of F1 the brewer pulls the pellicle and decides what the kombucha will become. For an F2, fruit, juice, ginger, flowers or herbs go in, and the vessel is closed. The Code counts this flavor infusion as a secondary fermentation when the flavorings bring fermentable sugar with them. The culture wakes up on that fresh sugar, and the carbon dioxide it makes has nowhere to go, so it dissolves into the drink.
The founder of Contraband Ferments walked Edible Brooklyn through it in 2017: a finished ferment moves to a new vessel with fresh fruit, dried fruit, whole spices, herbs or juices, and sits for two days to several weeks or months, tasted daily. Frozen berries, the founder found, worked best. The Code caps optional ingredients such as juice at 20 percent of the finished volume, so the tea stays the base.
Other routes to the same can
Not every brewer flavors in F2. Brew Dr. blends tea and botanicals in the steep, so they ferment from the start, and carbonates its kombucha cold in brite tanks rather than in the bottle. Others inject carbon dioxide at packaging, which the Code requires them to say on the label. A few brewers skip added flavor entirely and let the tea carry the drink. When you read a can, the questions are simple: what went in, and when.
Questions
What is the difference between F1 and F2 in kombucha?
F1 is open to the air and turns sweet tea sour. F2 is sealed, usually with fruit, juice or herbs, and builds flavor and carbonation.
How long is a second fermentation?
Anywhere from two days to several weeks or months. Contraband Ferments described that full range to Edible Brooklyn, tasting daily until the flavor was right.
Does every kombucha get a second fermentation?
No. Some brewers flavor in the steep before F1, some carbonate with injected carbon dioxide, and some bottle plain tea kombucha with no added flavor at all.
Is there a third fermentation?
The Kombucha Brewers International Code of Practice recognises a tertiary fermentation when kombucha is left to age in the bottle to change its flavor or fizz.
- Kombucha Brewers International, Kombucha Code of Practice
- Kombucha Brewers International, Kombucha Chemistry: What's in a brew?
- Edible Brooklyn, on secondary fermentation at Contraband Ferments, 2017
- Brew Dr. Kombucha, Process of Making Brew Dr. Kombucha
- PA Eats, profile of Baba's Brew, 2015
- Harrison and Curtin, Microorganisms, 2021
