First vs Second Fermentation in Kombucha: What's the Difference?

Kombucha brewing has two distinct stages, each with a different purpose, vessel, and outcome. First fermentation turns sweet tea into tangy kombucha; second fermentation turns still kombucha into a fizzy, flavoured drink. Knowing what each stage does — and what it doesn’t — lets you control the final result and troubleshoot when something goes wrong.

First fermentation (1F): building acidity

First fermentation is the main event. Bacteria and yeast in the SCOBY consume the sugar in your sweet tea — the yeast makes CO₂ and a little alcohol, and the bacteria convert those byproducts and some sugar into organic acids, mainly acetic and gluconic acid. Those acids give kombucha its tang and make the brew hostile to mould.

Vessel and cover A wide-mouth glass jar with a breathable cloth or coffee-filter cover. The surface bacteria need airflow to build a new SCOBY layer, so no sealed lid.

Duration 7–14 days at 20–26°C. Start tasting from day 7, aiming for a balance of sweetness and tang — enough acid to taste interesting, not vinegary.

Result Tangy, lightly effervescent, unflavoured kombucha with reduced sugar. Drinkable as-is; flavour comes from your tea, sugar, and starter liquid.

Key variables are sugar amount, starter-liquid ratio, room temperature, and SCOBY health — dial them in with the Kombucha Brewing Calculator.

Second fermentation (2F): carbonation and flavour

Second fermentation is optional and adds the fizz that makes kombucha feel like a drink rather than a tart tea. You bottle the finished 1F with a little sugar (or fruit juice) and seal it. With no escape for CO₂, pressure builds and the gas dissolves into the liquid — carbonation.

Vessel Airtight glass bottles — swing-top or screw-top. Plastic works too and doubles as a gauge: it firms up as pressure builds.

Duration 2–5 days at room temperature, then refrigerate. Warmer rooms carbonate faster. Taste and burp daily to check pressure.

Result Flavoured, carbonated kombucha. The added juice or sugar provides both flavour and fuel for carbonation, and is mostly consumed by the end.

Use the Second Fermentation Calculator to calculate sugar and juice for your target carbonation.

Side-by-side comparison

The two stages differ on nearly every axis except the underlying culture doing the work.

Purpose 1F builds acidity and reduces sugar; 2F builds carbonation and adds flavour.

Vessel 1F is an open jar with a breathable cover; 2F is sealed bottles.

Duration and sugar 1F runs 7–14 days on ~70g sugar per litre of tea; 2F runs 2–5 days on ~5–10g sugar per bottle.

SCOBY and outcome 1F needs the SCOBY and yields still kombucha; 2F does not, and yields fizzy kombucha. Only 1F is mandatory.

What can go wrong at each stage

! Watch for

1F too vinegary — fermented too long or too warm; taste earlier
1F too sweet — not long enough or too cold; extend a day or two
1F mould on top — insufficient starter liquid or contaminated equipment; discard the batch
2F flat bottles — poor seal, not enough sugar, or too cold
2F over-carbonated or gushing — too much sugar, too warm, or left too long

The most common 2F mistake is not checking the seals. A bottle that looks sealed but leaks slightly at the cap will never carbonate — all the CO₂ escapes. Press each cap firmly after sealing and listen for hissing.

Try the calculator

Second Fermentation Calculator

Match sugar and juice to your bottle size and target fizz — no guesswork, no gushers.

Open calculator →

Frequently asked questions

Can I skip second fermentation?+

Yes. First fermentation kombucha is perfectly drinkable — it's just still (uncarbonated). Second fermentation is optional and only needed if you want fizz or a more developed flavour.

How much sugar do I add for second fermentation?+

A common starting point is 5–10g of sugar per 500ml bottle, or 30–50ml of fruit juice. The Second Fermentation Calculator gives exact targets based on your desired carbonation level and bottle size.

Why did my 2F bottles not carbonate?+

The most common causes are too little sugar, a bottle seal that doesn't hold pressure, or too cool a room temperature. Check your seals first — even a small gap prevents pressure buildup.

Is second fermentation kombucha alcoholic?+

Kombucha contains a small amount of alcohol — typically 0.5–3% ABV in home brews depending on sugar and fermentation time. Second fermentation can push ABV slightly higher. Commercially sold kombucha is usually kept below 0.5% to avoid alcohol licensing requirements.

Do I need the SCOBY in second fermentation?+

No. The SCOBY stays behind in the 1F jar with your reserved starter liquid, ready for the next batch. 2F relies on the yeast and bacteria already suspended in the bottled kombucha, not the SCOBY itself.