Kombucha is fizzy because carbon dioxide is dissolved in it, either made by the yeast in a sealed second fermentation or injected by the brewer before packaging.
The Kombucha Brewers International Code of Practice allows 'naturally carbonated' or 'naturally effervescent' only when no forced carbonation was used, and requires forced carbonation to be labelled.
Kombucha's bubbles are carbon dioxide, and there are two ways to get it into the drink. The culture can make it: yeast ferments sugar, gives off the gas, and in a sealed vessel the gas dissolves into the kombucha. Or the brewer can add it, injecting carbon dioxide at packaging the way most soda is made. The trade's standard treats the two differently on the label.
| Natural carbonation | Forced carbonation | |
|---|---|---|
| Where the gas comes from | Yeast fermenting fresh sugar in a sealed vessel | Carbon dioxide injected before or during bottling |
| When it happens | Second fermentation, or bottle ageing | At packaging |
| Label words allowed | Naturally carbonated, naturally effervescent | Must disclose forced carbonation |
| What you notice | Finer bubbles that vary a little batch to batch | Steady, predictable fizz |
How a culture makes bubbles
In the open first fermentation, most of the gas the yeast makes simply drifts away. The Kombucha Brewers International chemistry primer describes the yeast splitting cane sugar with an enzyme called invertase and fermenting the result. For bubbles, a brewer closes the system. Fresh sugar goes in, usually as fruit, juice or a little more sugar, the vessel is sealed, and the gas builds up in the liquid. The Code of Practice calls this flavor infusion a secondary fermentation when the flavorings carry fermentable sugar.
The researchers behind a 2022 Frontiers in Microbiology study did the same thing in miniature: flasks sat loosely covered for seven days and were then sealed for five more, so carbon dioxide could build up on its own. Contraband Ferments described to Edible Brooklyn a second ferment of two days to several weeks with fresh or frozen fruit, tasted daily.
Why some brewers add the gas instead
Natural carbonation varies with temperature, sugar and the culture, so two batches can pour differently. Brewers who need every can to match, or who filter and pasteurize, often carbonate at the end. Brew Dr. describes a route borrowed from craft beer: it chills its kombucha to around 33 degrees and carbonates it gently overnight in brite tanks before bottling it cold. The Code allows the words naturally carbonated or naturally effervescent only if no forced carbonation was used, and requires forced carbonation to be disclosed.
What to listen and look for
A good pour hisses when the can opens and settles into a fine, lively bead rather than big soda bubbles that vanish in a minute. A raw kombucha that foams up the neck has usually just been warm. A flat one at home was usually under-sealed or under-fed in the second ferment, or bottled too cold for the yeast to work.
Questions
Why is kombucha fizzy?
Yeast in the culture makes carbon dioxide as it ferments sugar. In a sealed bottle or tank the gas cannot escape, so it dissolves into the drink.
Is kombucha naturally carbonated?
Some is and some is not. Brewers who inject carbon dioxide must say so on the label under the trade's Code of Practice.
Why is my homemade kombucha flat?
Usually the second fermentation was not sealed well, had too little fresh sugar for the yeast, or was too cold or too short.
Why does some kombucha foam over when opened?
Raw kombucha that warmed up keeps fermenting and builds pressure. Open it cold and slowly.
