If you’re familiar with kombucha brewing, you probably
know that at room temperature, the live yeasts present in
your brew will eat sugar that's present in your sweet tea
and convert it into carbon dioxide.
That happens in your brewing vessel during 1st fermentation
(F1) and in the bottle during 2nd fermentation (F2). But did
you know that even if you refrigerate your bottles, and even
if the yeasts go dormant while they’re in the fridge, you can
take your bottles out and leave them at room temp to boost
carbonation even further?
If you leave your homemade kombucha bottle sealed at
room temp even for a few hours, the yeasts will reactivate
and start eating the sugars again to produce more carbon dioxide. So as long as it's airtight, it'll start getting fizzier.
This is helpful in a couple scenarios:
Say you put a batch of kombucha in the fridge after F2-ing for 3 days. Then after it’s cooled down, you pop one open and realize it’s not quite as fizzy as you’d like. No worries! You can take the rest of the bottles from that batch and take them out of the fridge. Then, let them sit at room temp for another day or two to help bump up the carbonation. Just remember to refrigerate them again before opening so you don’t make a mess. (Cold liquids hold onto fizz better than room temp liquids, so a chilled bottle is less likely to make a mess and more likely to keep the carbonation in the liquid where you want it!)
This is also helpful if you open a bottle of kombucha and don’t finish it. If it’s flat and you want to re-carbonate it, you can just seal it up, let it sit for at least a few hours and it’ll gain back some of that lost carbonation. Just note that it may be a little less sweet than when you first open it (since the yeasties are eating up that sugar to create CO2), but hey, it beats having flat kombucha!
Note: I can really only say this works for homebrewed kombucha. This doesn’t necessarily apply to store-bought kombucha because who knows what yeast inhibitors or practices they use to halt their fermentation processes in the bottle. (They say they’re raw products, but the FDA doesn’t really regulate labels that say “raw” for accuracy.) So depending on the brand of kombucha you buy, you may or may not be able to re-fizz it after it’s gone flat.