The second ferment, the fizz window
Kombucha's first ferment belongs to the culture; the second belongs to you. The kombucha guide gives the window, one to three days at room temperature in sealed bottles with 10–20% juice by volume (UC's range; USU also says 1–3 days), and inside that window the yeast converts the added sugar into the carbonation the first ferment never builds under its loose cloth. (Brewers' logs often call this stage the 2F or second fermentation, same window, shorter name.) The work of the window is watching.
The setup, from the guide
The first ferment ends by bottling: the SCOBY comes out, and about a cup of finished liquid is reserved as the next batch's starter, the acid that defends the new brew. Into the bottles: the strained kombucha plus the juice, the guide's range runs 10–20% of the bottle's volume (UC); ginger presses its dose smaller. Seal with swing-tops, the guide's one non-negotiable being pressure-rated glass: plastic deforms, flimsy glass becomes a bomb, and the first opening of any carbonated batch happens over the sink.
The watching
Warm room, out of the sun. Burp the caps daily at the start, pressure builds with surprise on the first batch, and the burp both releases it and samples the fizz. The guide's variables, when the fizz lags: more sugar in the bottle, a warmer room, or a look at the starter's yeast strands. When the pour tastes right, the bottle goes to the cold shelf, the cold is what parks the fermentation where you like it. NCHFP's keeping times split here: plain kombucha, bottled and refrigerated, is for use within 30 days (that's the line on the app's plan card); once fruit or juice is mixed in after fermentation, NCHFP says to drink it right away or refrigerate it for no more than 2 days. NCHFP doesn't cover a room-temperature second ferment, so the flavored bottle gets the shorter line.
The vinegar-flat batch
Taste a batch gone over, flat-tart, past the peak, and the fix belongs to the NEXT batch: bottle earlier, from day 7. The window's edges are where every kombucha brewer calibrates; the log sheet's row (taste, fizz, date) is the calibration's paper trail.
Label the batch. — the cellar crew