Kombucha: first and second ferment

Kombucha is two fermentations stacked on top of each other. The first ferment (F1) turns sweet tea into tart, lightly fizzy kombucha using the SCOBY in an open jar. The second ferment (F2) takes that kombucha, adds a little sugar or fruit, and seals it in a bottle to build carbonation. Get the ratios and timing right and both stages are forgiving.

What the SCOBY is

SCOBY stands for "Symbiotic Culture Of Bacteria and Yeast" — the rubbery disc that floats on top of your brew. The yeast eats the sugar and makes alcohol and CO₂; the bacteria turn that alcohol into the acids that give kombucha its tang and protect it from spoilage. Just as important as the SCOBY itself is the starter liquid it comes packed in: that's mature, acidic kombucha, and it's what drops your fresh sweet tea to a safe low pH from day one.

First ferment: the sweet-tea recipe (1 gallon)

The standard 1-gallon batch is easy to remember:

The ratio that matters most is the starter tea: aim for roughly 10% of the batch volume (about a 9:1 sweet-tea-to-starter ratio). That acidity is your safety margin against mold while the culture wakes up.

Method: brew the tea hot, dissolve the sugar in it, then — this part is critical — let it cool to room temperature before adding the SCOBY. Hot tea will kill the culture. Combine the cooled sweet tea, the starter liquid, and the SCOBY in a wide glass jar, cover the mouth with a tight-woven cloth secured with a band (it needs to breathe but keep fruit flies out), and leave it somewhere out of direct sun.

F1 timing and temperature

• Time: about 7–12 days, sometimes longer. Start tasting around day 7.
• Temperature: kombucha is happiest at 72–84°F (22–29°C). Warmer ferments faster (and more sour); cooler is slower and can stall below ~68°F.
• How to judge it: taste, don't watch the calendar. It's ready when it's pleasantly tart with just a little residual sweetness. Too sweet = give it more days; vinegary = you went long (still usable, or save it as starter).

When it tastes right, set aside 1–2 cups as starter for your next batch (and a home for the SCOBY), then bottle the rest.

Second ferment: building the fizz

F2 is where kombucha gets its bottled carbonation. You're giving the yeast a little fresh sugar in a sealed bottle so the CO₂ it produces has nowhere to go but into the liquid:

• Bottle it: pour the finished kombucha into swing-top (Grolsch-style) bottles that can hold pressure, leaving about ½–1 inch of headspace.
• Add a little sugar source: fruit, juice, or a spoon of sugar. A good target is roughly 10–15% fruit/juice by volume — for example, 1–2 oz of fruit per 16 oz bottle.
• Seal and wait: leave bottles at room temperature for 2–4 days (longer if your room is cool). Warmer rooms carbonate faster.
• Then refrigerate: cold both slows fermentation and helps the CO₂ stay dissolved. Always chill before opening.