Kombucha

Why this method

Kombucha is sweetened tea fermented aerobically by a SCOBY (Symbiotic Culture Of Bacteria and Yeast). Yeasts invert sucrose and ferment the glucose and fructose to ethanol and CO₂; acetic-acid bacteria then oxidize that ethanol to acetic acid, dropping pH from roughly 4.5 to the finished 2.5-3.5 band. The whole process is protected from the first hour by acidifying the start with mature kombucha, so unwanted molds and bacteria never get a foothold. You run it in two stages: F1 (open, aerobic) builds the acid; F2 (sealed) builds flavor and carbonation.

Parameters

Parameter	Value
Sugar	50-75 g white sugar per liter of water
Tea	5-7 g loose black/green tea per liter (steep 5-7 min, then remove)
Starter (mature kombucha)	10-20% of total liquid volume
Starting pH (after starter)	below 4.5 (target 4.2 or lower)
F1 temperature	21-27°C / 70-80°F
F1 duration	7-14 days, to taste and pH
Finished pH	2.5-3.5
F2 (sealed)	21-27°C, 1-4 days
Cold storage / arrest	2-4°C

Equipment

• Wide-mouth glass jar or crock (1-4 L), never reactive metal
• Breathable cover (tight-weave cloth) plus a band for F1
• Calibrated pH meter (strips are too coarse near 4.6)
• Swing-top pressure-rated bottles for F2
• Healthy SCOBY plus reserved mature starter liquid

Method

Brew the sweet tea

Steep 5-7 g tea per liter in just-boiled water for 5-7 min, remove the tea, then dissolve 50-75 g sugar per liter. Cool to under 30°C before adding the culture, or the heat will kill the SCOBY.

Acidify the start (safety step)

Add 10-20% mature kombucha starter by volume, then drop in the SCOBY. Measure: the start should read below 4.5 (target 4.2 or lower). This acid head-start is the primary hurdle against mold and pathogens.

F1 — aerobic acid build

Cover with breathable cloth and band. Hold 21-27°C for 7-14 days. Acetic-acid bacteria need oxygen, so do not seal. Begin tasting and measuring pH around day 7.

Confirm the endpoint

When pH reaches 2.5-3.5 and the brew tastes pleasantly tart, F1 is done. Reserve at least 10-20% as starter for the next batch before you bottle.

F2 — flavor and carbonation

Bottle into sealed pressure-rated bottles with fruit, juice, or a little extra sugar (about 5-10 g/L). The residual yeast ferments this in a closed vessel, trapping CO₂. Hold 21-27°C for 1-4 days, then refrigerate at 2-4°C to arrest and hold the carbonation.

Caution

The acid head-start is the safety control, not optional. Always add at least 10% mature, acidic starter so the batch begins below pH 4.5. If you have no acidic starter, do not ferment — pasteurized vinegar to drop the start is a stopgap, never a flavor choice. Discard any batch growing fuzzy, raised, colored mold (blue/green/black). A flat, pale, smooth new SCOBY layer is normal, not mold; see Mold vs. Kahm. Taste is never a safety test — confirm pH with a meter.

Caution

F2 builds real pressure. Use pressure-rated bottles, “burp” them daily if you are unsure of the yeast load, and never over-sugar — sealed bottles can over-pressurize and burst. Refrigerate to stop the fermentation once carbonated.

Note

Finished kombucha typically carries below 0.5% ABV, but residual alcohol varies with sugar and time. Long, sugar-heavy, or sealed ferments can exceed that. This is a home-scale guide, not a labeling standard.

🫙 Sweet-tea formula

🛡️ pH safety calculator

See also

• Sweet-tea base formula — ratios and a scaling table
• Jun — the honey + green tea, lower-temperature variant
• Vinegar — the same ethanol-to-acetic step, taken to 4-5% acid
• Acidity and pH — why pH, not taste, is the safety number
• Sugar in fermentation and Microbial succession