What kombucha actually is

Kombucha is sweetened tea fermented by a mixed culture of yeasts and acetic acid bacteria. Understanding the division of labour makes every later decision obvious.

The two organisms

The yeasts eat sugar and produce ethanol and carbon dioxide. The bacteria eat that ethanol and produce acetic and gluconic acids. That is the whole engine: sugar in, acid out, alcohol as an intermediate rather than a product. A finished kombucha typically lands under 0.5% ABV.

The SCOBY is not the culture

The rubbery disc — a Symbiotic Culture Of Bacteria and Yeast — is mostly a cellulose mat the bacteria build as a raft. The living culture is in the liquid. This is why starter liquid matters more than the pellicle, and why a beautiful thick disc with no mature liquid ferments badly.

Two ferments, two purposes

• F1, open to the air under a cloth, 7–14 days: acidification. This is where sourness and the acid safety barrier develop.
• F2, sealed in a bottle, 2–5 days: carbonation and flavour. Sealed means the CO2 stays dissolved. This is the only stage where pressure builds — and the only stage that can be dangerous.

The pH line

Below pH 4.2, kombucha is inhospitable to the organisms that would make you ill. Above it, it is just sweet tea sitting on a counter. A pH strip costs almost nothing and removes all guesswork.

What it is not

Not a probiotic cure, not a detox, not a substitute for medicine. It is a genuinely good, cheap, low-waste drink that replaces soft drinks well. That is enough.

The next parts cover gear and sugar, then the method itself.