Kombucha and the SCOBY: How the Fermentation Works
Sweetened tea fermented by a symbiotic culture where yeasts ferment sugars to ethanol and CO2, and acetic acid bacteria oxidise that ethanol to acetic acid — the low pH being what protects the ferment from contamination. The rubbery surface disc is a pellicle of bacterial cellulose; the living culture is throughout the liquid. Yields acids, residual sugar, tea polyphenols, caffeine and trace ethanol.
**Kombucha** is a fermented tea, made by inoculating sweetened tea with a **SCOBY** — a *symbiotic culture of bacteria and yeast* — and leaving it to ferment for roughly one to three weeks. ## The two-stage fermentation The SCOBY is a genuine symbiosis with a division of labour: **Yeasts** — commonly *Brettanomyces*, *Zygosaccharomyces*, *Saccharomyces* and others — hydrolyse sucrose into glucose and fructose and ferment those to ethanol and carbon dioxide. **Acetic acid bacteria** — chiefly *Komagataeibacter*, *Acetobacter* and *Gluconobacter* — oxidise that ethanol to acetic acid, and glucose to gluconic acid. Their acid output keeps the pH low, which suppresses competing organisms and is the main reason a healthy ferment resists contamination. Some batches also carry lactic acid bacteria, though they are not the dominant group. ## The pellicle The rubbery disc that forms on the surface, usually called "the SCOBY", is more precisely a **pellicle** — a mat of bacterial cellulose secreted by the acetic acid bacteria. The living culture is distributed throughout the liquid, not confined to the disc, which is why a starter portion of mature liquid matters more for a new batch than the pellicle itself. The same bacterial cellulose is of independent interest as a material, and is the basis of *nata de coco* and of experimental textiles. ## What ends up in the glass Acetic and gluconic acids (the sourness), residual sugar, carbon dioxide, tea polyphenols, trace B vitamins, caffeine carried over from the tea, and **trace ethanol** — typically under 0.5% by volume in commercial products, though home brews and long ferments can exceed that. Fermentation time is the main control: longer means more acid, less sugar, and a sharper drink. Taken far enough it simply becomes vinegar. ## Live cultures and pasteurisation Raw, unpasteurised kombucha contains live organisms. Many commercial products are pasteurised for shelf stability and consistency, which kills them. Whether the live cultures deliver a clinical benefit is a separate question — see Is Kombucha a Probiotic? What the Clinical Trials Actually Show. **Home-brewing cautions.** The low pH that protects the ferment also leaches metals: brewing or storing kombucha in glazed ceramic or lead-containing vessels risks lead contamination, and glass or food-grade stainless steel is the standard advice. Contamination by mould is the other common failure, distinguishable from the normal pellicle by fuzzy growth and colour.