SCOBY sounds a bit like science fiction, but it is real fermentation. Behind the word is a living community of bacteria and yeasts that gives kombucha its character. Read this alongside our Kombucha guide if you also want to know what that means for flavour and sugar.

Concept and structure

SCOBY stands for Symbiotic Culture Of Bacteria and Yeasts: a community that converts sugar into organic acids, aromas and carbon dioxide. A SCOBY is not a single organism, and botanically not a fungus, even though it is often called a kombucha mushroom. It is closer to a lichen: several species living together.

In home brewing, a gelatinous, pancake-shaped mat often floats on the surface, beige to brown depending on the tea. Holes, bubbles and uneven patches are normal. The same culture can also exist in liquid form, as ours does, in a well-aerated environment that allows a stable fermentation profile and consistent quality.

Who lives in a SCOBY

Not one species, but a small ecosystem. The genera most often described in research are acetic acid bacteria such as Komagataeibacter, Acetobacter and Gluconobacter, which convert alcohol into acetic and gluconic acid and build the cellulose; lactic acid bacteria of the Lactobacillus genus; and yeasts including Saccharomyces cerevisiae, Brettanomyces and Pichia species, which metabolise sugar into CO2 and alcohol for the bacteria to work on.

The exact composition differs from culture to culture and even shifts within a single SCOBY over time. That is one reason no two kombucha batches taste exactly alike.

How fermentation works

The roles are divided: yeasts ferment sugar into alcohol, while acetic and lactic acid bacteria convert that into fine organic acids. The result is freshness, balance and a characteristic tang.

Why oxygen matters

A SCOBY needs air. In open vessels the solid mat floats at the surface, where oxygen is most available. Oxygen keeps the acetic acid bacteria active and lets them form acetic acid, the basis of the typical kombucha taste.

Why layers form

In an open vessel a new cellulose layer forms at the surface with each cycle, a daughter SCOBY that can be separated and used on its own. In a liquid culture the same happens microscopically, without a visible mat.

Healthy or not

A healthy SCOBY is beige, light brown or slightly translucent, feels firm and slippery, and smells sour and vinegary. Brown strands hanging underneath are normal yeast. A SCOBY that sinks instead of floating is usually fine too. Warning signs are mould, meaning dry, fuzzy patches in green, black, blue or pink growing on the surface, and a foul or acetone-like smell. When in doubt, discard and start again.

Where to get one

Three routes are common: take a daughter SCOBY from another brewer, buy from a specialist supplier, or grow your own from raw, unpasteurised kombucha without added flavourings. The steps are in our guide to brewing kombucha at home.