What exactly is a SCOBY?

That jelly-like disc floating in your kombucha has a name and a job. Here is what it is, how it forms, and how to keep it happy.

An acronym, not an animal

SCOBY stands for Symbiotic Culture Of Bacteria and Yeast. It is a living community, not a mushroom and not a jellyfish, even though it looks a little like both. The bacteria and the yeasts work as a team: the yeasts turn sugar into alcohol and carbon dioxide, and the bacteria turn that alcohol into the tangy acids that make kombucha taste like kombucha.

How the disc forms

The rubbery layer that grows on top is called the pellicle. It is cellulose, woven by the bacteria as they work, more like the house the culture builds than the culture itself. A fresh layer forms with every batch, which is why an older SCOBY ends up looking like a stack of pancakes.

Keeping it alive

A SCOBY is happiest kept in enough of its own acidic liquid, the starter tea, at room temperature, out of direct sun and away from reactive metal. Between batches it can rest in a jar of sweet tea, sometimes called a SCOBY hotel, on the counter or in the fridge. Feed it tea and sugar and it will keep going for years, passed from friend to friend.

Trust your nose. Kombucha should smell sharp and vinegary, clean and a little sour. It should never smell rotten.

When to worry

Brown stringy bits hanging underneath and bumps on the surface are all completely normal. What is not normal is dry, fuzzy, raised mould, blue, green or black, sitting on top like it would on old bread. That is the rare sign to compost the batch and start again. It does not happen often when the tea is acidic enough.

A SCOBY is about the closest most of us get to keeping a pet microbe. Look after it, and it looks after your kombucha.