A SCOBY is a living mat made from cellulose, which bacteria produce, plus the bacteria and yeasts that live in and on it. The mat is only part of the culture. The liquid underneath holds much of the rest.
The cellulose mat
The thick, pale layer people photograph is called a pellicle. It is also known as a “mother”, a “mushroom” or a “jellyfish”, although it is not a mushroom. The pellicle is made of bacterial cellulose, a tough, water-rich fibre that bacteria produce at the surface of the liquid. It takes the shape of the container it grows in.
Studies of kombucha mats often identify bacteria in the genus Komagataeibacter (formerly Gluconacetobacter), which is known for making cellulose. For the full cast of microbes, see what bacteria ferment kombucha.
The bacteria
Acetic acid bacteria are the main acid-makers in a SCOBY. They turn the alcohol made by the yeasts into acetic acid, and they make other acids too. That lowers the pH, which makes the brew less hospitable to mold and other unwanted microbes. It is why a healthy brew tastes more tart over time, and why sweet tea is vulnerable at first, before it has turned acidic.
The yeasts
Yeasts are fungi. They feed on sugar and produce small amounts of alcohol and carbon dioxide. The carbon dioxide gives kombucha its fizz. BBC Good Food notes that a SCOBY can contain several yeast species, including Brettanomyces bruxellensis, Schizosaccharomyces pombe and Saccharomyces cerevisiae, the baker’s and brewer’s yeast. Which ones are present varies.
The brown strands hanging from a SCOBY are colonies of yeast. They are normal.
The liquid
The culture doesn’t live only in the mat. Much of it lives in the starter liquid under the SCOBY. That is why a SCOBY kept in plenty of liquid stays healthy better than one kept dry.
What isn’t in a SCOBY
- Added sugar. Sugar is food for the culture, not an ingredient. Most of it is used up during fermentation.
- Tea. Tea is the food the culture grows on, not a part of the mat.
- Flavourings or essential oils. A SCOBY is grown from plain tea, sugar, water and kombucha. Flavourings and oils can slow the culture down.
Why every SCOBY is different
Each SCOBY reflects the environment it grew in, so the mix of microbes varies from one to another. Two SCOBYs can look and behave differently. The colour of the mat depends partly on the tea: black tea gives a darker mat, and green tea a lighter one. See green tea or black tea for kombucha for how the two compare.
What this means for your brew
- Keep the culture plain. Use plain black tea, sugar and water for the first brew, and save flavourings for the bottle. How to grow a SCOBY from scratch has the full method.
- Look at the whole culture. A sinking SCOBY is usually fine, as long as the liquid is getting more tart. See why does my SCOBY sink?
- Know what healthy looks like. Brown strands and uneven colour are normal. Fuzzy mold is not. Is it mold or a healthy SCOBY? explains the differences.