What a SCOBY is, and how a batch gets made

Most people meet the word SCOBY through a picture of a pale disc floating on sweet tea. Bon Appétit once called it a jellyfish-like starter. That disc makes a good photo, but it is only part of what a brewer means by the word. Once you know the rest, you can follow the whole process.

What the letters stand for

SCOBY is short for symbiotic culture of bacteria and yeast. The trade group Kombucha Brewers International covers it in its Code of Practice. The Code says the culture comes in two forms, and a brewer can start a batch with either one or both. The liquid form is often called starter liquid or backslop. It is simply finished kombucha saved from the last batch. The solid form goes by pellicle or mother culture, and the Code calls it a matrix of bacterial cellulose and yeast.

Bacteria build that mat, according to a 2021 paper by Keisha Harrison and Chris Curtin at Oregon State University. Forming it takes at least one bacterium from the genus Komagataeibacter, which makes cellulose and acetic acid. The mat floats at the surface of the tea. Up there, the microbes inside it can reach oxygen from above and nutrients from below.

Harrison and Curtin also checked who lives in there, using DNA sequencing on 103 SCOBY from brewers, mostly in North America. The most common residents were the yeast Brettanomyces and the bacterium Komagataeibacter. Even so, the cultures sorted into four distinct types. The university's summary points out how odd that is. Most other fermented drinks tend to be run by a single dominant organism. Each SCOBY type is a very different mix of yeast and bacteria working together.

How a batch gets made

Every batch begins as a pot of tea, and the Code of Practice lays out what follows. Leaves are steeped in hot or cold water and then taken out. Steeping time is up to the producer. Sugar goes in next. When the sweet tea reaches a suitable temperature, the culture is added. The Code says typically no less than 10 percent of the batch for that culture. A lab study in Frontiers in Microbiology copied the traditional method and used 12 percent kombucha from an earlier batch.

Fermentation needs air and time, and the same setup works at very different sizes. A review in Reviews in Agricultural Science describes a vessel under a breathable cloth, fermented for 7 to 14 days. Brew Dr. began with 5 gallon containers and now runs 9,000 gallon tanks. Each tank has a cloth-topped vent port, the room stays at about 75 degrees, and a batch takes around 2 to 4 weeks. At the small end, PA Eats described Baba's Brew running a seven day first ferment. The Code explains the spread by listing what moves the timing. That list includes vessel size and shape, temperature, humidity, the type of tea and how much culture went in.

Several changes are easy to notice while this goes on. According to the Code, the liquid starts to smell acidic and gas bubbles appear. A new SCOBY begins forming across the surface. The color also lightens the longer it ferments.

What the yeast and bacteria are doing

The chemistry works like a relay between the two kinds of microbe. A Kombucha Brewers International article on brew chemistry explains the first leg. Yeast carry an enzyme called invertase, which splits cane sugar into simpler sugars. Yeast also give off carbon dioxide. Bacteria then make organic acids, mainly acetic and gluconic acid. A 2026 summary of work from Wroclaw Medical University credits those acids for the familiar tart flavor.

Those acids do not all taste alike, and the same trade group article gives numbers for them. Acetic acid has a pungent sourness you can detect at 175 milligrams per liter. Lactic acid adds a tang with very little aroma, and you notice it at 400. Citric acid gives a tart note at only 60. The Code puts the finished drink at a pH of about 2.3 to 3.8.

Aroma comes out of the partnership too, as the Frontiers study showed. The researchers split a culture into single microbes and had 12 trained panelists smell the results. Pairing the yeast Brettanomyces bruxellensis with the bacterium Acetobacter indonesiensis gave stronger apple juice notes than either one alone. The authors call that apple juice aroma characteristic of kombucha.

Deciding it is done, and what comes next

There's no universal finish line, because it depends on taste. The Code says flavor moves from pleasantly fruity and tangy after a few days to vinegary after a longer ferment. Harrison and Curtin note that producers set their own end points, based on the sweet and acid balance they want.

When the brewer calls it, the Code says the new pellicle comes off and may be used for later batches. A flavor infusion can happen at this point. It counts as a secondary fermentation if the flavorings bring fermentable sugar with them. Filtering to remove excess yeast is allowed at any stage.

Bubbles arrive by one of two common routes. In the Frontiers study, flasks sat loosely covered for seven days and were then sealed for five more, so carbon dioxide could build up on its own. Brew Dr. takes a route used in craft beer. It chills the kombucha to around 33 degrees, carbonates it gently overnight in brite tanks, and bottles it cold.

Cold storage is the last step for most of what reaches a shelf. The Code calls for packaged kombucha to stay between 40 and 48 degrees Fahrenheit through distribution. That slows further fermentation and helps keep off-flavors away. Pasteurized or otherwise shelf-stable versions do not need refrigeration.