What is a SCOBY? How the kombucha culture ferments tea

What a SCOBY is made of and what it does

SCOBY comes from “symbiotic culture of bacteria and yeast”. It describes a community of microbes that share the sweet tea; “tea fungus” and “kombucha mushroom” are older names for the culture. Yeasts such as Brettanomyces, Saccharomyces and Zygosaccharomyces convert sugars into alcohol and carbon dioxide. Acetic acid bacteria, including Komagataeibacter and Acetobacter, convert alcohol into acetic acid, while some bacteria also make gluconic acid from glucose. These reactions reduce the sweetness and give kombucha its sour taste. The growing acidity helps limit unwanted microbes, although it doesn’t make a contaminated batch safe. The mix of microbes differs between cultures, so a SCOBY isn’t a single species you can identify by looking at the disc.

Komagataeibacter bacteria make cellulose, the material also found in plant fibres. It collects as a wet mat at the surface, where the bacteria have access to oxygen; brewers call this mat the pellicle. In a 2021 study, Harrison and Curtin examined 103 starter cultures used by commercial kombucha brewers, mostly in North America, and found differences in their microbial composition. Those results describe the cultures they sampled. They don’t show which microbes cause a particular flavour in your jar, and the authors called for further research into the links between culture composition and the finished drink.

How starter liquid and the pellicle help a batch

You can brew without a pellicle if you have active, acidic starter liquid. It supplies microbes as well as acid, and a new cellulose layer can form as the tea ferments. For the starter quantities, use the measured ratio in the first-batch guide rather than judging by the size of the disc. A pellicle contains microbes and holds some liquid, but its thickness doesn’t tell you whether the culture will acidify fresh tea. A thick layer can come from repeated growth; a thin one can accompany a fermenting batch. Keep the starter liquid when you transfer a pellicle to another jar.

Fresh sweet tea needs acidification when you begin a batch. Starter liquid lowers its pH and supplies the culture that continues fermenting the tea; clean equipment and suitable brewing conditions still matter. The BC Centre for Disease Control’s guidance gives pH checks for fermentation, because smell and appearance alone can’t confirm that a batch has acidified enough. How to make kombucha explains the starter ratio and the checks to use. Measure the tea and starter separately so you can repeat the recipe or trace a failed batch.

Checking the pellicle and the liquid

• Colour from off-white to tan. It darkens with age and with black tea, and can pick up colour from the tea.
• Texture smooth and rubbery, or bumpy and jelly-like. Both are normal.
• Brown, wet strands or clumps hanging underneath or caught in the pellicle commonly come from yeast. They don’t confirm that the culture is still fermenting.
• Holes, ridges, trapped bubbles and thin patches can occur as cellulose grows. A pellicle may float at an angle or sink; its position doesn’t measure culture activity.
• A translucent, wet, glossy film on the surface of the liquid. That is the newest layer forming.

Check the exposed surface for dry, fuzzy, raised patches, which can appear white, green, blue or black. If you find mold, discard all the liquid and the pellicle, then clean the vessel before starting with an uncontaminated culture. Removing only the spotted layer won’t salvage the batch. Kombucha mold compares mold with wet yeast deposits and new cellulose so you can check what you’re seeing.

Why new cellulose layers form

As bacteria make cellulose at the surface, a new pellicle may attach to the old one or form above it while the old disc sinks. You can end up with several layers after repeated batches, but their growth rate and thickness vary with the culture and brewing conditions. Separate spare layers with clean hands or utensils, keeping the starter liquid with any culture you plan to use. You can keep a layer for brewing and move the extras to a SCOBY hotel. If you share a culture, include enough acidic starter liquid for the recipient’s recipe and tell them which tea and quantities you used.

Older layers can darken with tea stains and become thick or leathery. Colour alone doesn’t tell you when to replace one, and choosing a paler layer won’t repair spoiled starter liquid. If a batch develops mold or an unusual rotten smell, discard the liquid and pellicles together rather than peeling away the oldest layer and brewing again. For a culture that no longer acidifies fresh tea, check the starter quantity and temperature before your next attempt with a replacement culture. Keeping dates and batch notes helps you see when the problem began.

How to grow a SCOBY from a bottle of kombucha

1. Choose unpasteurised, unflavoured kombucha with live cultures. Check the label and storage instructions; a drink sold as kombucha may have undergone processing that inactivates its culture. For a reliable first brew, you can instead get starter liquid intended for brewing from a supplier.
2. For a small test batch, brew 2.5 g of plain black tea in 500 ml of water and dissolve 30 to 35 g of sugar in it. Let the tea cool to room temperature before adding the culture.
3. Add 50 to 100 ml of the bottled kombucha to the cooled tea in a clean, food-safe glass jar. These quantities scale down the first-batch recipe. Cover the opening with a tightly woven cloth secured with an elastic band, keeping insects out while allowing air through.
4. Keep the jar at 20 to 26 °C (68 to 79 °F), out of direct sunlight. Record the starting pH and check acidification as described in the first-batch guide. A thin surface film may appear, but there’s no fixed day when it must form.
5. Use the liquid for another small batch only after the test batch has acidified and you’ve checked it for contamination. BCCDC recommends a finished-drink pH below 4.2 and above 2.5; don’t use a sour smell alone as your test. Increase the batch size after you’ve confirmed that the culture ferments fresh tea.

A bottle that contains living microbes won’t necessarily work well as brewing starter. Don’t keep adding sweet tea to a test batch that fails to acidify, or wait for a thick pellicle to prove it is ready. If mold appears, discard everything and clean the jar. Get a fresh brewing culture if the test fails; the fermentation troubleshooting guide explains how starter quantity and temperature affect a batch. This test checks fermentation activity, not the full microbial composition of the bottle.

Buying or borrowing a SCOBY

Ask a brewer for unflavoured starter liquid from a batch that has fermented normally, plus a pellicle if they have one to spare. Tell them how much tea you plan to brew so they can supply the quantity your recipe needs. With a purchased culture, follow the supplier’s instructions and check whether it arrives fresh in liquid or dried for rehydration. A fresh culture should arrive moist in acidic liquid; dried products need their own preparation steps. Reject any culture with mold or an unusual rotten smell. Ask the supplier about a damaged package or a fresh culture that arrives dry before using it. The source of the culture and its storage history matter more than how large or pale the disc looks.

Storing spare SCOBYs: the SCOBY hotel

A SCOBY hotel holds spare pellicles and starter liquid between batches. Put them in a clean glass jar with enough unflavoured kombucha from a healthy batch to cover the layers, then secure a cloth over the opening and keep the jar at room temperature. Fermentation can continue during storage, making the liquid more acidic. That acidity doesn’t establish how active the stored culture will be when you add fresh tea. Before using hotel liquid for a larger brew, check the jar for contamination and confirm its activity in a small batch if you’re unsure of its storage history.

• Check the liquid level and exposed surface regularly. Follow the culture supplier’s feeding instructions, using cooled sweet tea; record what you add and when, since evaporation and fermentation vary with the room.
• If yeast sediment builds up, transfer the pellicles and retained starter liquid to a clean jar with clean utensils. Don’t rinse the culture to make it look paler; wet yeast deposits alone aren’t a reason to discard it.
• Keep an active SCOBY hotel at room temperature. Refrigeration slows the culture and it may need time to recover, but a short stay in the fridge does not automatically ruin it.
• Secure a breathable cloth over the opening, because acetic acid bacteria need oxygen during fermentation. Keep insects out and check that the cover stays in place.
• Keep the reserve culture separate from suspect batches, using clean utensils for each jar. After a moldy batch, restart only with an uncontaminated culture; a hotel that shared contaminated utensils needs checking too.

Before a brewing break, check that the stored pellicles have enough liquid to stay covered and that the cloth is secure. For a longer absence, arrange for someone to inspect the jar and follow your feeding instructions. Check the culture when you return, and test its fermentation activity before starting a large batch if it has been stored for a long time. How long kombucha lasts also covers storage of finished bottles and brewing batches.

Reusing a culture between batches

You can reuse starter liquid and pellicles while the culture continues to ferment fresh tea without signs of contamination. There’s no guaranteed lifespan, and a new cellulose layer doesn’t reset the culture’s storage history. Keep a record of the batches you draw starter from, including any interruptions or changes in temperature. If a culture dries out, freezes or spends a long time in the fridge, its appearance won’t tell you whether it will recover. Use a small test batch to check fermentation, following the pH checks in the brewing guide. Discard contaminated cultures and replace starter that fails to acidify the tea rather than repeatedly feeding a failed batch.

Frequently asked questions