Is your SCOBY moldy? How to tell, and what to do

This is the most frequently asked question on every kombucha forum: “is this spot on my SCOBY mold, or is it something harmless?” The good news is that real mold is usually clearly recognizable once you know what to look for.

The definitive signs of mold

Mold typically is:

• Dry and fuzzy, with a texture more like mold you’d see on bread or cheese than the SCOBY’s smooth, gel-like surface.
• Colored — typically appearing as green, black, blue, or white fuzzy patches.
• Only on top of the SCOBY, on the part that is not in contact with the liquid — mold spores land from the air and can only take hold and grow where the surface is dry.
• Often gives off an unpleasant, “musty” smell, clearly distinguishable from kombucha’s characteristic tangy, vinegary aroma.

If you see these signs, the safe decision is clear: throw out the whole batch and the SCOBY too. Don’t try to “cut out” the moldy part — mold’s root-like structures (hyphae) can be present in the culture beyond the visible spot.

What’s often mistaken for mold (but isn’t)

• Brown strands or spots. These are typically yeast colonies that form at the bottom or inside the SCOBY. They’re harmless and don’t indicate a problem.
• A new layer on top. After every cycle, a thin layer grows that’s sometimes a slightly different color or thickness — this is normal growth, not contamination.
• Bubbles under or around the SCOBY. Carbon dioxide production is a natural part of active fermentation.
• An uneven, wavy surface, or slightly brownish discoloration from the tea’s tannins — these are cosmetic differences, not quality defects.

Why does mold happen in the first place?

The risk of mold typically increases under the following conditions:

• Too low acidity at the start — with too little or weak starter liquid, the sweet tea acidifies more slowly, leaving a longer window for unwanted microorganisms to take hold.
• Inadequate covering — if the weave of the cloth or paper towel is too loose, insects or dust can get in, potentially carrying mold spores with them.
• Too cool an environment — slower fermentation likewise means a longer “vulnerable” period.
• Dirty equipment — an improperly washed vessel or hands can introduce contamination into the system.

How to reduce the risk next time

• Always use enough fresh starter liquid (about 10% of the final volume), so the sweet tea reaches a safe acidity quickly.
• Choose a tightly woven but breathable cloth, or several layers of paper towel, for the covering, secured tightly with a rubber band.
• Keep the vessel in a stable, room-temperature spot, away from drafts and direct sunlight.
• Check the batch visually on a regular basis (every 2–3 days) so you notice early if something’s off.

If you’re unsure about a particular spot, the best rule of thumb is: when in doubt, throw it out. A lost batch is a small loss compared to a potential health risk.