Yeast and bacteria divide labor in the SCOBY
A SCOBY (Symbiotic Culture of Bacteria and Yeast) is a living fermentation ecosystem where wild yeast (usually Saccharomyces cerevisiae and relatives) converts glucose into ethanol, and bacteria (usually Acetobacter aceti and others) immediately convert that ethanol into acetic acid. The two organisms create a mutualistic relationship: yeast provides food (ethanol) for bacteria, and bacteria's acid production creates an environment where yeast thrives but most spoilage organisms cannot.
The result is a mildly alcoholic, tart beverage. Typical kombucha fermentation produces 0.5-3% ABV and 0.3-1% acetic acid depending on fermentation length and temperature.
Cellulose production and long-term fermentation
The SCOBY also produces cellulose (the white rubbery layer that forms on the surface), which serves as a physical barrier protecting the ferment from oxygen and mold. This 'mother' layer can be repurposed to start new batches. Kombucha fermentation typically runs 7-30 days. Longer fermentation drives more sugar consumption and more acid production, resulting in a drier, sharper flavor. The SCOBY's anaerobic environment, acidity, and the presence of beneficial yeast and bacteria make contamination rare.