Yeast & Bacteria Fermentation: The Living Culture Behind Fermented Drinks

What SCOBY actually means

SCOBY stands for Symbiotic Culture of Bacteria and Yeast. In practice, people use the term in two ways:

• The culture: the active community of microorganisms (yeasts + bacteria) that performs fermentation.
• The pellicle: the cellulose layer that can form on the surface during fermentation. It’s often called a “tea fungus” in older naming, but it’s not a fungus in the biological sense. It’s mostly bacterial cellulose produced by bacteria living in the system.

One important detail: the kombucha microbial community doesn’t live only in the “mat.” Research describes kombucha as a two-part system—microbes in the floating cellulosic biofilm and microbes in the liquid broth. Both matter for how fermentation behaves.

Who’s in the SCOBY: bacteria and yeast roles

Kombucha fermentation is a collaboration. Yeasts and bacteria don’t do the same job—they trade outputs, and that’s what makes kombucha different from simple sweet tea.

Yeasts primarily contribute by fermenting sugars into ethanol and CO₂. CO₂ contributes to carbonation, while ethanol becomes a key substrate for bacteria.

Acetic acid bacteria (AAB) use oxygen and can oxidize ethanol into acids—especially acetic acid—which contributes to tartness and the overall “structure” of the drink. Certain bacteria in kombucha are also well known for producing cellulose, which forms the pellicle.

It’s also normal for studies to report different compositions across cultures. That variation isn’t a problem by itself—kombucha isn’t a single standardized strain product. It’s a mixed ecosystem that needs control to achieve consistent results.

Why SCOBY composition matters (taste, consistency, and “live” behavior)

The microbial makeup of a SCOBY affects:

• Acidity profile (how sharp or rounded the tartness feels).
• Carbonation potential (how much CO₂ is produced and retained).
• Aroma and finish (esters and other fermentation-derived compounds can shift the sensory profile).
• Pellicle formation (how quickly it forms and how thick it becomes).
• Batch-to-batch stability (how predictable fermentation is under the same parameters).

Even with the same SCOBY, fermentation conditions can steer outcomes. Reviews and experimental studies consistently point to variables like tea choice, sugar concentration, time, temperature, and oxygen availability as key drivers of what dominates and what gets produced.

This is why “live” beverages require a different mindset than standard soft drinks. The product is built by biology, so the producer’s job is to make biology behave predictably—through sanitation, stable parameters, and deliberate process control.