This is where the science gets genuinely interesting. Sourdough is not just wild yeast — it is a specific community of yeast species, each with distinct properties, competing and cooperating in a wet flour environment.

The dominant species in most mature wheat and rye starters is Kazachstania humilis, formerly known as Candida humilis or Candida milleri. It is acid-tolerant in a way that commercial Saccharomyces cerevisiae is not, which is why it thrives in the acidic environment that the bacteria create. The panel below covers the two main players.

For those interested in going deeper, Gobbetti and Gänzle’s Handbook on Sourdough Biotechnology (2013) is the standard reference text. Dense, but worth it.