Kahm yeast

Kahm yeast is a wild yeast that can grow on the surface of a brine when it is exposed to air, usually appearing as a thin, white, sometimes wrinkled or bubbly film. It is a common sight in home fermentation, especially in warmer rooms or jars that are opened frequently, and it forms because yeast, unlike lactic acid bacteria, thrives in the presence of oxygen.

While kahm yeast itself is generally considered harmless, it is worth skimming off promptly because it can alter the flavor of a ferment over time, giving it a slightly off or yeasty taste, and a thick layer can make it harder to see what is happening underneath. It typically reforms if the surface stays exposed to air, so many fermenters find that fitting an airlock lid or keeping the vegetables fully submerged reduces how often it shows up.

The key distinction to learn is between kahm yeast and mold. Kahm yeast is flat, white, and sits on the surface, while mold is typically fuzzy, raised, and can appear in shades of green, black, pink, or blue. Widely followed home fermentation guidance treats fuzzy or colored growth as a sign to discard the batch, since mold can send roots down into the ferment in ways that are not always visible.