White Film on Your Ferment: Kahm Yeast or Mold?
A white film on a ferment looks alarming and usually is not: in most jars it is kahm yeast, a harmless surface yeast that shows up wherever liquid meets air. Telling it from mold takes ten seconds once you know the two or three features that matter. Keeping it from coming back is mostly about keeping vegetables submerged and the salt percentage in the right band - the brine calculator and the vegetable salt guide cover that math.
Kahm yeast: flat, white, wipeable
Kahm yeast looks like a thin white cloth or a milky swirl stretched across the surface - flat, matte, sometimes wrinkled where it dries at the jar edge. It wipes or skims off in sheets. It thrives on contact with air, so it favors jars where vegetables poke above the brine or the jar sits warm and undisturbed. Remove it, keep everything under liquid, and the ferment carries on unaffected.
Mold: fuzzy, colored, rooted
Real mold grows up: fuzzy or hairy texture, rounded colonies with visible centers, and color - white fuzz may precede the blue-green of the classic bread mold, or arrive as black, pink or orange spots. It prefers solid footholds, which is why it starts on cabbage leaves or garlic floating at the surface. Mold sends roots below the visible spot, so a jar with fuzzy growth goes to the compost whole.
Prevention: three habits
First, submersion - a glass weight, a folded cabbage leaf, or a zip bag of brine keeps solids under liquid, where mold cannot breathe. Second, salt in the 2-5% by-weight band, mixed honestly: the salt volume vs weight guide explains why spoons drift and grams do not. Third, a cool room or a quick daily check - surface trouble starts fastest in warm, forgotten jars. The kimchi time calculator and the temperature-time calculator translate room temperature into an honest schedule, which is exactly when jars get checked.
Questions
Is kahm yeast safe to eat?
The film itself is not dangerous - kahm yeast is just an aerobic yeast taking advantage of an open surface. The problem is flavor: a thick kahm layer makes brine sharp and yeasty-tasting. Skim it off, rinse anything that touched it, and get the vegetables back under the brine. If the taste of the remaining ferment has turned sharply yeasty, it is a quality loss, not a safety one.
What color mold means throw the jar away?
Any mold that is not a flat, white-to-cream film: black, green, blue, pink or orange spots, or anything fuzzy and raised like a carpet. Fuzzy circles growing from a point, especially on vegetables poking above the brine, mean spores have penetrated below the surface. Discard the whole jar - mold roots spread farther than they show.
What salt percentage keeps the surface clean?
For vegetable ferments, staying inside the standard 2-5% by-weight band does most of the work: strong enough to slow unwanted surface microbes, gentle enough for Lactobacillus. Below 2% the surface community grows fast; above 5% you slow your friends along with the intruders. The brine calculator keeps the percentage honest by weight, not by spoon.