Troubleshooting: Mold vs Kahm Yeast

Something white showed up on your brine. Before you panic or before you ignore it, make one call: kahm yeast or mold? Kahm is a cosmetic nuisance you skim and move on from. Mold is a safety problem that ends the batch. The two look different, and the difference is decidable in about ten seconds with your eyes.

The single discriminating feature is texture, read against the surface of the brine.

The decision table

Feature	Kahm yeast — skim it	Mold — discard it
Texture	Flat, smooth, sometimes wrinkled; lies on the brine like a skin	Fuzzy, raised, three-dimensional; visible hairs or spores
Color	White, cream, off-white, faint tan	White and fuzzy, or any blue / green / black / pink
Edge	Connected film, irregular but smooth-edged	Discrete spots or colonies that grow outward
Penetration	Surface only	Sends root-like hyphae down into the food
Smell	Yeasty, slightly funky	Musty, earthy, “old basement”
Safety call	Harmless — quality issue only	Unsafe — throw the batch out

The trap is white fuzzy mold, which beginners read as “just kahm.” Kahm is never fuzzy. If it has any three-dimensional fuzz, it is mold, regardless of color.

Caution

If the growth is fuzzy or colored (blue, green, black, pink), discard the entire batch — not just the visible patch. Mold hyphae penetrate below the surface and can produce mycotoxins you cannot see, smell, or taste, so skimming the top does not make the rest safe. Do not taste to decide. When in doubt, throw it out.

Why each one appears

Both organisms are aerobic — they need oxygen, so they only colonize the exposed surface where food or brine meets air. That single fact drives every cause and every fix.

Cause	Why it lets aerobes in	Fix
Food floating above the brine	Exposed surface = oxygen	Submerge everything; use a weight and a follower
Too warm	Speeds yeast/mold ahead of the acid drop	Hold 18–22 °C / 64–72 °F
Under-salted	Weak selective pressure for the lactic bacteria	Hit 2.0–2.5 % salt by weight
Stalled / slow acidification	pH lingers high, surface stays hospitable	Confirm bubbling within 1–2 days; warm slightly if stalled
Long open headspace	Big air interface	Minimize headspace; airlock or burp daily

Info

Kahm yeasts (Pichia, Candida, and relatives) oxidize the lactic and acetic acid your ferment just made, raising surface pH and flattening flavor. So kahm is not only ugly — left unchecked it slowly un-does the acidity that protects the jar. Skim it promptly and get the food back under the brine.

Prevention, in one line

Keep it submerged, salted to 2.0–2.5 % by weight, and at 18–22 °C — that’s the whole game. Submersion removes the oxygen aerobes need; salt and the right temperature get the lactic bacteria to drop the pH fast, closing the window before anything colonizes the top.

Tip

A glass or ceramic weight plus a cabbage-leaf “follower” keeps shreds down. A tight headspace and a daily burp (or an airlock) starves the surface of oxygen. If a thin white film still forms, skim it the day you see it — kahm spreads across the surface but never down.

See also

• Aerobic vs anaerobic — why submersion is the master variable
• Safety First — the mold rule and the pH 4.6 line in context
• Spoilage vs safe — the full “is it safe?” decision guide
• Brine salinity calculator — dial in the 2.0–2.5 % salt floor