2026-09-05

Is This Mold on My Starter?

Fuzzy and colored means throw it out, jar and all. Flat and liquid means it's hooch and it's fine. Here's how to tell the difference in about five seconds.

If it's fuzzy, raised, and colored, throw the whole thing out. If it's flat, it isn't mold.

That distinction is the entire answer, and it's the one thing worth being conservative about in sourdough. Everything else on this site is about rescuing a bad loaf. This page is about food safety, so the advice is: when it's mold, don't negotiate with it.

What you're seeing

Look at the surface of your starter under good light, from the side as well as from above.

Mold sits on top of the starter as a distinct raised colony. It has texture — fuzzy, powdery, or velvety, sometimes with visible fine filaments. It's usually circular and grows outward from a center. It's colored: pink, orange, salmon, red, black, green, blue-green, or a fuzzy white that's clearly three-dimensional rather than a film. It very often starts on a dried crust of starter on the wall of the jar, above the paste, or around the rim under the lid.

Not mold, version one: a layer of thin liquid pooled on top, flat and level like water in a glass, clear or straw or gray or brown or nearly black. That's hooch — alcohol from fermentation. It's harmless and it means your starter is hungry. Full explanation at what the liquid on top of your starter is.

Not mold, version two: a flat white or grayish skin across the surface, wrinkled or patchy, with no height and no fuzz. That's most likely kahm yeast. It's harmless.

The test that resolves almost every case: does it have height and texture, or is it flat? Fuzz and color mean discard. Flat means you're fine.

Answer for the jar in front of you

1 · Does it have height, or is it flat?

Look from the side, not just from above. This is about a patch on the surface having height of its own, fuzzy, powdery or velvety. A domed, bubbly starter is not a raised patch.

2 · What color is it?

Side view of your jar, as you have described it.

Hooch

Not mold. Completely fine.

What it is

A flat, level liquid layer is hooch: alcohol and water that separated out once the yeast ran out of flour to eat. Color tells you how long it has been, not how dangerous it is. Clear or straw is hours past peak, gray or brown is a day or two at 68–72°F (20–22°C), nearly black is weeks, often in the fridge at 38–40°F (3–4°C).

What to do

Pour it off for a milder starter, or stir it in for a tangier one. Either is correct. Then feed it: discard down to 20 g (0.7 oz) and add 50 g (1.8 oz) flour and 50 g (1.8 oz, 50 ml) water. Expect it to be sluggish on the first feed and normal by the second, roughly 8–12 hours apart at 78°F (26°C).

The whole hooch answer, including the fridge case →

The cause: airborne spores landed on exposed food

Mold spores are in the air of every kitchen, constantly. They're not a sign your kitchen is dirty or that you did something wrong.

They need three things to grow into a visible colony: moisture, food, and time without competition. A healthy, actively fed starter denies them the third. Lactic acid bacteria drop the pH to around 3.5–4.5 and produce acids and other antimicrobial compounds, and most molds can't establish themselves in that. It's the same reason sourdough bread keeps longer than commercial-yeast bread.

Mold gets a foothold anywhere the acid isn't. Dried starter smeared on the upper wall of the jar has lost its moisture and its microbial activity, and is now just flour paste sitting in air. Same for the rim and the underside of the lid. That's why the colony almost always appears above the paste line rather than in the middle of a well-fed starter.

Neglect accelerates it. A starter unfed for weeks exhausts its flour, its populations crash, acid production stalls, and the protection weakens.

Why you cannot scrape it off

This is the most common bad advice on the internet, and it's worth understanding why it's wrong.

What you see on top is the fruiting body — the part that produces spores. Underneath, the mold grows as hyphae, a network of microscopic threads that push down and outward through the medium it's feeding on. In a soft, wet substrate like starter, those threads spread far beyond the visible patch, and they're invisible to you. Removing the top inch removes the part you can see and leaves the network.

Some molds also produce mycotoxins, which are chemical compounds rather than organisms. They diffuse through the whole jar and they're heat-stable at bread temperatures, so baking doesn't remove them.

Hard cheese can be trimmed with a wide margin because its density limits how far hyphae travel. A jar of paste has no such structure and no safe margin.

The fix

Discard the entire contents. All of it, not just the top.
Deal with the jar. The simplest option is to throw it away, especially plastic, which holds scratches that harbor spores. If you want to keep a glass jar, wash it in hot soapy water, then sterilize it: boil for 10 minutes, or run it through a dishwasher sanitize cycle, or bake dry glass at 250°F (120°C) for 30 minutes. Do the same for the lid if it's heatproof, or replace it.
Sterilize or replace anything the starter touched — spatula, spoon, scale bowl.
Wipe down the area it was sitting on.
Start a new starter, or get 20 g (0.7 oz) of a friend's.

How to keep it from happening again

Clean jar, clean tools. Move your starter to a fresh jar every week or two. It takes 30 seconds and removes the dried crust that mold actually grows on.
Wipe the walls. After feeding, run a clean spatula or a damp paper towel around the inside above the paste line so nothing dries there.
Feed more often. Once a day at room temperature above 70°F (21°C), twice a day above 78°F (26°C), weekly in the fridge at 38–40°F (3–4°C). A well-fed starter defends itself.
Lid on, but loose. Sealed tight, pressure builds and condensation collects. Fully open, the surface dries into a crust and spores drift straight in. A loose lid or a screwed-on ring with a lifted lid is right.
Don't top up with fruit, juice, or dairy. They add sugars and lower the acidity that's protecting the culture.

Is it saveable?

No. Not by scraping, not by feeding it back to health, not by baking the mold out. A moldy starter is finished.

But this is the least costly failure in sourdough. A moldy starter is not a personal failure and it isn't a lost investment. A new one from flour and water is active in about a week: mix 50 g (1.8 oz) whole wheat or rye flour with 50 g (50 ml) water, and feed it once a day at 75–78°F (24–26°C). It'll be sluggish for three or four days, hit a false rise around day 3, go quiet, then start doubling reliably by day 7 to 10. If you can get a spoonful from someone else's jar, you're baking in two days.

The only thing worth mourning is a decades-old culture, and the fix for that is a backup: dry a thin smear of starter on parchment, crumble it into a sealed bag, and it stays viable in a cupboard for years.

Something else wrong?

Start at the diagnosis hub if you're not sure which one you have.

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