Sourdough starter mold, hooch, and the smells in between

Fuzzy or colored growth — white, green, blue, or black — is sourdough starter mold, and the whole jar goes in the bin. Pink or orange streaks mean the same. Everything else, including grey liquid and a nail-polish smell, is a hungry culture, not a sick one.

How to tell sourdough starter mold from everything else

Mold is a texture problem, not a smell problem. It's raised, fuzzy, and dry-looking, and it starts on the driest real estate in the jar — the crust of old starter smeared above the fill line, or the underside of an unwashed lid. Colors run white, sage green, blue-green, or black. Pink or orange streaks and slimy pink patches aren't fungal, but they end the same way.

Everything a healthy culture does is flat and wet: a dark layer on the surface, a stripe of liquid halfway down the glass, a leathery skin on top. If you can't tell whether a white patch is fuzz or flour, tilt the jar under a light — flour dust lies flat and smears when you nudge it, mold stands up.

Don't scrape. The visible colony is the fruiting part; the network underneath runs further than you can see. Discard everything, wash the jar in hot soapy water, and rebuild — a starter from scratch takes a week or two, which is annoying, not a crisis.

Hooch: the liquid on top of your sourdough starter

That grey-brown layer is hooch — mostly ethanol and water with dissolved acids, produced once the yeast has burned through the available flour. It's harmless, and the thing bakers most often mistake for spoilage. Color tells you how long the starve has run: clear or straw means recently, grey means a while, near-black means days. Sourdough starter hooch that reads pink or orange isn't hooch at all — see above.

Your two options aren't equivalent. Pour the liquid off and you remove accumulated acid, so the next few feeds run milder. Stir it back in and the starter runs more sour. Either way, feed immediately afterward, and feed more flour than last time. A fridge starter that throws hooch after a week or two is behaving exactly as expected and needs nothing beyond a feed or two at room temperature before you bake.

When your sourdough starter smells like acetone or nail polish

Same message as hooch, further along. As the culture runs out of sugars it produces ethyl acetate and related esters, which read as solvent, nail polish remover, or pear drops. A starter that smells like acetone has been hungry for hours, not minutes.

The correction is food, not rescue, and you usually need only one of these:

• Feed more often. Around 78°F (26°C), a 1:1:1 feed by weight peaks in roughly four to six hours and is running on empty by eight to twelve.
• Feed a bigger ratio. Moving from 1:1:1 to 1:5:5 or 1:10:10 buys most of a day.
• Move it somewhere cooler. A 65–68°F (18–20°C) shelf stretches the same feed over far more hours.

What should sourdough starter smell like at each stage

There's no single correct smell, because it travels through a cycle. Right after a feed it's close to neutral — wet flour with a milky edge. Climbing toward peak it turns yeasty and faintly sweet, like bread dough or the head of a fermenting beer. At peak, roughly doubled with a domed, bubbled surface, a well-fed culture smells tangy in a plain-yogurt way: clean and lactic.

Past peak it sharpens, in order: vinegar, then alcohol, then solvent. If your sourdough starter smells like vinegar and stays there, acetic acid has taken over, which is favored by stiff low-hydration starters, cool rooms, and long gaps between feeds. Wetter, warmer, and more frequent pushes it back toward the lactic end. Neither end is wrong — some bakers chase the vinegar for a more sour crumb — but a starter you're about to bake with should smell tangy, not caustic. The only smell worth real suspicion is putrid rather than sharp, and it almost always comes with something visible.

Matching the signal to the fix

What you see or smell	What it means	What to do
Grey or amber liquid on top	Ate through the flour hours ago	Pour off or stir in, then feed a larger ratio
Acetone, nail polish remover, pear drops	Same starve, further along	Feed sooner, or switch 1:1:1 to 1:5:5
Sharp vinegar that never softens	Acetic acid dominant	Warmer, wetter, more frequent feeds
Beer, wine, bruised apple	Alcohol from a culture just past peak	Feed now; nothing is wrong
Plain yogurt, faintly sweet, domed and bubbly	Balanced and at peak	Bake
Fuzz of any color, pink or orange streaks	Mold or bacterial contamination	Discard it all, wash the jar, rebuild

Every row but the last is a timing problem, not a health problem, and timing is easier to read off a log than to reconstruct from memory — the gap between feeds is usually what drifted, not the flour or the jar. Sourdough Tracker is a sourdough baking companion app for iPhone if you'd rather not carry that in your head.

Once the interval fits the temperature, the smell follows within two or three feeds. If it doesn't on consistent ratios, suspect the flour or the room before contamination.

Keeping mold out of the jar

Mold needs a dry, undisturbed surface, which is why it starts on residue rather than in the starter itself. Wipe the walls down to the fill line at every feed and move to a clean jar weekly. Cover loosely — a lid set on top, a ring without its seal — so gas escapes without trapping condensation against dried crust. Keep it away from fruit bowls and other open ferments, and never return a used spoon.

FAQ

Can I scrape mold off a sourdough starter and keep the rest?

No. Fungal growth extends well past the visible colony, so the flour underneath is already colonized. Bin the jar, wash the glass in hot soapy water, and rebuild from a dried backup or from scratch.

Is the liquid on top of sourdough starter safe?

Yes, as long as it's clear, straw, grey, or brown. It's alcohol and acid from a culture that ran out of food. Pour it off for a milder starter or stir it in for a sourer one, then feed.

What does it mean when a sourdough starter smells like nail polish?

Starvation. Those solvent esters build up after the peak, so smelling it every time means the feed is too small or too far apart for your kitchen's temperature. Move to a 1:5:5 ratio or feed twice a day and it clears within a couple of cycles.

My new starter smells like cheese or gym socks — is it ruined?

Probably not. Around days two to four, a young starter often passes through a bacterial phase with a genuinely foul smell before acid-loving organisms take over. Keep feeding on schedule; it usually resolves in a few days.

Can a starter in the fridge grow mold?

It can, but slowly, and usually on smears above the flour line rather than in the culture. Scrape the walls clean before it goes in, use a lid that isn't airtight, and feed every one to two weeks.