Sourdough starter smells like acetone? It is just hungry
A nail-polish smell means your starter has run out of food and is hungry, not spoiled. Feed it — a bigger feed like 1:2:2 or 1:5:5 — once or twice a day, and the smell goes within a day or two. At 75°F (24°C) a 1:2:2 feed should be ready in about 4½–8½ hours.
Grey liquid (hooch) comes with hunger too. Pour it off for a milder starter, or stir it in for a tangier one.
2
Keep a little, feed a lot
Keep a spoonful and feed it 1:2:2 or 1:5:5 — more fresh flour for the yeast to eat.
3
Feed it on time
Feed it again around its peak for a couple of days, instead of letting it sit long after it falls.
4
Warm and whole grain help
A warmer spot and a spoon of rye or whole wheat get it back to a sweet, yeasty smell faster.
Why
When the yeast and bacteria run out of food they make other by-products — some of which smell like acetone or vinegar. It is a sign of hunger, and a few good feeds put it right.
Questions
Is a starter that smells like acetone safe?
Yes — the smell alone is not a problem. Only mold means throw it away, jar and all: fuzzy spots, or pink, orange or red streaks.
Why does it smell like vinegar?
That is the acid from the bacteria — also a hungry starter, or one kept long in the fridge. A couple of feeds soften it.
It still smells after three days of feeding
Try a bigger feed and a warmer spot, and use filtered water. A starter a few weeks old settles into its own smell.
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