Starter Smells Like Acetone or Nail Polish? Here's Why

Short answer: if your starter smells like acetone or nail polish remover, it's been over-fermented (hungry). The yeast ran out of food and is producing alcohol byproducts. Feed it now.

What "acetone smell" tells you

A starter that smells:

• Sharply alcoholic
• Like nail polish remover (acetone)
• Like vodka or harsh wine

…has gone past its peak and is breaking down its remaining food into alcohol and acid. The yeast is stressed.

This is a hunger signal, not a health crisis. It's fixable in one or two feeds.

Why it happens

After a feed, the starter's microbes consume sugars from the flour. They produce CO2 (rise), lactic acid (yogurt smell), and acetic acid (vinegar smell). Eventually:

• Sugar runs out
• Yeast switches to fermenting alcohol
• Acetic acid accumulates
• The starter stops rising

You smell the alcohol/acetone phase as a sharp punch.

When acetone smell appears

If your starter smells acetone-y at 18h, it just needs feeding. If it smells acetone-y at 6h, your starter is too active for the feed ratio (feed bigger or warmer).

The 4 fixes ranked

1. Just feed it

Take the starter, discard most of it (down to 10–20g), and feed:

• 50g flour
• 50g water

Within 12 hours, the smell should return to normal (yeasty, mildly tangy).

2. Use a larger feed ratio

If you regularly find your starter at acetone-stage, the feed ratio is too small for how often you feed.

Shift from 1:1:1 to 1:5:5:

• 10g starter
• 50g flour
• 50g water

This gives the yeast more food and extends the time between feeds without acetone development.

3. Cool it down

A starter at 80°F goes through its food in 6 hours. The same starter at 68°F takes 12 hours.

If your kitchen is hot:

• Move starter to a cool corner
• Keep in a wine cooler
• Refrigerate between feeds

4. Refrigerate

For a starter you don't bake with daily:

• Feed normally (1:1:1)
• Wait 1–2 hours
• Refrigerate

The cold pauses fermentation. You can then feed weekly without acetone.

A quick test

Smell the starter every 2 hours after a feed:

• 2h: faint wheat
• 4h: stronger wheat, slight rise
• 6h: yeasty peak
• 8h: tangy
• 10h: acetic
• 12h: alcoholic
• 14h+: acetone

This test maps your starter's fermentation cycle. From it you can plan the feed schedule.

Acetone vs. healthy aged starter

A long-rested starter (24h since feed) might smell:

• Sharply acidic + alcoholic = healthy but hungry
• Sour but pleasant = fine
• Truly bad: rotten, sweaty, vomit-like = bacterial issue

Acetone alone is fine. Combined with off smells, dispose and start over.

Acetone in stored starters

A starter in the fridge for 2+ weeks often smells acetone-y when you open it. This is normal.

Pour off the brown liquid (hooch) on top. Refresh:

• Discard most
• Feed 1:1:1
• Wait 12 hours
• Repeat
• Should be back to normal in 2 cycles

Acetone right after feed

If your starter smells acetone within hours of feeding:

• Starter is unusually active
• Or the previous feed was too small
• Or the temperature is very warm

Increase feed ratio or move to cooler spot.

Don't discard the starter

Acetone smell isn't a death sentence. The microbes are alive. Feed and they'll recover.

Many bakers panic at the first whiff of acetone and start over from scratch. Don't. A 2-feed reset fixes it.

Bake from acetone-smelling starter?

Not ideal but technically possible:

• Refresh once (12h before bake)
• Build a levain from refreshed starter
• The levain's fresh yeast does the work; the residual acetone smell from the starter dissipates

A bake from a refreshed-once starter is fine. A bake from acetone-smelling starter directly will produce dense, alcoholic bread.

Summary table

Acetone is a normal phase, not an emergency. Adjust schedule and it goes away.