Why Does My Sourdough Starter Smell Like Acetone Beginner Troubleshooting

A sourdough starter is a living culture of wild yeast and lactic acid bacteria that transforms flour and water into a powerful leavening agent. When cared for properly, it develops a tangy, yogurt-like aroma—pleasant, slightly sweet, and promising good bread. But if you open your jar and are met with a sharp, nail-polish-remover scent, you're not alone. The acetone smell in a sourdough starter alarms many beginners, often leading them to wonder if their starter has gone bad or needs to be discarded.

The truth is, an acetone odor is not a death sentence—it’s a signal. It indicates that your starter is under stress, likely due to infrequent feeding or extended time without fresh nutrients. This article breaks down the science behind the smell, offers actionable steps to correct it, and equips you with long-term maintenance strategies so your starter thrives.

What Causes the Acetone Smell?

The distinct smell of acetone (similar to nail polish remover or overripe fruit) comes from the metabolic byproducts of hungry microbes in your starter. When your starter runs out of food—mainly the starches and sugars in flour—the bacteria begin breaking down stored energy reserves, producing acetic acid and other volatile compounds, including acetoin and diacetyl, which can manifest as an acetone-like aroma.

This process is part of the natural fermentation cycle, but it becomes noticeable when:

• The starter hasn’t been fed in several days.
• It’s stored at room temperature without regular refreshment.
• Too much time passes between feedings, especially in warm environments.

Karen Bell, head instructor at the Sourdough School in the UK, explains:

“An acetone smell is your starter telling you it’s starving. It’s not dead—it’s asking for food. Respond promptly, and it will bounce back stronger.”

In technical terms, the dominant bacteria in sourdough—Acetobacter and certain strains of Lactobacillus—produce acetic acid under low-sugar conditions. As fermentation continues anaerobically (without oxygen), ketone bodies like acetone can form through incomplete metabolism. While safe, this is a sign your microbial ecosystem is imbalanced due to nutrient deprivation.

Is an Acetone-Smelling Starter Still Safe to Use?

Yes, in most cases. A sourdough starter with an acetone odor is typically not spoiled unless accompanied by visible mold, pink streaks, or a foul rotting smell. The presence of a dark liquid on top—often called “hooch”—is a related indicator of hunger and is completely normal. Hooch is mostly alcohol (ethanol) produced by yeast activity and can be stirred back in or poured off before feeding.

However, using a severely underfed starter may result in weak rise and overly sour bread. Reviving the culture before baking ensures better performance and flavor balance.

Tip: Never discard your starter at the first sign of hooch or acetone smell. With proper feeding, it can recover within 24–48 hours.

Step-by-Step Guide to Fixing an Acetone-Smelling Starter

If your starter smells sharply acidic or chemical-like, follow this structured revival process. Consistency is key—don’t skip feedings, even if progress seems slow at first.

1. Discard all but 25g of starter. Remove any dark hooch or discolored layers. Keep only the healthy, elastic portion from the middle or bottom.
2. Feed 1:1:1 ratio (starter:flour:water). Add 25g all-purpose or whole grain flour and 25g lukewarm water (around 75–80°F / 24–27°C). Mix well, cover loosely, and leave at room temperature.
3. Repeat every 12 hours. Feed again after 12 hours, discarding down to 25g before each feeding. Use whole grain flour (rye or whole wheat) for the first two feeds if possible—its higher nutrient content helps jumpstart microbial activity.
4. Monitor for signs of life. Look for bubbles, expansion (ideally doubling), and a pleasant sour aroma. These indicate active fermentation returning.
5. Switch to maintenance schedule. Once your starter reliably doubles within 6–8 hours after feeding, reduce feedings to once daily if keeping at room temperature, or store in the fridge with weekly feedings.

Most starters recover fully within two to three days of consistent feeding. Patience matters—microbial populations need time to rebalance after starvation.

Common Mistakes That Cause Acetone Buildup

Understanding what leads to acetone production helps prevent recurrence. Below are frequent missteps made by new sourdough bakers:

Mistake	Why It’s a Problem	Solution
Infrequent feeding (every 3+ days at room temp)	Starves microbes, triggering ketone production	Feed every 12–24 hours if kept out
Using only white flour consistently	Lacks micronutrients found in whole grains	Rotate in rye or whole wheat flour weekly
Storing starter in a tightly sealed jar	Traps gases, increases pressure and acidity	Use a loose lid or cloth cover
Irregular feeding times	Disrupts microbial rhythm	Stick to a consistent schedule
Leaving starter in a hot environment (>85°F/29°C)	Speeds fermentation, depletes food faster	Keep in a cooler spot (70–75°F / 21–24°C)

Real Example: Reviving a Neglected Starter

Sophie, a home baker in Portland, left her sourdough starter unattended for ten days while traveling. Upon returning, she found a thick layer of grayish hooch and a strong solvent-like odor. Worried it was ruined, she almost tossed it—but decided to try reviving it.

She poured off the hooch, stirred the remaining starter, and began feeding it twice daily with equal parts rye flour and water. By day two, small bubbles appeared. On day three, it doubled in size within seven hours. Within four days, the acetone smell had faded, replaced by a clean, fruity tang. She used it successfully in a loaf of country sourdough with excellent oven spring and flavor.

Sophie’s experience reflects a common scenario: neglect doesn’t equal death. With timely intervention, even a deeply starved starter can return to full vitality.

Preventative Maintenance Checklist

To avoid future acetone issues, adopt these habits as part of your sourdough routine:

• ✅ Feed your starter regularly—even if not baking.
• ✅ Use whole grain flour at least once a week for added nutrition.
• ✅ Store at a stable temperature (70–75°F / 21–24°C) if keeping at room temperature.
• ✅ If refrigerating, feed weekly and warm to room temperature before baking.
• ✅ Discard and refresh before the starter collapses after peaking.
• ✅ Label feeding times to stay consistent.
• ✅ Keep a backup in the freezer (dry or liquid) for emergencies.

Tip: Mark your calendar or set phone reminders for feedings during the initial weeks. Building routine prevents lapses that lead to acetone formation.

When to Worry: Signs Your Starter May Be Beyond Saving

An acetone smell alone is not cause for alarm. However, certain red flags suggest contamination or irreversible damage:

• Mold growth: Fuzzy patches in green, black, or pink hues.
• Foul, rotten odor: Like sewage or rotting eggs (distinct from sour or vinegary notes).
• Pink or orange streaks: Indicates harmful bacterial colonies.
• No activity after 5 days of feeding: Even with ideal conditions, no bubbles or rise.

If any of these appear, it’s safest to discard the starter and begin anew. Otherwise, trust the process—most issues stem from care gaps, not failure.

Storage Strategies to Minimize Starvation

Your storage method directly affects how quickly your starter consumes its food and whether acetone develops. Choose based on your baking frequency:

Room Temperature (Daily Baking)

Keep your starter on the counter and feed it every 12–24 hours. Ideal for those baking 2–3 times per week. Use a breathable cover (cloth or paper towel) secured with a rubber band to allow gas exchange.

Refrigeration (Occasional Baking)

Store in a loosely sealed container in the fridge. Feed once a week. Always bring to room temperature and feed 2–3 times before baking to reactivate. This slows microbial activity and reduces acetone risk between uses.

Freezing or Drying (Long-Term Backup)

For extended breaks, dry a portion of starter on parchment and store in an airtight container, or freeze a small amount. Rehydrate with water and feed over several days to revive. This safety net prevents total loss during travel or illness.

“Having a backup changed everything for me. I used to panic if I missed a feeding. Now I know I can always restart.” — James Reed, artisan baker and fermentation educator

Frequently Asked Questions

Can I bake with a starter that smells like acetone?

You can, but results may be suboptimal. Bread might have excessive sourness and poor rise. It’s better to revive the starter first through consistent feeding until it’s active and bubbly.

Why does my starter smell like acetone only on top?

The surface is exposed to more oxygen, encouraging aerobic bacteria that produce sharper acids. Stirring the starter before feeding redistributes gases and balances pH. Always stir in any hooch unless it’s excessively dark or odorous.

Does using different flours affect the smell?

Yes. Whole grain flours (especially rye) ferment faster and support diverse microbes, often reducing off-smells. White flour-only starters may weaken over time and develop stronger acetone notes when hungry. Rotating flour types promotes resilience.

Conclusion: Listen to Your Starter, Don’t Fear the Smell

An acetone odor in your sourdough starter isn’t a mistake—it’s feedback. It tells you that your microbial community is alive, active, and simply running low on fuel. Rather than reacting with concern, respond with care. Feed it, maintain consistency, and observe how it responds. Over time, you’ll learn its rhythms, recognize its healthy signs, and build confidence in nurturing this living ingredient.

Sourdough is as much about relationship as it is about recipe. Each feeding strengthens that bond. Whether you’re troubleshooting now or preparing for future success, remember: every experienced baker has poured off hooch and revived a sleepy starter. You’re not failing—you’re learning.

💬 Have a revival story or tip? Share your experience below. Your insight could help another beginner keep their starter alive and thriving.