Why Is My Sourdough Starter Smelling Like Acetone Common Causes And Fixes

If you've opened your sourdough starter only to be greeted by a sharp, chemical-like odor reminiscent of nail polish remover, you're not alone. The acetone smell—often described as fruity, metallic, or solvent-like—is surprisingly common among both new and experienced bakers. While it may seem alarming, this scent doesn’t mean your starter is ruined. Instead, it's a clear signal that your culture is under stress and needs attention.

Sourdough starters are living ecosystems composed of wild yeast and lactic acid bacteria. When conditions aren't ideal, these microorganisms shift their metabolic behavior, producing different byproducts—including acetone. Understanding why this happens and how to correct it can save your starter and improve your bread-making results.

What Causes the Acetone Smell in Sourdough Starters?

The acetone-like aroma typically comes from the accumulation of ketones, particularly diacetyl and acetoacetate, which form when your starter runs out of readily available sugars. These compounds are natural byproducts of fermentation, especially when the balance between yeast and bacteria is disrupted.

When a starter is hungry—meaning it hasn’t been fed in a while—the microbes begin breaking down stored starches and proteins more aggressively. This extended fasting period forces them into a state of metabolic adaptation, where they produce alcohol and ketones instead of the usual lactic and acetic acids that give sourdough its pleasant tang.

In short: an acetone smell usually means your starter is starving.

Tip: A small amount of alcohol or acetone scent after several days unfed is normal. If it's overwhelming or persists after feeding, intervention is needed.

Key Microbial Processes Behind the Smell

Lactic acid bacteria (LAB), such as Leuconostoc and Lactobacillus, dominate healthy sourdough cultures. They ferment sugars into lactic acid, contributing to flavor and acidity. Yeasts like Saccharomyces exiguus produce carbon dioxide and ethanol. Under optimal conditions, these organisms coexist in harmony.

However, when food (flour) is scarce, LABs slow down, and certain yeasts or secondary bacterial strains may produce higher levels of volatile organic compounds. One such compound is acetone, formed during the breakdown of fatty acids or amino acids when glucose is limited.

“An acetone odor is nature’s way of telling you the microbial community is running low on fuel. It’s a hunger signal, not a death sentence.” — Dr. Karl DeSautels, Fermentation Scientist, University of California Cooperative Extension

Common Causes of Acetone Odor (and How to Fix Them)

Several factors can lead to an acetone smell. Identifying the root cause is essential for restoring your starter’s vitality.

1. Infrequent Feeding or Long Storage Without Refreshment

This is the most common reason. If your starter sits for 5–7 days without feeding—especially at room temperature—it will exhaust its food supply. As the microbes consume all available sugars, they enter survival mode, producing alcohol and ketones.

Solution: Resume regular feedings every 12 hours using equal parts (by weight) water and flour. Stick to unbleached all-purpose or whole grain flour initially, as they contain more nutrients and microbes to help rebuild balance.

2. Refrigeration Without Proper Preparation

Storing a starter in the fridge slows fermentation but doesn’t stop it entirely. Over weeks, even refrigerated starters deplete their resources. Pulling one out after months and expecting peak performance leads to off smells.

Solution: Before refrigerating, always feed your starter and let it peak (rise and begin to fall). For long-term storage, refresh it weekly. When reviving, discard most of the starter and feed twice daily at room temperature for 3–5 days.

3. Using Only White Flour

Refined white flours lack the bran and germ found in whole grains, which provide B-vitamins, minerals, and complex carbohydrates that support diverse microbial life. A diet solely of white flour can weaken bacterial populations over time.

Solution: Incorporate whole rye or whole wheat flour into 1–2 feedings per week. These flours are rich in nutrients and promote robust bacterial activity, helping suppress unwanted metabolic pathways.

Tip: Rye flour is highly recommended for reactivating sluggish starters due to its high enzyme and nutrient content.

4. High Ambient Temperatures

Warm environments accelerate fermentation. At temperatures above 78°F (26°C), yeast activity increases dramatically, consuming food faster and increasing alcohol production—which can degrade into acetone-like compounds.

Solution: Move your starter to a cooler spot (ideally 70–75°F or 21–24°C). Consider feeding more frequently (every 12 hours) if keeping it at room temperature in a warm kitchen.

5. Contamination or Imbalanced Microbiome

While rare, exposure to mold, soap residue, or chlorinated water can disrupt your starter’s ecosystem. An imbalanced microbiome may allow less desirable bacteria or yeasts to thrive, altering odor profiles.

Solution: Use filtered or bottled water if your tap water is heavily chlorinated. Always use clean utensils and containers. If contamination is suspected (pink streaks, mold), discard and restart.

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

If your starter currently smells like nail polish remover, follow this structured recovery plan:

1. Discard all but 20g of your starter. Keeping too much old culture prolongs recovery.
2. Feed with 40g water and 40g flour (preferably whole rye or whole wheat for first feeding).
3. Wait 12 hours at room temperature (~72°F/22°C).
4. Repeat feeding (discard down to 20g again, then add 40g water + 40g flour).
5. Continue twice-daily feedings for 3–5 days, switching to all-purpose flour after Day 2.
6. Observe signs of life: Bubbles, expansion (doubling), and a pleasant sour or yogurty aroma.
7. Test readiness: Drop a spoonful into a glass of water. If it floats, it’s ready to bake with.

Most starters show improvement within 48 hours. Full recovery typically takes 3–5 days of consistent care.

Do’s and Don’ts When Managing a Smelly Starter

Do	Don’t
Feed regularly, especially if kept at room temperature	Leave your starter unfed for more than 3–4 days at room temp
Use unbleached, preferably whole-grain flour occasionally	Rely only on bleached white flour long-term
Store in a loosely covered container to allow gas exchange	Seal tightly in a jar—pressure buildup can damage microbes
Keep at stable, moderate room temperature	Place near heat sources like ovens or radiators
Smell and observe changes daily to catch issues early	Ignore strong off-odors or discoloration

Real Example: Recovering a Neglected Starter

Sarah, a home baker in Portland, left her sourdough starter in the fridge for six weeks while traveling. Upon returning, she found a layer of grayish liquid (hooch) on top and a strong acetone smell underneath. Worried it was dead, she nearly discarded it.

Instead, she followed a revival protocol: she poured off the hooch, discarded all but 20g of the thick paste, and fed it with equal weights of water and rye flour. She repeated the feeding every 12 hours for three days. By Day 2, bubbles appeared. By Day 4, the starter doubled in size within 6 hours and produced a loaf of sourdough with excellent oven spring and flavor.

Sarah now maintains her starter weekly, even when baking infrequently, and keeps a backup in the freezer.

Frequently Asked Questions

Is an acetone smell dangerous?

No, the presence of acetone is not harmful. It indicates a starved but likely still viable starter. As long as there’s no mold, pink hue, or rotten odor, your culture can be revived safely.

Can I bake with a starter that smells like acetone?

Technically yes, but the resulting bread may have off-flavors and poor rise. It’s better to revive the starter first through consistent feeding until it becomes active and aromatic (pleasantly sour or fruity).

Why does my starter produce so much hooch?

Hooch (the dark liquid on top) is alcohol produced during fermentation. Frequent hooch formation signals underfeeding. To reduce it, increase feeding frequency or store in the fridge with periodic refreshments.

Prevention Checklist: Keep Your Starter Healthy

• Feed every 12 hours if kept at room temperature
• Use high-quality, unbleached flour
• Incorporate whole grain flour once a week
• Store in a breathable or loosely capped container
• Keep in a cool, stable environment (70–75°F / 21–24°C)
• Revive refrigerated starters with 3–5 consecutive feedings before baking
• Monitor aroma and rise daily during active use
• Maintain a backup (dry or frozen) for long absences

Tip: Label your feeding schedule on the jar with tape. Even simple notes like “Fed 8 AM” help track timing and prevent neglect.

Conclusion: Listen to Your Starter

Your sourdough starter communicates through smell, texture, and rise. An acetone odor isn’t a failure—it’s feedback. With consistent feeding, proper flour choices, and environmental awareness, you can restore balance and maintain a vibrant culture for years.

Don’t rush the revival process. Patience and routine are key. Once stabilized, your starter will reward you with reliable leavening power and complex flavor in every loaf.

💬 Have a starter rescue story or tip? Share your experience in the comments below—your insight could help another baker save their culture!