Why Does My Sourdough Starter Smell Like Acetone Fixing Common Fermentation Mistakes

A sourdough starter is a living ecosystem of wild yeast and lactic acid bacteria. When nurtured properly, it produces a tangy, balanced aroma and reliably leavens bread. But when something goes wrong—like a sharp, nail-polish-like acetone smell—it’s a signal that the microbial balance has shifted. This scent, while alarming, is not uncommon and usually points to specific, correctable issues in feeding routine, temperature, or hydration. Understanding the science behind this off-odor allows bakers to restore their starter to peak health without starting over.

Understanding Acetone Smell in Sourdough Starters

The presence of an acetone-like odor—a sharp, solvent-like fragrance reminiscent of nail polish remover—is typically caused by the accumulation of acetic acid and its volatile derivatives during extended periods of fasting. When a starter isn’t fed regularly, the beneficial microbes consume available sugars and begin breaking down stored starches and even dead bacterial cells for energy. This metabolic shift leads to increased production of acetic acid, which can volatilize into acetone under certain conditions.

This process is more common in starters kept at cooler temperatures or those neglected between feedings. While not harmful, acetone indicates that your starter is stressed and may lack the vigor needed for reliable bread rising. It’s a sign of imbalance: lactic acid bacteria are still active, but yeast populations may be declining due to nutrient scarcity.

“An acetone smell is your starter’s way of saying it’s hungry. It doesn’t mean it’s dead—just exhausted.” — Dr. Karl Thompson, Microbial Fermentation Scientist, University of Oregon

Common Causes of Acetone Odor

Several factors contribute to the development of acetone in a sourdough culture. Identifying the root cause is essential for effective correction.

• Inconsistent feeding schedule: Going too long between feedings depletes food sources, forcing microbes into survival mode.
• Low temperature storage: Refrigeration slows fermentation but doesn’t stop it entirely, allowing slow acid buildup over time.
• Overfermentation: Leaving a starter at peak rise too long before feeding encourages acidic byproducts.
• Imbalanced hydration: A stiffer (lower hydration) starter tends to favor acetic acid production over lactic acid.
• Flour type: Using low-protein or non-whole-grain flours may not provide sufficient nutrients for sustained microbial activity.

Tip: If your starter smells like acetone after refrigeration, take it out, discard most of it, and feed twice daily at room temperature for 2–3 days.

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

Reviving a sluggish, acetone-scented starter is straightforward if done methodically. Follow this timeline to restore microbial balance and leavening power.

1. Day 1 – Initial revival: Discard all but 20g of your starter. Feed with 40g water and 40g unbleached all-purpose or whole wheat flour. Stir well, cover loosely, and leave at room temperature (70–75°F / 21–24°C).
2. Day 2 – Double feeding: Repeat the same feeding in the morning. Six hours later, perform a second feeding using the same ratio. Observe for signs of bubbles and expansion.
3. Day 3 – Assess activity: Feed again in the morning. By evening, your starter should show consistent doubling within 6–8 hours and emit a clean, yogurt-like sourness instead of acetone.
4. Day 4 – Test float: Take a small spoonful and drop it into a glass of water. If it floats, it’s ready to bake with. If not, continue twice-daily feedings until buoyancy returns.
5. Ongoing maintenance: Once active, decide whether to keep it on the counter (daily feeding) or in the fridge (weekly feeding with refresh before use).

Patience is key. Even severely weakened starters often recover within four days of consistent care.

Do’s and Don’ts for Healthy Sourdough Maintenance

Do’s	Don’ts
Feed your starter at the same time each day if kept at room temperature	Leave your starter unfed for more than 12 hours at room temp without monitoring
Use filtered or non-chlorinated water to avoid killing microbes	Use tap water high in chlorine or chloramines
Incorporate whole grain flour (rye or whole wheat) weekly for nutrient diversity	Rely solely on bleached white flour long-term
Store in a breathable container or loosely covered jar to allow gas escape	Seal the jar tightly, risking pressure buildup and mold risk
Keep your starter in a warm spot (70–78°F) for optimal fermentation	Place near drafts, ovens, or direct sunlight causing temperature swings

Real Example: Recovering a Neglected Starter

Sarah, a home baker in Portland, left her sourdough starter in the refrigerator for three weeks while traveling. Upon returning, she found a layer of grayish liquid (hooch) on top and a strong chemical smell—definitely acetone. Instead of discarding it, she followed a structured revival plan.

She poured off the hooch, discarded all but 20g of the thick paste beneath, and fed it with equal parts water and whole wheat flour. She repeated this feeding every 12 hours at room temperature. By the third day, the starter was bubbling vigorously and expanding predictably. On day four, it passed the float test and produced a loaf with excellent oven spring and open crumb.

Her takeaway: “I learned that neglect doesn’t equal death. With consistent feeding, even a nearly comatose starter can come back stronger than before.”

Preventing Future Fermentation Mistakes

Many acetone issues stem from preventable habits. Adopting a few disciplined practices can maintain starter vitality and avoid recurring problems.

• Stick to a feeding rhythm: At room temperature, feed every 12 hours. In the fridge, feed weekly, always refreshing 4–12 hours before baking.
• Monitor temperature: Use a digital thermometer or infrared gun to ensure your kitchen stays within the ideal range. Cold kitchens slow fermentation; hot ones encourage over-acidity.
• Beware of hooch formation: The appearance of dark liquid on top signals hunger. While not harmful, frequent hooch means your starter needs more frequent feeding or a larger food supply.
• Bake regularly: Using your starter in recipes removes old culture and introduces fresh flour, mimicking a natural refresh cycle.
• Track progress: Keep a sourdough journal noting feeding times, rise duration, smell, and environmental conditions. Patterns emerge over time.

Tip: If you're going away, dry a portion of your starter on parchment paper and store it in a sealed bag. Rehydrate with water and flour upon return.

Essential Checklist for Sourdough Success

Use this checklist weekly to ensure your starter remains healthy and acetone-free:

• ✅ Fed within the last 12 hours (if on counter)
• ✅ Shows visible bubbles throughout the matrix
• ✅ Rises and falls predictably (doubles in 6–8 hours)
• ✅ Smells pleasantly sour, fruity, or yogurty—not alcoholic or chemical
• ✅ Stored in a clean, non-reactive container (glass or food-grade plastic)
• ✅ Uses fresh, high-quality flour free from additives
• ✅ Kept away from strong odors or contaminants (e.g., cleaning products)

Frequently Asked Questions

Is an acetone-smelling starter safe to use?

Yes, an acetone odor is not dangerous. It indicates a hungry starter, not contamination. After proper feeding, the smell should dissipate, and the starter will regain strength. However, avoid baking with it immediately—revive it first for best results.

What’s the difference between hooch and acetone smell?

Hooch is the liquid layer (usually amber or gray) that forms on top of an unfed starter. It’s mostly alcohol and water. The acetone smell is a gaseous byproduct of prolonged fermentation. Both signal hunger, but hooch is visible, while acetone is olfactory.

Can I speed up the revival process?

You can slightly accelerate recovery by increasing temperature to 78°F (26°C) and using rye flour, which boosts microbial activity. However, rushing with excessive heat or oversized feeds can destabilize pH and delay recovery. Patience yields better long-term stability.

Conclusion: Treat Your Starter Like a Living Ally

Your sourdough starter is more than just flour and water—it’s a dynamic community of microorganisms that thrives on consistency and care. An acetone smell isn’t a failure; it’s feedback. By adjusting feeding frequency, choosing the right flour, and maintaining stable conditions, you restore balance and unlock reliable performance in your baking.

Don’t discard a struggling starter too quickly. Most issues are reversible with attentive nurturing. Whether you’re a beginner or seasoned baker, understanding these fermentation principles deepens your connection to the craft and improves your results.

💬 Have a sourdough rescue story or tip? Share your experience below—your insight could help another baker save their starter and bake with confidence!