Why Does My Sourdough Starter Smell Like Acetone What It Means And How To Fix It

If you’ve ever opened your sourdough jar and caught a whiff of nail polish remover, you’re not alone. That sharp, chemical-like acetone smell is surprisingly common—and while it’s not dangerous, it’s definitely a signal from your starter. This odor doesn’t mean your culture is dead or ruined, but it does indicate that something in its environment or feeding routine needs attention. Understanding why this happens, what it tells you about microbial balance, and how to correct it can make the difference between a thriving starter and one that underperforms in your baking.

Sourdough starters are living ecosystems composed of wild yeast and lactic acid bacteria. When balanced, they produce pleasant aromas—tangy, fruity, yogurty, or even slightly sweet. But when stressed, they emit byproducts like acetic acid and ethanol, which can break down into volatile compounds resembling acetone. The good news? This issue is usually reversible with simple adjustments.

What Causes the Acetone Smell in Sourdough Starters?

The acetone-like odor typically arises during periods of starvation. When your starter runs out of accessible carbohydrates (from flour), the microbes begin consuming their own waste products and stored energy reserves. One such metabolic pathway results in the production of ketones—including acetone—as a byproduct of fermentation under low-nutrient conditions.

This process is similar to ketosis in humans: when glucose is scarce, the body burns fat and produces ketones. In a sourdough starter, when flour is depleted, the microbes shift into survival mode and start breaking down complex molecules, leading to off-odors.

Several factors contribute to this state:

• Infrequent feeding: Going too long between feedings starves the culture.
• Low temperature: Cold environments slow microbial activity, extending digestion cycles and increasing the chance of over-fermentation.
• High hydration levels: Very wet starters ferment faster and may exhaust food more quickly.
• Type of flour: Some flours, especially low-protein or whole grain varieties, provide less consistent nutrition.

Tip: If your starter smells like acetone after being refrigerated, it's likely been dormant too long without feeding. Refresh it with two consecutive room-temperature feeds.

Is an Acetone Smell Dangerous?

No, an acetone smell is not harmful. It does not indicate mold, pathogens, or spoilage. Your starter is still safe to use, though performance may be compromised. Yeast activity might be sluggish, and leavening power reduced due to microbial fatigue.

However, if accompanied by pink or orange streaks, fuzzy growth, or a rotten-meat smell, discard the starter immediately—those are signs of contamination. Acetone alone is just a metabolic red flag, not a health risk.

“An acetone odor is nature’s way of telling you the microbial community is hungry. It’s a sign to adjust feeding frequency, not to panic.” — Dr. Laura Chen, Food Microbiologist at the Fermentation Research Institute

How to Fix an Acetone-Smelling Starter: A Step-by-Step Guide

Reviving a stressed starter requires consistency and patience. Follow these steps to restore balance and eliminate the chemical odor.

1. Discard most of the starter. Keep only about 20–30 grams of the original culture. This reduces acidity and resets the microbial load.
2. Feed with equal parts water and flour by weight. Use unbleached all-purpose or bread flour for reliable nutrition. For example: 30g starter + 30g water + 30g flour.
3. Repeat every 12 hours at room temperature (70–75°F / 21–24°C). Consistent feeding prevents re-starvation.
4. Observe changes in rise and aroma. After 2–3 feedings, you should notice increased bubbling and a tangy, yogurt-like scent replacing the acetone.
5. Test for readiness. Perform a float test: drop a small spoonful into a glass of water. If it floats within 6–8 hours of feeding, it’s strong enough to bake with.
6. Resume regular maintenance. Once stable, either continue daily feeding or return to refrigeration with weekly refreshments.

For severely neglected starters (e.g., those stored for weeks without feeding), it may take up to five days of twice-daily feedings to fully recover. Don’t rush the process—each cycle dilutes accumulated acids and rebuilds healthy populations of yeast and lactobacilli.

Preventing Acetone Odors: Best Practices

Prevention is simpler than recovery. By establishing a consistent care routine, you can avoid acetone smells altogether. Consider these guidelines:

Practice	Do	Avoid
Feeding Frequency	Feed every 12 hours if kept at room temperature	Going more than 24 hours without feeding
Storage Temperature	Keep between 70–75°F (21–24°C) for active use	Leaving near heat sources or drafty windows
Flour Type	Use unbleached all-purpose or rye flour for stability	Relying solely on low-gluten alternatives like coconut flour
Hydration Level	Maintain 100% hydration (equal flour and water by weight)	Creating overly runny starters without adjusting feed frequency
Refrigeration Routine	Feed weekly; always bring to room temp before baking	Storing for months without refreshing

Tip: Label your feeding schedule on a calendar or set phone reminders. Even experienced bakers forget when life gets busy.

Real Example: Recovering a Forgotten 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 dark liquid (hooch) on top and a strong solvent-like smell. She poured off the hooch, discarded all but 25g of the thick paste below, and fed it with 25g water and 25g all-purpose flour. She repeated this every 12 hours for three days.

By day two, bubbles appeared. By day four, the starter doubled in size within 6 hours and passed the float test. Though the first loaf was slightly dense, the second rose beautifully. Sarah now keeps a feeding log and sets biweekly alarms to refresh her refrigerated starter—even when she’s not baking.

Her experience shows that neglect isn’t fatal. With structured care, even a deeply dormant starter can return to full vitality.

Frequently Asked Questions

Can I bake with a starter that smells like acetone?

You can, but results may be suboptimal. The yeast may lack strength, leading to poor rise and dense crumb. It’s better to revive the starter first through regular feeding until it becomes active and aromatic again.

What is the brown liquid on top of my starter?

That’s called “hooch”—a form of alcohol produced when the starter runs out of food. It’s harmless and a clear indicator of hunger. Pour it off or stir it back in before feeding. Frequent hooch formation suggests underfeeding.

Does the acetone smell mean my starter is dead?

No. An acetone smell means your starter is stressed, not dead. As long as there are no signs of mold or putrid odors, it can almost always be revived with proper feeding. True death is rare unless exposed to extreme heat or contaminants.

When to Be Concerned: Signs Beyond Acetone

While acetone is normal under stress, other symptoms warrant caution:

• Pink or orange discoloration: Indicates harmful bacterial growth. Discard immediately.
• Fuzzy mold patches: Usually white, green, or black. Never attempt to scoop out mold—start fresh.
• Rotten egg or sewage smell: Suggests sulfur-producing bacteria. Unhealthy and best discarded.
• No bubbles after multiple feedings: Could mean insufficient microbial life. Try introducing rye flour or pineapple juice (for pH adjustment) to encourage regrowth.

In contrast, grayish color, clear hooch, or mild vinegar/acetone scents are generally safe and correctable.

Tip: If reviving a weak starter, try using rye flour for one or two feedings. Its high nutrient content and natural enzymes help jump-start microbial activity.

Long-Term Maintenance Checklist

To keep your sourdough starter healthy and acetone-free, follow this checklist:

• ✅ Feed at least once every 12 hours if kept at room temperature
• ✅ Store in a breathable container (loosely covered lid or jar with burp-cap)
• ✅ Use filtered or non-chlorinated water to avoid killing microbes
• ✅ Maintain consistent flour type unless intentionally changing
• ✅ Bring refrigerated starters to room temperature and feed 2–3 times before baking
• ✅ Discard and refresh regularly—even if not baking—to prevent stagnation
• ✅ Monitor for rapid hooch formation or weak rise, adjusting schedule as needed

Consistency matters more than perfection. A starter fed slightly late will survive; one ignored for weeks will require rehabilitation. Think of it like tending a garden: occasional lapses happen, but routine care yields the best harvest.

Conclusion: Listen to Your Starter’s Signals

Your sourdough starter communicates through smell, texture, and rise. An acetone odor isn’t a failure—it’s feedback. It tells you the ecosystem is out of balance, likely due to hunger or infrequent care. With prompt action, you can restore its vigor and continue producing flavorful, well-risen loaves.

Baking with sourdough is as much about relationship as recipe. The more attentively you respond to its needs, the more reliably it will perform for you. Whether you're troubleshooting an old starter or refining your maintenance routine, remember: every sourdough journey includes hiccups. What matters is how you respond.

💬 Have a sourdough rescue story or tip? Share your experience in the comments—your insight could help another baker save their starter today.