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

If you've opened your sourdough starter only to be greeted by a sharp, nail-polish-like odor, you're not alone. That unmistakable acetone smell can be alarming, especially if you're new to sourdough baking. But this scent doesn’t mean your starter is dead or ruined—it’s actually a common signal from your microbial culture indicating it's under stress. Understanding why this happens and how to respond quickly can save your starter and get your baking back on track in as little as 24 hours.

Sourdough starters are living ecosystems made up of wild yeast and lactic acid bacteria. When balanced, they produce pleasant aromas—tangy, fruity, yogurty, or even slightly cheesy. But when imbalanced, certain metabolic byproducts dominate, and one of the most noticeable is acetone. This compound forms during periods of extended fasting or nutrient scarcity. The good news? With a few simple adjustments, you can restore balance and return your starter to peak health.

What Causes the Acetone Smell?

The primary reason your sourdough starter smells like acetone is **hunger**. When your starter runs out of accessible carbohydrates (mainly flour), the microbes begin consuming their own waste products for energy. One of these metabolic processes produces acetic acid, which further breaks down into acetone, especially in low-moisture or high-acid environments.

This typically occurs when:

• You’ve gone more than 48–72 hours without feeding it (if kept at room temperature).
• Your starter is stored in the refrigerator for several days or weeks without refreshment.
• The ratio of starter to fresh flour and water is too high, meaning there isn't enough food relative to microbial load.
• It’s been fed infrequently or with insufficient flour.

Acetone production is part of a survival mechanism. As lactobacilli break down leftover acids for energy, ketone bodies—including acetone—are released. While not harmful, this signals that your starter is past its peak activity and needs immediate attention.

“An acetone smell is nature’s alarm bell. It means the microbial community has shifted into conservation mode due to lack of food.” — Dr. Laura Chen, Microbial Fermentation Scientist, University of California, Davis

How to Fix an Acetone-Smelling Starter (Step-by-Step)

Reviving a hungry starter is straightforward and usually takes just one to two feedings. Follow this timeline for rapid recovery:

1. Discard all but 25g of starter. Remove the dark liquid (hooch) on top—it’s alcohol formed from fermentation—and include it in your discard unless it’s gray or moldy.
2. Feed 1:1:1 ratio (starter:flour:water). Mix 25g starter with 25g unbleached all-purpose or whole grain flour and 25g lukewarm water (about 75°F/24°C).
3. Wait 4–6 hours at room temperature. Do not refrigerate. Watch for bubbles and expansion.
4. Repeat the feeding. Discard down to 25g again and feed another 25g flour + 25g water.
5. Evaluate after 12 hours. A healthy response includes doubling in size, domed surface, and a pleasant tangy or fruity aroma.
6. Use or store appropriately. If baking, use at peak rise. If storing, refrigerate only after full activation.

Within 24 hours, most starters return to normal function. If no activity occurs after two full feedings, consider refreshing with whole rye or whole wheat flour—its higher nutrient content often jumpstarts sluggish cultures.

Tip: Always use lukewarm water (not hot) when reviving a starter—excessive heat can damage beneficial microbes.

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

Action	Do	Don’t
Feeding Frequency	Feed every 12 hours if keeping at room temperature.	Leave unfed for more than 72 hours at warm temps.
Flour Choice	Use unbleached all-purpose, whole wheat, or rye for revival.	Switch frequently between very different flours mid-revival.
Hooch Handling	Stir it back in or pour off—both are safe.	Panic—hooch is normal during dormancy.
Storage After Revival	Refrigerate only after it doubles reliably post-feeding.	Stick a weak starter back in the fridge immediately.
Smell Assessment	Trust process—acetone fades with consistent feeding.	Assume it’s dead based on smell alone.

Real Example: Reviving a Neglected Starter

Jessica, a home baker in Portland, forgot about her starter for nine days while traveling. She returned to find a thick layer of dark hooch and a strong chemical odor reminiscent of nail polish remover. Worried she’d lost it forever, she poured off the hooch, discarded all but 20g of the sludge, and fed it 1:1:1 with whole wheat flour and filtered water. After six hours, small bubbles appeared. She repeated the feeding 12 hours later. By the next morning, the starter had doubled, smelled pleasantly sour, and passed the float test. She baked a successful loaf that evening.

Her experience shows that even severely neglected starters often survive. The key was prompt action and consistent feeding—not giving up at the first sign of imbalance.

Preventing Acetone Smell: Long-Term Maintenance Tips

While occasional hunger is normal, frequent acetone development suggests a need for better routine management. Here’s how to keep your starter happy and odor-balanced:

• Establish a feeding schedule. If kept at room temperature, feed every 12 hours. For refrigerated storage, feed weekly and always activate before baking.
• Size matters. Maintain a smaller volume (e.g., 50–100g total) to reduce waste and make feeding manageable.
• Use quality flour. Unbleached flour with natural enzymes supports robust microbial growth.
• Monitor temperature. Cool kitchens slow fermentation; aim for 70–78°F (21–26°C) for optimal activity.
• Label and log. Track feeding times and rise times to anticipate behavior and detect issues early.

Tip: Keep a dedicated jar with measurement lines and a breathable lid—this makes regular observation and feeding easier and less messy.

Frequently Asked Questions

Is an acetone-smelling starter safe to use?

Yes. The smell itself isn’t dangerous. However, a weak or inactive starter may not leaven bread properly. Always wait until it’s bubbly, risen, and aromatic (pleasantly sour) before using. If it hasn’t revived after two feedings, continue feeding until active.

Can I bake with hooch present?

You can stir hooch back into the starter, but it indicates underfeeding. While safe, using a hungry starter may result in overly acidic dough and poor rise. Best practice: revive first, then bake.

Why does my starter smell like acetone even after feeding?

If the odor persists beyond two feedings, the environment may be too cold, the flour too low in nutrients, or contamination could be present. Try switching to whole rye flour and ensure the temperature is above 70°F. If black or pink mold appears, discard entirely.

Checklist: Quick Fix for Acetone-Smelling Starter

1. ✅ Discard down to 25g of starter (including or excluding hooch).
2. ✅ Feed with 25g flour + 25g lukewarm water (1:1:1 ratio).
3. ✅ Wait 4–6 hours at room temperature (no refrigeration).
4. ✅ Repeat feeding: discard to 25g, feed again.
5. ✅ Observe for doubling and bubbles within 12 hours.
6. ✅ Use only when active and fragrant—never while still smelling like chemicals.
7. ✅ Store in fridge only after full revival and stabilization.

Follow this checklist diligently, and your starter should regain vitality quickly. Consistency beats perfection—regular care prevents most problems before they start.

Final Thoughts: Trust the Process

The acetone smell in your sourdough starter is not a failure—it’s feedback. Your microbes are communicating that they need food. Respond with timely feedings, patience, and the right conditions, and they’ll reward you with reliable rise and rich flavor. Many experienced bakers have faced this exact issue, and nearly all starters can be rescued with basic care.

Remember, sourdough is as much about rhythm as it is about recipe. Once you establish a routine that matches your lifestyle—whether daily baking or weekly loaves—your starter will thrive. And that sharp, chemical odor will become nothing more than a distant memory of a lesson learned.

💬 Have a starter rescue story or tip? Share your experience in the comments—your insight could help someone save their sourdough life!