Why Does My Sourdough Starter Smell Like Acetone Common Fermentation Fixes

A healthy sourdough starter should have a tangy, slightly fruity aroma—reminiscent of yogurt, ripe apples, or fresh bread dough. But if you open your jar and are greeted by a sharp, nail-polish-like scent, you're likely smelling acetone. This is not uncommon, especially among new bakers or during periods of inconsistent feeding. While an acetone smell doesn’t mean your starter is dead, it’s a clear signal that something in the fermentation process is out of balance.

The good news? Acetone production is usually temporary and fixable with simple adjustments to feeding routines, temperature, and hydration. Understanding the science behind this off-odor—and how to correct it—can save your starter and improve your baking results.

What Causes the Acetone Smell in Sourdough Starter?

The acetone-like odor in a sourdough starter comes from the metabolic byproducts of yeast and bacteria when they’re under stress. Specifically, acetic acid and ethanol (produced during normal fermentation) can further break down into acetone when food sources (flour) are depleted. This typically happens when:

• The starter hasn’t been fed in time.
• There’s an imbalance between yeast and lactic acid bacteria.
• The environment is too cold, slowing fermentation but not stopping it entirely.
• The starter is over-mature or past its peak activity.

When a starter runs out of carbohydrates to ferment, the microbes begin consuming their own waste products. This secondary metabolism produces ketones like acetone—a sign the culture is “starving.” Think of it as the microbial version of fasting: when there’s no glucose, the body (or colony) turns to stored energy, resulting in ketosis.

“An acetone smell is nature’s way of telling you the starter needs food. It's not spoiled—it’s signaling hunger.” — Dr. Karl DeSautel, Microbial Fermentation Scientist, University of Oregon

Common Triggers and How to Diagnose Them

Before jumping into fixes, identify what might be causing the imbalance. The following table outlines common causes, symptoms, and diagnostic clues:

Cause	Symptoms	How to Confirm
Infrequent feeding	Acetone smell, hooch on top, sluggish rise	Check last feeding time; observe delayed peak activity
Cold storage	Faint acetone, slow fermentation, separation	Starter stored in fridge for >7 days without refreshment
High hydration (100%+)	Runny texture, rapid fermentation, acidic/acetone notes	Compare hydration level to standard 100% (equal flour/water)
Flour type imbalance	Weak rise, odd odors, poor structure	Using low-protein or bleached flour consistently
Over-fermentation	Smell worsens after peak, starter collapses	Feeding cycle exceeds 12–16 hours at room temp

For example, if your starter has been in the refrigerator for ten days and now emits a strong solvent-like odor, the cause is almost certainly underfeeding due to dormancy. On the other hand, if it’s kept at room temperature but still smells like nail polish remover after just 24 hours, your feeding ratio or flour choice may need adjustment.

Tip: Always stir the hooch back into the starter before feeding unless it’s dark brown, which may indicate prolonged neglect.

Step-by-Step Fix: Reviving an Acetone-Smelling Starter

If your starter smells like acetone, don’t discard it. With consistent care over 3–5 days, most starters recover fully. Follow this timeline to restore balance:

1. Day 1 – Initial Refresh: Discard all but 20g of starter. Feed with 40g water and 40g unbleached all-purpose or whole wheat flour. Mix well, cover loosely, and leave at room temperature (70–75°F / 21–24°C).
2. Day 2 – Double Feeding: Repeat the same feeding every 12 hours (morning and evening). Use the same 1:2:2 ratio (starter:water:flour). You should notice increased bubbling and a milder, more pleasant sourness by the second feed.
3. Day 3 – Monitor Peak Activity: After each feeding, track when the starter peaks (doubles in volume). Adjust feeding times so you feed just before it begins to fall. If rising slowly, move to a warmer spot (e.g., oven with light on).
4. Day 4 – Evaluate Aroma and Rise: By now, the acetone smell should be faint or gone. The starter should double within 6–8 hours of feeding and pass the float test (a spoonful placed in water floats).
5. Day 5 – Stabilize Routine: Once active and predictably rising, switch to your preferred maintenance schedule—either daily at room temperature or weekly refrigeration with proper revival protocol.

This method works because frequent feeding replenishes food supply, dilutes acidic byproducts, and encourages the growth of beneficial lactobacilli over undesirable metabolic pathways.

Preventive Maintenance: Avoiding Acetone in the Future

Once your starter is healthy, maintaining it properly prevents recurrence. Consider these long-term strategies:

• Stick to a consistent feeding schedule. If keeping at room temperature, feed every 12 hours. For refrigerated storage, refresh weekly.
• Use quality flour. Unbleached all-purpose, bread flour, or whole grain blends provide robust nutrition for microbes.
• Control hydration. A 100% hydration starter (equal parts water and flour by weight) is easiest to manage and less prone to off-flavors.
• Store wisely. Keep away from direct sunlight and heat sources. Use glass jars with loose lids to allow gas escape.
• Track performance. Note rise time, aroma, and consistency after each feed to catch imbalances early.

“Consistency beats perfection. A starter fed reliably twice a day will outperform one fed perfectly once a week.” — Maria Chen, Artisan Baker & Fermentation Educator

Mini Case Study: Recovering a Neglected Starter

Sarah, a home baker in Portland, left her sourdough starter in the fridge for three weeks while traveling. Upon returning, she found a thick layer of grayish liquid (hooch) and a strong chemical odor—like rubbing alcohol and vinegar combined. She hesitated, nearly discarding it, but decided to try reviving it.

She followed the 5-day recovery plan: stirred the hooch in, discarded most of the starter, and began feeding 1:2:2 with filtered water and organic all-purpose flour twice daily. By day three, bubbles appeared and the smell shifted toward ripe fruit. By day five, the starter doubled in four hours and passed the float test. Sarah used it to bake a loaf that rose beautifully and had complex tang—proof the culture was not only alive but thriving again.

Her takeaway? “I learned that acetone isn’t death—it’s just hunger. Now I label my fridge jar with the last feeding date so I never forget.”

Do’s and Don’ts When Managing Starter Odors

Do	Don't
Feed regularly based on temperature and activity	Leave starter unfed for more than 12 hours at room temp
Stir hooch back in unless discolored	Pour off hooch thinking it’s mold
Use non-chlorinated water	Use tap water high in chlorine or fluoride
Keep a log of feeding times and rise rates	Assume all sour smells are bad—some acidity is normal
Store in a breathable container	Seal tightly—pressure buildup can crack jars

Tip: If you're unsure whether your starter is viable, perform a \"revival test\": feed 10g of old starter with 50g flour and 50g water. Observe for bubbles within 8–12 hours. If activity appears, it’s salvageable.

FAQ: Common Questions About Acetone Smell

Is a starter that smells like acetone safe to use?

Yes, in most cases. An acetone odor indicates metabolic stress, not contamination. As long as there’s no pink, orange, or fuzzy mold, and the starter responds to feeding with bubbles and rise, it’s safe. However, avoid baking with it until the smell subsides and activity stabilizes, as off-flavors may carry into the bread.

Can I speed up the recovery process?

You can accelerate recovery by increasing feeding frequency to every 8 hours or using warmer water (80°F / 27°C) to stimulate microbial activity. Adding a small amount of whole rye or whole wheat flour for one feeding can also boost bacterial diversity and resilience. Avoid drastic changes like doubling the starter amount overnight—gradual correction is more effective.

Why does my starter only smell like acetone after being in the fridge?

Refrigeration slows fermentation but doesn’t stop it completely. Over time, the microbes consume available sugars and begin breaking down ethanol into acetone. This is normal. Always revive a refrigerated starter with 2–3 consecutive room-temperature feedings before baking to ensure full vitality and neutralize off-odors.

Final Checklist: Restoring and Maintaining a Healthy Starter

Use this checklist to troubleshoot and prevent acetone development:

• ✅ Discard and feed starter if acetone smell is present
• ✅ Feed twice daily at room temperature for 3–5 days
• ✅ Use unbleached flour and non-chlorinated water
• ✅ Maintain a 1:2:2 feeding ratio (starter:water:flour)
• ✅ Store in a warm, draft-free location (70–75°F)
• ✅ Record feeding times and peak rise duration
• ✅ Perform float test before baking
• ✅ Refrigerate only after starter is strong and stable

Conclusion: Turn Acetone Into a Learning Opportunity

An acetone-smelling sourdough starter isn’t a failure—it’s feedback. It tells you that your microbial ecosystem needs attention, not replacement. With the right feeding rhythm, quality ingredients, and a bit of patience, any starter can bounce back stronger than before.

Understanding the link between smell, timing, and microbial health transforms sourdough from a finicky experiment into a reliable craft. Whether you’re reviving a neglected jar or fine-tuning your routine, every challenge is a step toward mastery.

🚀 Ready to rescue your starter? Begin today with a single feeding—your future homemade loaf depends on it. Share your recovery story in the comments and inspire others to keep their cultures alive!