Why Does My Sourdough Starter Smell Like Acetone And Can I Save It

If you’ve opened your sourdough starter only to be greeted by a sharp, nail-polish-like odor reminiscent of acetone, you’re not alone—and your starter isn’t necessarily doomed. This smell is a common concern among home bakers, both new and experienced. While it may seem alarming, an acetone aroma typically signals a stressed or underfed culture rather than irreversible spoilage. Understanding the science behind this scent, identifying its causes, and knowing how to respond can mean the difference between tossing your starter and restoring it to vibrant health.

Sourdough starters are living ecosystems composed of wild yeast and lactic acid bacteria. When maintained properly, they produce a balanced mix of acids, alcohol, and gases that contribute to their signature tangy aroma and leavening power. But when imbalances occur—due to infrequent feeding, temperature fluctuations, or nutrient depletion—byproducts like acetic acid and ethanol can accumulate, leading to off-putting odors such as acetone. The good news? In most cases, a smelly starter can be revived with consistent care.

What Causes the Acetone Smell in Sourdough Starters?

The primary reason for an acetone-like smell is hunger. When a sourdough starter runs out of available carbohydrates (from flour), the microbes begin metabolizing stored energy reserves, producing ketones—including acetone—as waste products. This process is similar to ketosis in humans during fasting. In microbial terms, it’s a survival mechanism: no food means the bacteria and yeast break down complex molecules into simpler ones, releasing volatile compounds with strong odors.

Acetone is primarily produced by Acetobacter species and certain strains of Lactobacillus, both of which are naturally present in sourdough cultures. Under normal conditions, these bacteria coexist harmoniously, contributing to flavor complexity. However, when the environment becomes too acidic or starved, their metabolic output shifts toward more aggressive acid and ketone production.

Other contributing factors include:

• Infrequent feeding: Going too long between feedings allows microbes to exhaust food sources.
• Low hydration: A stiff or dry starter has less available water for microbial activity, concentrating acids and odors.
• High temperatures: Warm environments accelerate fermentation, increasing metabolic byproducts.
• Over-maturation: Leaving a starter at peak rise too long leads to autolysis (self-digestion) and off-smells.
• Flour type: Whole grain flours ferment faster due to higher mineral and enzyme content, potentially accelerating acetone formation.

Tip: If your starter smells like acetone, don’t discard it immediately. Most often, it just needs to be fed regularly for a few days to rebalance.

Can You Save a Sourdough Starter That Smells Like Acetone?

Yes—almost always. An acetone odor is a warning sign, not a death sentence. Unlike moldy or putrid smells (which suggest contamination), acetone indicates metabolic stress, not spoilage. With proper feeding and environmental adjustments, your starter can return to full vitality within 2–4 days.

The key is consistency. Irregular feeding patterns confuse the microbial community, allowing undesirable byproducts to dominate. By resuming a predictable feeding schedule using fresh flour and filtered water, you reintroduce nutrients and dilute accumulated acids, giving beneficial microbes a chance to rebound.

Starters kept in the refrigerator are especially prone to acetone development because cold slows yeast activity more than bacterial metabolism, creating an imbalance. Even if stored cold, a neglected starter can survive for weeks or even months—though revival may take slightly longer.

“An acetone smell is nature’s way of telling you your starter is hungry. It’s not broken—it’s asking for food.” — Dr. Karl DeSaulniers, Fermentation Scientist and Author of *The Art of Sourdough*

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

Follow this timeline to restore your starter’s balance and eliminate the chemical odor:

1. Day 1 – Initial Feed: Discard all but 20g of your starter. Feed it 40g of water and 40g of 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 Daily Feeding: Repeat the same feeding in the morning. Then, 12 hours later, perform another identical feed. This begins rebuilding microbial diversity.
3. Day 3 – Monitor Activity: Look for bubbles, expansion (ideally doubling), and a pleasant sour or fruity aroma. Continue twice-daily feedings regardless of appearance.
4. Day 4 – Assess Progress: If rising well and smelling more like yogurt or ripe fruit, reduce to once-daily feeding if storing in the fridge. If still sluggish or smelly, continue twice-daily feedings.
5. Day 5–6 (if needed): Persist with regular feedings. Most starters recover fully by Day 5.

After revival, consider maintaining your starter at room temperature if baking frequently (every 1–3 days), or store it in the fridge with weekly feedings if baking less often. Always bring refrigerated starters to room temperature and feed 2–3 times before baking to ensure peak leavening power.

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

Do	Don’t
Feed consistently every 12–24 hours if at room temperature	Leave your starter unfed for more than 5 days at room temp
Use unbleached flour (white, whole wheat, or rye)	Use bleached or self-rising flour, which can inhibit microbial growth
Stir down the starter before each feeding to incorporate oxygen	Seal the container tightly—use a loose lid or cloth cover to allow gas escape
Keep your starter in a warm spot (70–78°F) for optimal fermentation	Place near heat sources like ovens or radiators, which cause overheating
Smell and observe changes daily to catch issues early	Ignore separation or dark liquid (hooch) for extended periods

Tip: The dark liquid (hooch) that sometimes forms on top is alcohol from fermentation. Pour it off or stir it in before feeding—it’s not harmful.

Real Example: Recovering a Neglected Refrigerator Starter

Sarah, a home baker in Portland, returned from a two-week vacation to find her sourdough starter had developed a strong solvent-like smell and a layer of grayish hooch on top. Worried she’d lost months of cultivation, she nearly discarded it. Instead, she followed a structured revival plan: she poured off the hooch, saved 20g of the base, and began feeding it twice daily with equal parts water and all-purpose flour.

By Day 2, small bubbles appeared. On Day 3, the starter doubled in size within 8 hours and emitted a clean, tangy scent. By Day 4, she used it to bake a successful loaf of sandwich bread. “I learned that neglect doesn’t equal failure,” Sarah said. “Now I label my fridge jar with the last feeding date so I don’t forget again.”

This case illustrates a crucial point: even after prolonged dormancy, a starter’s microbial community remains resilient. With patience and routine, it can reestablish equilibrium quickly.

Preventing Future Acetone Development

Once your starter is healthy, maintain its balance with proactive habits:

• Stick to a feeding schedule: At room temperature, feed every 12–24 hours. In the fridge, feed weekly.
• Adjust ratios based on climate: In hot weather, increase feeding frequency or reduce starter retention (e.g., feed 1:2:2 instead of 1:1:1).
• Use rye or whole wheat periodically: These flours contain more nutrients and enzymes, boosting microbial vigor.
• Track progress: Keep a log of feeding times, rise times, and aroma notes to identify patterns.
• Refresh before storage: Always feed and wait for peak activity before refrigerating.

Avoid drastic changes in flour type or hydration unless intentional. Sudden shifts can destabilize the microbiome, leading to temporary off-odors—even if not acetone-related.

Frequently Asked Questions

Is an acetone smell dangerous?

No. Acetone is a natural metabolic byproduct and not harmful in this context. It does not indicate pathogenic bacteria or mold. As long as there’s no pink, orange, or fuzzy growth, your starter is safe to revive.

Why does my starter smell like nail polish remover after being in the fridge?

Cold storage slows yeast activity more than bacterial metabolism, allowing acid-producing bacteria to dominate temporarily. Upon feeding at room temperature, the yeast population rebounds, restoring balance. This is normal and expected.

Can I bake with a starter that smells like acetone?

Not effectively. A hungry starter lacks sufficient active yeast for reliable leavening. Bread may not rise properly and could have an overly sharp taste. Always refresh until bubbly, active, and pleasantly sour before baking.

Essential Checklist for Starter Health

Use this checklist to maintain a thriving sourdough culture:

• ✅ Feed every 12–24 hours if kept at room temperature
• ✅ Use non-chlorinated water (filtered or bottled if tap is chlorinated)
• ✅ Store in a breathable container (jar with loose lid or cloth cover)
• ✅ Discard and feed regularly to prevent over-acidification
• ✅ Observe rise and fall cycle to time feedings correctly
• ✅ Smell for clean sourness—not rot, mold, or persistent chemicals
• ✅ Label with last feeding date, especially for refrigerated starters

Conclusion: Your Starter Is Worth Saving

An acetone smell in your sourdough starter is a clear signal—but not a terminal one. It reflects a temporary imbalance caused by hunger or inconsistent care, not contamination or failure. With methodical feeding and attention to environment, nearly every starter can be brought back to life. The resilience of sourdough cultures is remarkable; some documented starters have survived decades through careful stewardship.

Treat your starter not as a finicky experiment but as a living partner in your baking journey. Each adjustment you make deepens your understanding of fermentation and strengthens your results. Whether you're reviving a neglected jar or fine-tuning your maintenance routine, remember: a little patience and consistency go a long way.

💬 Have a sourdough success story or tip? Share your experience below—your insight could help another baker save their starter and keep baking.