Why Does My Sourdough Starter Smell Like Acetone And How To Bring It Back To Health

A sourdough starter is a living culture of wild yeast and lactic acid bacteria that transforms flour and water into the foundation of naturally leavened bread. When healthy, it has a pleasant tang—like yogurt or ripe fruit. But if you open your jar and are hit with a sharp, nail-polish-like scent reminiscent of acetone, something’s off. This smell is not normal for an active, balanced starter and signals imbalance or neglect. The good news: acetone odor doesn’t mean your starter is dead. With proper care, most starters can be revived within 24 to 72 hours.

The acetone smell typically comes from an overproduction of acetic acid and other volatile compounds during fermentation. It often occurs when the starter runs out of food or sits too long between feedings. Understanding why this happens—and knowing how to correct it—is essential for anyone maintaining a home sourdough culture.

What Causes the Acetone Smell?

Sourdough starters rely on a symbiotic relationship between wild yeast (such as Saccharomyces exiguus) and lactic acid bacteria (mainly Lactobacillus species). These microorganisms consume the starches and sugars in flour, producing carbon dioxide (which makes bread rise), ethanol (alcohol), and organic acids (which give sourdough its flavor).

When a starter goes unfed for too long, the available food supply diminishes. As simple sugars run low, microbes begin breaking down more complex compounds, including ethanol. Certain strains of Acetobacter and other bacteria metabolize alcohol into acetic acid—the primary component of vinegar—and other ketones such as acetone. This metabolic shift creates the sharp, solvent-like odor many bakers find alarming.

This process is more common in cooler environments where microbial activity slows but doesn't stop. In refrigerated starters, fermentation continues at a reduced rate, allowing acidic byproducts to accumulate without being diluted by fresh feedings.

Tip: Always stir your starter before smelling it. Sometimes the dark liquid on top (called \"hooch\") concentrates odors, but mixing it in gives a better sense of overall health.

How to Bring Your Starter Back to Health

Reviving an acetone-smelling starter is usually straightforward. The core principle: reintroduce regular feedings with fresh flour and water to dilute accumulated acids and replenish nutrients. Here's a step-by-step plan to restore balance.

Step-by-Step Revival Timeline

1. Day 0 – Assessment & First Feeding: Discard all but 20g of your starter. Mix with 100g of room-temperature water and 100g of unbleached all-purpose or whole wheat flour. Stir well, cover loosely, and leave at warm room temperature (75–80°F / 24–27°C).
2. Day 1 – Second Feeding (12 hours later): Repeat the same feeding ratio. By now, you may see small bubbles forming and a slight rise. The acetone smell should begin to fade.
3. Day 2 – Third and Fourth Feedings (every 12 hours): Continue twice-daily feedings. Observe increased activity—bubbles, doming, expansion to double or triple in size within 4–8 hours after feeding.
4. Day 3 – Evaluate Readiness: If your starter consistently doubles within 6–8 hours and smells pleasantly tangy or fruity, it’s ready to bake with. If not, continue feeding every 12 hours until stable.

During this revival phase, avoid using the starter for baking. Its leavening power will be weak until the microbial community regains strength. Patience is key—forcing a bake too early leads to dense loaves and frustration.

Common Mistakes That Worsen the Problem

Even experienced bakers sometimes make errors that prolong recovery or cause recurring issues. Recognizing these pitfalls helps prevent future setbacks.

• Irregular feeding schedule: Skipping feedings or extending intervals beyond 48 hours (at room temp) allows acid buildup.
• Using cold water or flour: Cold ingredients slow fermentation, delaying recovery.
• Over-retention during discard: Keeping too much old starter dilutes the fresh inoculum needed for rapid growth.
• Storing in too cool a spot: Drafty kitchens or corners far from ambient warmth inhibit consistent activity.
• Assuming hooch means death: Many discard their starter when they see gray or brown liquid on top. Hooch is a sign of hunger—not failure.

Issue	Do	Don’t
Acetone smell	Feed twice daily with equal parts flour and water by weight	Add sugar or honey to “feed” it faster
Hooch formation	Stir it in before discarding and feeding	Pour it off and assume the starter is ruined
Slow rise	Place near heat source (oven with light on, top of fridge)	Move to warmer room erratically; aim for consistency
Storage	Refrigerate only after stability is confirmed	Store in fridge without weekly refreshments

Preventive Care: Maintaining a Balanced Starter Long-Term

Once your starter is healthy again, preventing future acetone episodes requires routine and awareness. A few proactive habits go a long way.

If kept at room temperature, feed your starter every 12 to 24 hours depending on ambient temperature and hydration level. Warmer kitchens accelerate fermentation, requiring more frequent attention. For those who don’t bake daily, refrigeration is practical—but only after establishing a robust culture.

Before refrigerating, ensure your starter peaks reliably—doubling within 6–8 hours post-feeding. Then feed it, let it sit at room temp for 1–2 hours, and place it in the fridge. Even in cold storage, feed it at least once a week: remove, discard most, refresh with new flour and water, let it breathe at room temp for several hours, then return to refrigeration.

Tip: Use whole grain flours (rye or whole wheat) occasionally during revival—they contain more nutrients and minerals that boost microbial vitality.

Choosing the Right Flour

Flour choice impacts both aroma and performance. While all-purpose flour works fine for maintenance, switching temporarily to rye or whole wheat during revival introduces additional enzymes and food sources that help struggling cultures rebound faster.

Rye flour, in particular, is rich in pentosans and soluble fibers that support bacterial growth. Many professional bakers keep a small rye-based starter specifically for boosting lethargic cultures.

Expert Insight: What Microbiologists Say

The science behind sourdough fermentation has advanced significantly in recent years. Researchers now understand that starter health depends not just on presence of microbes, but on their ratios and metabolic outputs.

“An acetone odor indicates a stressed ecosystem. The yeasts are likely dormant or dying off, while acid-producing bacteria dominate. Regular feeding restores pH balance and allows yeasts to repopulate.” — Dr. Kirsten Shockey, Fermentation Scientist and Author of *The Art of Fermentation*

This imbalance isn’t permanent. Unlike spoiled food, a sourdough starter rarely hosts harmful pathogens because the acidic environment naturally suppresses invaders like mold or dangerous bacteria. So even if neglected for weeks, most starters can bounce back with consistent effort.

Mini Case Study: Reviving a Forgotten Fridge Starter

Sarah, a home baker in Portland, returned from a month-long trip to find her sourdough starter covered in gray hooch and reeking of nail polish remover. She almost tossed it—but decided to try one revival cycle first.

She poured off excess hooch, stirred the thick paste beneath, and measured out 20g. Using 100g each of lukewarm water and whole wheat flour, she fed it and placed it near her oven. After 12 hours, there were tiny bubbles. She repeated the feeding. By the second day, the starter rose nearly triple in volume and smelled like green apples. On day three, she used it in a boule—with excellent oven spring and open crumb.

Sarah now maintains a strict weekly feeding schedule and labels her jar with the last feed date. Her lesson: “Smell isn’t destiny. Action is.”

Checklist: How to Diagnose and Fix an Acetone-Smelling Starter

1. ✔️ Check for hooch and stir it in (unless moldy or pink)
2. ✔️ Discard down to 20g of starter
3. ✔️ Feed with 100g water and 100g flour (preferably whole grain initially)
4. ✔️ Place in warm spot (75–80°F / 24–27°C)
5. ✔️ Repeat feeding every 12 hours for 2–3 days
6. ✔️ Monitor for doubling within 6–8 hours post-feeding
7. ✔️ Confirm readiness with float test: drop ½ tsp in room-temp water—if it floats, it’s ready
8. ✔️ Resume baking or store properly (room temp with daily feeding or fridge with weekly refreshment)

Frequently Asked Questions

Is acetone smell dangerous?

No. Acetone is a natural byproduct of extended fermentation and is not harmful in this context. It indicates an imbalanced culture, not contamination. As long as there’s no mold, pink streaks, or foul rotting odor, your starter is safe to revive.

Can I use a smelly starter for baking?

Technically yes, but results will be poor. Bread may lack rise, have excessive sourness, or collapse. Wait until the starter consistently doubles after feeding and smells pleasantly tangy before baking.

Why does my starter produce hooch so quickly?

Hooch forms when the starter consumes all available sugars. Rapid hooch production suggests either too much starter retained during discard, insufficient feeding frequency, or high temperature accelerating metabolism. Adjust feeding ratios or move to a slightly cooler location.

Conclusion: Your Starter Is Resilient—Trust the Process

A sourdough starter that smells like acetone isn’t broken—it’s communicating. It’s telling you it’s hungry, stressed, or overdue for care. This odor is a common hurdle, not a failure. With structured feedings, warmth, and attention, nearly every starter can recover fully.

Think of your culture as a pet or plant: it thrives on consistency. Once you establish a rhythm—whether daily at room temperature or weekly in the fridge—problems like acetone become rare. And when they do appear, you’ll know exactly what to do.

💬 Have a revival success story? Share your experience below—your tip might help another baker save their starter tonight.