Why Does My Sourdough Starter Smell Like Acetone Is It Dead Or Hungry

If you've ever opened your sourdough starter only to be greeted by a sharp, nail-polish-like odor reminiscent of acetone, you're not alone. This smell can be alarming—especially if you're new to sourdough baking—but it doesn't necessarily mean your starter is dead. In fact, this scent is usually a clear signal from your microbial culture, indicating its current state and needs. Understanding why this happens—and what to do about it—can save your starter and improve your baking results.

Sourdough starters are living ecosystems composed primarily of wild yeast and lactic acid bacteria. These microorganisms feed on the carbohydrates in flour, producing carbon dioxide (which makes bread rise) and organic acids (which give sourdough its tang). When conditions shift—such as feeding frequency, temperature, or flour type—the balance of these microbes changes, often altering the aroma. Acetone is one such compound that can emerge under specific circumstances.

What Causes the Acetone Smell?

The acetone-like odor in a sourdough starter typically comes from the metabolic byproducts of bacteria, particularly certain strains of Lactobacillus. When your starter runs low on food (flour), the microbes begin to break down stored energy reserves through a process called ketosis. During ketosis, fatty acids are metabolized into ketone bodies—one of which is acetone. This same compound is responsible for the fruity or solvent-like smell sometimes noticed in human breath during fasting or low-carb diets.

In sourdough terms, acetone forms when:

• The starter hasn’t been fed in several days.
• There’s an overpopulation of bacteria relative to available food.
• The starter has been stored at warm temperatures, accelerating fermentation.
• It’s made with high-extraction or whole-grain flours, which contain more fats and complex carbohydrates.

This doesn’t mean your starter is dead. In most cases, it’s very much alive—just starving.

Tip: If you see hooch (a dark liquid) on top and detect an acetone smell, your starter is signaling hunger—not death. A consistent feeding schedule will usually restore balance within 24–48 hours.

Is Your Starter Dead or Just Hungry?

Distinguishing between a dormant but salvageable starter and a truly dead one is crucial. The presence of an acetone smell leans heavily toward \"hungry,\" not \"dead.\" Here’s how to assess its condition:

Sign	Hungry Starter	Dead Starter
Smell	Acetone, alcohol, sharp tang	No smell, or foul/rotten odor
Appearance	May have hooch; bubbles possible after stirring	Grayish, moldy, pink film, or complete separation with no activity
Response to Feeding	Begins bubbling within 4–8 hours	No activity after 24 hours, even with fresh flour and warm environment
Texture	Thick but stirrable	Slime-like, blackened, or crusty

A healthy starter may slow down in cooler environments or after extended periods without feeding, but it rarely dies unless exposed to contaminants, extreme heat, or chemical cleaners. The acetone smell is part of a natural survival mechanism—not a death rattle.

“An acetone odor is a sign of metabolic stress, not failure. It means the bacteria are adapting to scarcity. With proper feeding, the culture rebounds quickly.” — Dr. Karl Weiss, Microbial Fermentation Scientist, University of Copenhagen

How to Revive a Hungry Sourdough Starter

Reviving a starter that smells like acetone is usually straightforward. Follow this step-by-step guide to restore vitality and prepare it for baking.

1. Pour off any hooch. If there’s a layer of dark liquid on top, discard it. Hooch is alcohol produced during fermentation and indicates prolonged fasting.
2. Discard most of the starter. Keep about 20–30 grams of the original culture. This ensures you’re working with active microbes without overwhelming them with waste products.
3. Feed with fresh flour and water. Use equal parts (by weight) of unbleached all-purpose or whole wheat flour and lukewarm water (around 75–80°F / 24–27°C). For example: 25g starter + 50g flour + 50g water.
4. Stir well and cover loosely. Use a breathable lid or cloth secured with a rubber band to allow gas exchange while keeping debris out.
5. Repeat every 12 hours. Feed twice daily at consistent intervals. Within 24–48 hours, you should notice increased bubbling, a yeasty aroma, and rising volume.
6. Check for ripeness. A ripe starter will double in size within 4–8 hours of feeding, pass the float test (a spoonful placed in water floats), and smell pleasantly sour or fruity—not harsh or chemical.

After two to three days of regular feeding, your starter should return to peak performance. If using whole grain flour contributed to the acetone smell, consider switching to white flour temporarily during revival to reduce fat content and stabilize pH.

Preventing Future Acetone Buildup

While occasional acetone notes aren’t harmful, frequent occurrences suggest room for improvement in maintenance. Here’s how to keep your starter balanced and thriving:

Tip: Store your starter in a transparent container with measurement markings. This makes it easier to track growth, discard accurately, and spot early warning signs like unusual coloration or separation.

• Feed consistently. Even if refrigerated, feed weekly. For daily use, maintain at room temperature with twice-daily feedings.
• Adjust feeding ratios. If your starter peaks too quickly or crashes with acetone, try increasing the amount of flour relative to starter (e.g., 1:2:2 ratio of starter:flour:water).
• Monitor temperature. Warm kitchens accelerate fermentation. If your home exceeds 78°F (26°C), consider feeding more frequently or storing the starter in a slightly cooler spot.
• Use quality flour. Fresh, unbromated flour provides better nutrition for microbes. Avoid bleached or enriched flours, which may inhibit growth.
• Don’t ignore hooch. While not dangerous, persistent hooch means your starter is regularly running out of food. Adjust your feeding schedule accordingly.

Mini Case Study: Reviving a Forgotten Starter

Sophia, a home baker in Portland, left her sourdough starter unattended for nine days while traveling. Upon returning, she found a thick layer of gray hooch and a strong acetone smell. Worried it was ruined, she nearly discarded it—but decided to try reviving it first.

She poured off the hooch, saved 25g of the sludge, and began feeding it 1:2:2 with all-purpose flour and filtered water every 12 hours. By the second day, small bubbles appeared. On the third day, the starter doubled in size within six hours and passed the float test. Within four days, she baked a successful loaf of sandwich bread.

Her takeaway: “I learned that sourdough is resilient. That acetone smell wasn’t the end—it was a cry for food. Now I label my jar with feeding times and set phone reminders.”

Frequently Asked Questions

Can I bake with a starter that smells like acetone?

Technically yes, but not ideally. An acetone-smelling starter is weak and imbalanced. Baking with it may result in poor rise, dense crumb, or overly sharp flavor. Always refresh and wait for full activity before using in recipes.

Does acetone mean my starter has bad bacteria?

No. Acetone is a natural metabolic byproduct, not a sign of contamination. Harmful bacteria would present as pink streaks, orange spots, or putrid odors. Acetone, while unpleasant, is safe and reversible.

Should I throw out my starter if it keeps smelling like acetone?

Not unless it shows signs of mold or spoilage. Recurring acetone smell points to inconsistent feeding or environmental factors. Adjust your routine instead of starting over—you’ll lose valuable microbial diversity built over time.

Expert Checklist: Maintaining a Healthy Sourdough Starter

To prevent acetone buildup and ensure long-term success, follow this actionable checklist:

✅ Feed on schedule

Every 12 hours at room temp; weekly if refrigerated.

✅ Use a 1:2:2 feeding ratio

1 part starter, 2 parts flour, 2 parts water (by weight).

✅ Store properly

Room temperature for daily use; fridge for infrequent bakers.

✅ Discard before feeding

Maintain manageable volume and microbial balance.

✅ Observe daily

Look for rise, bubbles, and aroma changes.

✅ Use clean tools

Always use non-metal utensils and sanitized jars to avoid interference.

✅ Switch flour if needed

White flour stabilizes; whole grains boost flavor but increase acidity.

Conclusion: Listen to Your Starter

Your sourdough starter communicates through smell, texture, and behavior. An acetone odor isn’t a death sentence—it’s feedback. More often than not, it means your starter is simply hungry and waiting for nourishment. With attentive care, consistent feeding, and a bit of patience, you can restore its vigor and continue creating delicious, naturally leavened bread.

Don’t let a temporary imbalance discourage you. Some of the most robust starters have weathered weeks of neglect and still bounced back stronger. Treat your culture with respect, respond to its cues, and it will reward you with years of flavorful baking.

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