Why Does My Sourdough Starter Smell Like Acetone And When Should I Worry

A sourdough starter is a living culture of wild yeast and lactic acid bacteria that transforms flour and water into a powerful leavening agent. When cared for properly, it develops a pleasant tangy aroma—similar to yogurt or ripe fruit. But if you open your jar and are met with a sharp, nail-polish-like scent reminiscent of acetone, it can be alarming. This smell doesn’t necessarily mean your starter is ruined, but it does signal an imbalance in the microbial ecosystem. Understanding why this happens—and when it’s time to intervene—is essential for maintaining a healthy, active starter.

What Causes the Acetone Smell in Sourdough Starter?

The primary reason your sourdough starter smells like acetone is due to the metabolic byproducts of bacteria during periods of starvation. When your starter runs out of fresh food (i.e., flour), the beneficial microbes begin consuming their own waste products and stored energy reserves. One such compound produced during this phase is acetic acid, which has a pungent, vinegar-like or solvent-like odor often mistaken for acetone.

This typically occurs when:

• You’ve gone longer than usual between feedings.
• The ambient temperature is high, accelerating fermentation and depleting nutrients faster.
• Your feeding ratio uses too little flour relative to the existing starter volume.
• The starter is stored in the refrigerator for extended periods without refreshment.

In scientific terms, the acetone-like smell stems from the breakdown of pyruvate under anaerobic conditions—a normal part of bacterial metabolism in Lactobacillus species. As oxygen levels drop and fermentable carbohydrates become scarce, these bacteria shift toward alternative metabolic pathways, producing more acetic acid and trace ketones that contribute to the sharp scent.

Tip: If your starter smells like acetone after refrigeration, don’t panic—it's common. A few consistent room-temperature feedings usually restore balance.

When Should You Worry About the Smell?

While an acetone odor alone isn't cause for immediate concern, it becomes problematic when accompanied by other warning signs. Think of the smell as your starter’s way of saying, “I’m hungry.” Ignoring repeated hunger signals weakens the colony over time, making it less effective at leavening bread.

Here’s when you should take action:

1. No rise after feeding: If your starter fails to double in size within 4–8 hours after being fed at room temperature, its strength may be compromised.
2. Persistent hooch with dark color: The liquid layer (hooch) on top turning brown or black instead of clear or light amber indicates prolonged neglect.
3. Mold presence: Any fuzzy spots—white, green, pink, or black—are definite red flags requiring disposal.
4. Foul rotting odor: Beyond acetone, a putrid, sewage-like stench suggests harmful bacterial contamination.
5. Failure to recover after multiple feedings: After three consecutive feedings every 12 hours at warm room temperature, there’s still no sign of bubbling or expansion.

Acetone smell without these symptoms? Likely just a hungry starter. Combine it with any of the above, and it’s time to reassess your maintenance routine.

How to Fix an Acetone-Smelling Starter

Reviving a neglected starter is usually straightforward. The goal is to reestablish a balanced population of yeast and bacteria by providing regular nourishment and optimal conditions.

Step-by-Step Recovery Process

1. Discard most of the starter: Keep only about 20–30g of the original culture. This reduces acidity and gives new microbes room to grow.
2. Feed with equal parts flour and water: Use 1:1:1 ratio (starter:flour:water by weight). For example: 25g starter + 25g all-purpose or whole grain flour + 25g lukewarm water.
3. Repeat every 12 hours: Feed consistently at room temperature (70–78°F / 21–26°C).
4. Observe for activity: Look for bubbles, expansion, and a slightly sweet-tart aroma within 4–6 hours post-feeding.
5. Double the starter once active: Once rising predictably, increase feeding amounts gradually to build volume for baking.

Within 2–3 days of consistent feeding, most starters regain full vitality. Whole grain flours (like rye or whole wheat) can accelerate recovery because they contain more nutrients and minerals that support microbial growth.

“Sourdough starters are resilient. Even after weeks in the fridge with a strong acetone layer, they often bounce back with proper feeding.” — Dr. Karl DeSaulniers, Fermentation Scientist and Artisan Baker

Prevention: Best Practices for a Healthy Starter

Preventing acetone buildup comes down to consistency and understanding your starter’s needs based on environment and usage frequency.

Feeding Frequency Guidelines

Storage Method	Recommended Feeding Interval	Notes
Room Temperature (Daily Baking)	Every 12–24 hours	Feed before or after peak rise; ideal for daily bakers.
Refrigerated (Occasional Use)	Once weekly or before use	Always refresh 1–2 times at room temp before baking.
Infrequent Use / Long Storage	Every 2 weeks minimum	May develop acetone; discard hooch and feed twice before use.

Do’s and Don’ts for Maintaining Balance

Do	Don’t
Use filtered or non-chlorinated water	Use chlorinated tap water (can inhibit microbes)
Store in a loosely covered container	Seal tightly (traps gases, increases pressure)
Stir down before each feeding	Ignore separation or hooch formation
Keep at stable room temperature	Place near heat sources or drafty windows
Label feeding times	Guess when last fed

Tip: Stirring your starter once or twice between feedings helps distribute acids evenly and prevents excessive buildup in one area.

Real Example: Recovering a Forgotten Refrigerated Starter

Sophie, a home baker in Portland, returned from a two-week vacation to find her sourdough starter topped with a deep brown liquid and smelling sharply of nail polish remover. She was ready to discard it—until she recalled reading about acetone production during dormancy.

She followed a simple protocol:

• Removed the dark hooch layer.
• Retained 25g of the cloudy paste beneath.
• Fed it 25g all-purpose flour and 25g water, mixing well.
• Covered loosely and left at 75°F (24°C).

After 12 hours, small bubbles appeared. She fed again using the same ratio. By the third feeding, the starter doubled in size within six hours and emitted a clean, tangy fragrance. Two days later, she baked a successful loaf of sourdough boule.

Sophie now maintains a feeding schedule even when traveling—either prepping a fresh batch before leaving or storing a backup in the freezer.

FAQ: Common Questions About Acetone Smell

Is it safe to bake with a starter that smells like acetone?

Yes, as long as there’s no mold or foul rotting odor. However, performance may be reduced. For best results, feed the starter 1–2 times and wait for full rise before using it in recipes.

Why does my starter produce hooch so quickly?

Hooch forms rapidly when the starter runs out of food. Frequent hooch indicates either infrequent feeding or a high ratio of starter to fresh flour. Adjust your feeding schedule or reduce the amount of mature starter retained.

Can I use the hooch instead of pouring it off?

You can stir it back in occasionally to retain flavor complexity, especially if it's clear or light amber. Dark hooch contains higher concentrations of alcohol and acids and should be discarded to avoid overwhelming the dough.

Conclusion: Listen to Your Starter

Your sourdough starter communicates through smell, texture, and behavior. An acetone-like odor is not a death sentence—it’s feedback. It tells you that your culture is alive but underfed. With attentive care, nearly every starter can be revived and maintained in peak condition.

The key lies in routine: regular feedings, proper storage, and observation. Whether you bake daily or once a month, adapting your maintenance strategy to your lifestyle ensures longevity and reliability. Treat your starter not just as an ingredient, but as a partner in the art of bread-making—one that rewards patience and attention with flavor, nutrition, and tradition.

💬 Have a revival story or tip? Share your experience below—your insight could help another baker save their starter!