Why Does My Sourdough Starter Smell Like Acetone?

Quick Answer

A sourdough starter that smells like acetone or nail polish remover is hungry -- it has consumed all available sugars and shifted into a stressed fermentation mode. The smell comes from ethyl acetate, an ester produced when ethanol (from yeast fermentation) reacts with acetic acid (from bacterial fermentation) in an unfed starter. It is not ruined. Feed it with fresh flour and water at a 1:5:5 ratio (starter to flour to water), let it ferment at room temperature, and repeat every 12 hours for 2-3 days. The acetone smell will disappear and the starter will return to its normal yeasty, mildly tangy aroma.

What Is Actually Happening in the Jar

A sourdough starter is a complex ecosystem. It contains wild yeast (primarily Saccharomyces and Candida species) and lactic acid bacteria (primarily Lactobacillus and Leuconostoc species) living together in a flour-and-water medium. When you feed the starter, the yeast and bacteria consume the sugars released from starch by enzymatic activity. The yeast produces carbon dioxide and ethanol. The bacteria produce lactic acid and acetic acid.

In a well-fed, healthy starter, these metabolic products are in balance. The starter smells pleasantly yeasty and mildly sour, like good beer or ripe fruit. The pH sits between 3.5 and 4.5, and the microbial population is thriving and vigorous.

When the food runs out -- because the starter has not been fed for too long relative to its activity level and the ambient temperature -- the ecosystem shifts. The yeast, unable to ferment sugars, begins to die off or go dormant. The acetic acid bacteria, which thrive in more acidic and oxygen-rich conditions, become dominant. The ethanol that accumulated from yeast fermentation begins to react with the surplus acetic acid through a process called esterification, producing ethyl acetate.

Ethyl acetate is the compound responsible for the nail polish remover or acetone smell. It is actually a different chemical from acetone (which is a ketone), but our noses perceive them similarly. The smell is a clear chemical signal that the starter's ecosystem is out of balance -- too much acid, not enough fresh food, declining yeast populations.

How Serious Is It?

Not very. This is one of the most recoverable states a starter can be in. The yeast and bacteria are stressed but alive. They are not dead. Unlike some irreversible problems in fermentation -- like a dough that will not pass the windowpane test due to destroyed gluten -- this is purely a feeding issue, and feeding solves it.

However, the longer the starter stays in this state, the harder recovery becomes. A starter that has been neglected for a week or two at room temperature will recover with a few feedings over 2-3 days. A starter that has been neglected for months may take a week or more of consistent feeding, and in extreme cases, the yeast population may be so depleted that it never fully recovers.

The Recovery Process

The key to bringing back an acetone-smelling starter is aggressive feeding with a high ratio of fresh flour to old starter. This dilutes the accumulated acid and ethanol, provides abundant fresh food for the surviving organisms, and resets the pH to a range where yeast can outcompete acid-producing bacteria.

Discard most of the starter. Take just a tablespoon (about 20 grams) of the smelly starter and put it in a clean jar. Discard the rest. This step is important -- you want to minimize the amount of accumulated acid and ethanol in the new mixture.

Feed at a 1:5:5 ratio. Add 100 grams of flour and 100 grams of water to the 20 grams of starter. Stir well. This high ratio of fresh flour means there is plenty of food and the pH of the mixture is closer to neutral, giving the yeast a better environment to reactivate.

Use room temperature. Keep the starter at 70-78 degrees Fahrenheit. Warmer temperatures favor yeast activity over bacterial activity, which is what you want during recovery. Do not put it in the refrigerator -- that will slow the yeast even further.

Repeat every 12 hours. Discard all but a tablespoon and feed again at the 1:5:5 ratio. Within the first feeding, the acetone smell should diminish noticeably. By the second or third feeding, it should be replaced by a milder, yeastier smell. By the fourth or fifth feeding, the starter should be rising predictably and smelling like healthy, active sourdough.

Watch for the rise. The reliable sign that recovery is complete is not smell alone but leavening activity. A recovered starter should at least double in volume within 4 to 6 hours of feeding at room temperature. When it consistently does this over 2-3 consecutive feedings, it is ready to bake with.

Why Some Starters Are More Prone to This

Room temperature storage with infrequent feeding. A starter kept at room temperature needs feeding at least once every 24 hours, and in warm kitchens (above 75 degrees), twice a day. If you bake less frequently, store the starter in the refrigerator and feed it once a week. The cold slows metabolism enough that weekly feeding is sufficient.

High hydration starters. A starter with a lot of water relative to flour (100% hydration or higher) ferments faster and runs out of food quicker than a stiffer starter. The extra water also facilitates faster chemical reactions, including esterification.

Whole grain flour. Whole wheat and rye flours provide more enzymatic activity and more nutrients than white flour, which makes the starter more vigorous -- but also means it burns through its food supply faster. If you feed with whole grain flour, you may need to feed more frequently or use a higher ratio of flour to starter.

The Hooch Question

Along with the acetone smell, you may notice a layer of dark liquid on top of the starter. This is hooch -- a mixture of ethanol, water, and organic acids that separates out when the starter has exhausted its food. Hooch is another indicator of a hungry starter, and it is normal.

Some people stir the hooch back into the starter before feeding, and some pour it off. Stirring it back in adds acidity, which produces a more sour bread. Pouring it off and then feeding produces a milder starter. During recovery from an acetone state, pour it off -- you want to reduce acidity, not add to it.

Can You Bake with a Starter That Smells Like Acetone?

Technically, yes. The bread will not be toxic. But the results will be poor. A stressed starter with depleted yeast populations will not leaven bread well -- you will get a dense, flat, excessively sour loaf. The acetic acid dominance produces a harsh, sharp sourness rather than the balanced tanginess of good sourdough. It is the difference between properly soured coffee with pleasant acidity and coffee that just tastes wrong.

Take the 2-3 days to recover the starter properly. The bread you make with a healthy, vigorous starter will be dramatically better.