What a sourdough starter actually is
A sourdough starter is a living mixture of wild yeast and bacteria that you keep in a jar and feed regularly. It is not a packet of commercial yeast—it is a culture you maintain yourself. The yeast and bacteria live together in flour and water, and when you feed them, they multiply and produce gas (which makes bread rise) and acids (which give sourdough its tangy flavor).
The main players are Saccharomyces cerevisiae (a wild yeast) and Lactobacillus (a lactic acid bacterium). These two organisms have a working relationship: the bacteria produce acids that create an environment where the yeast thrives, and the yeast produces byproducts the bacteria can use. You do not need to do anything special to attract them—they are already present on grain and in the air around your kitchen.
When you first mix flour and water and leave it on the counter, wild microbes begin to colonize it. After about five to seven days of regular feeding, the culture becomes stable and strong enough to leaven bread reliably. At that point, you have a working starter.
Key Takeaways
- A sourdough starter is a living culture of wild yeast and bacteria that you feed with flour and water every few days or every week, depending on how you store it.
- The yeast produces carbon dioxide gas that makes dough rise, while the bacteria produce acids that flavor the bread and create the sour taste.
- A new starter takes five to seven days of daily feeding before it is ready to bake with, and you will see bubbles and smell a yeasty or vinegary odor when it is active.
- You keep a starter alive by feeding it regularly—either at room temperature (every 12 hours) or in the refrigerator (once a week), depending on your baking schedule.
- The starter works because the yeast and bacteria consume the flour and water you feed them, reproduce, and produce gas and acid as waste products.
How the yeast and bacteria feed and reproduce
When you mix flour and water, you are providing food and a habitat. The yeast and bacteria in the flour and air begin to eat the starches and sugars in the flour. As they consume the food, they reproduce—each cell divides and creates more cells. This is why your starter grows larger and bubblier over time.
The yeast breaks down sugars and produces two main waste products: carbon dioxide (the gas that makes bread rise) and ethanol (alcohol, which mostly evaporates during baking). The bacteria break down starches into acids, primarily lactic acid and acetic acid. These acids are what give sourdough its distinctive sour flavor and also lower the pH of the dough, which slows down the growth of unwanted molds and bacteria.
The bacteria and yeast do not compete—they help each other. The acids the bacteria produce make the environment more acidic, which is exactly where the yeast prefers to live. In return, the yeast produces compounds that the bacteria can use. This partnership is why a healthy starter smells pleasantly yeasty or vinegary, not rotten.
Why you have to feed your starter regularly
The yeast and bacteria consume the flour and water you give them. Once the food runs out, they cannot reproduce anymore, and the culture weakens. If you never feed your starter, it will eventually die because the microbes will starve and the culture will be colonized by molds or unwanted bacteria.
Feeding means adding fresh flour and water. When you do, you are replenishing the food supply and diluting the waste products (the acids) that have accumulated. The microbes then have room to grow again. A typical feeding ratio is equal parts starter, flour, and water by weight—for example, 50 grams of starter, 50 grams of flour, and 50 grams of water.
How often you feed depends on where you store it. At room temperature (around 70°F), a starter consumes its food quickly and needs feeding every 12 hours or so. In the refrigerator, the cold slows down the microbes' metabolism dramatically, so you only need to feed once a week. Many home bakers keep their starter in the fridge and feed it once a week, then take it out and feed it at room temperature for a few hours before baking.
The signs that your starter is ready to use
A new starter is ready to bake with when it is consistently doubling in size within 4 to 8 hours of feeding. You will see bubbles throughout the mixture, and it will smell noticeably yeasty or vinegary. Some starters develop a layer of liquid on top called hooch—this is alcohol produced by the yeast, and it is a sign the culture is active (though you can stir it back in or pour it off).
Before you use a new starter for bread, feed it once or twice and wait for it to peak—that is, to reach its maximum height and show lots of bubbles. This usually happens 4 to 8 hours after feeding, depending on room temperature. At that point, the yeast population is at its highest and most active, so your dough will rise reliably.
An established starter that you have been maintaining for weeks or months will have a rhythm. You will learn when it peaks after feeding, and you can time your baking around that. Some bakers feed their starter in the morning, let it sit at room temperature all day, and bake in the evening when it has peaked.
How temperature affects your starter's speed
Yeast and bacteria are living organisms, and they work faster when it is warm and slower when it is cold. At 75°F to 80°F, a starter will double in size in 4 to 6 hours. At 65°F to 70°F, it might take 8 to 12 hours. At 55°F or below, the microbes are nearly dormant, which is why refrigerated starters only need feeding once a week.
This is why bakers talk about "bulk fermentation time" and "proofing time"—these are not fixed hours but depend on your kitchen temperature. A dough made with starter will rise faster on a warm day than on a cold day, even if you use the same amount of starter and the same flour. Understanding this helps you predict when your dough will be ready to shape and bake.
If your kitchen is very cold (below 65°F), you can speed up fermentation by placing your dough in a warmer spot—near a sunny window, on top of the refrigerator, or in an oven with the light on. Conversely, if your kitchen is very warm (above 80°F), you may need to slow fermentation by using cooler water or by placing dough in a cooler spot, because the yeast will work so fast that the dough may overproof before you are ready to bake.
What happens when you use starter in dough
When you mix active starter into bread dough, you are introducing a large population of yeast and bacteria into the flour and water. The microbes continue to eat and reproduce, producing gas and acid just as they do in the jar. The gas gets trapped in the gluten network of the dough, creating bubbles and making the dough rise.
The bacteria also continue to produce acids, which develop flavor and also strengthen the gluten by breaking down proteins slightly. This is why sourdough has a more complex flavor than bread made with commercial yeast alone, and why the crumb (the interior texture) is often more open and irregular.
The amount of starter you use affects how fast the dough rises. A larger amount of starter (called a "stiff levain" or "liquid levain" depending on the ratio) will make dough rise faster. A smaller amount will make it rise more slowly, which gives the bacteria more time to develop flavor. Professional bakers often use a small amount of starter and let the dough ferment for 12 to 18 hours or more to build flavor.
Keeping your starter healthy long-term
A starter that is fed regularly and stored properly can last for decades or even longer. Some home bakers have starters that have been passed down through families. The key is consistency: feed it on a schedule, use clean jars and utensils, and do not expose it to extreme temperatures or direct sunlight.
If your starter develops mold (fuzzy growth on the surface) or smells like nail polish or acetone, it has been contaminated and should be discarded. A healthy starter smells pleasantly sour or yeasty, never rotten or chemical. If you neglect your starter for weeks and it looks dried out or has a dark liquid on top, you can often revive it by feeding it several times over a few days—the yeast and bacteria are usually dormant but not dead.
Some bakers keep a "backup" starter in the freezer or refrigerator as insurance. You can freeze a portion of active starter in an ice cube tray, then thaw and feed it if your main starter ever fails. This is optional but useful if you bake sourdough regularly and do not want to start from scratch.
Frequently Asked Questions
How long does it take to create a starter from scratch?
A new starter usually becomes active and ready to bake with in five to seven days of daily feeding. You will see bubbles and smell yeast by day three or four, but the culture is not stable enough to leaven bread reliably until it has been fed consistently for at least a week. Some starters take longer—up to two weeks—depending on the flour you use and your kitchen temperature.
Can I use my starter right after I feed it?
No. You should wait until the starter has peaked—that is, risen to its maximum height and started to show bubbles throughout. This usually happens 4 to 8 hours after feeding, depending on temperature. If you use starter before it peaks, the yeast population is still growing and may not be strong enough to leaven your dough properly.
What does hooch mean, and is it bad?
Hooch is the clear or brownish liquid that sometimes sits on top of your starter. It is alcohol produced by the yeast, and it is not bad—it is a sign your starter is active. You can stir it back in or pour it off, depending on preference. Some bakers use the hooch because it adds extra flavor.
Why does my starter smell like vinegar or nail polish?
A vinegary smell is normal and comes from the acetic acid the bacteria produce. A smell like nail polish or acetone usually means the starter has been neglected and the yeast is stressed. Feed it regularly for a few days and the smell should improve. If the smell is rotten or moldy, discard the starter and start over.
Can I bake with a starter straight from the refrigerator?
Not reliably. A refrigerated starter is dormant and needs to be fed and brought to room temperature before use. Feed it, let it sit at room temperature for 4 to 8 hours until it peaks, then use it in your dough. If you use it cold and inactive, your dough may not rise properly.