How Does a Sourdough Starter Work?

A sourdough starter is a tiny, stable ecosystem living in a jar of flour and water. Wild yeast makes the carbon dioxide that lifts the bread, while lactic acid bacteria make the acids that give sourdough its tang and keep unwanted microbes out. Every time you feed it, you hand both groups a fresh meal, and they rise, peak and collapse on a timetable you can learn to read.

What is actually living in a sourdough starter?

Two kinds of microbes, and they outnumber each other wildly. In a mature starter, lactic acid bacteria (LAB) typically outnumber yeast by something like 10 to 100 to one. They come from the flour itself, from the air, and from your hands and kitchen.

The yeasts are usually a handful of species. Saccharomyces cerevisiae, the same species as commercial baker's yeast, shows up often, but so do acid-tolerant wild yeasts such as Kazachstania humilis (once called Candida milleri). Their job is fermentation: they break sugars down into carbon dioxide and a little ethanol.

The bacteria belong mostly to the lactobacilli, a group recently split into new genus names such as Fructilactobacillus sanfranciscensis (the famous San Francisco bug) and Lactiplantibacillus plantarum. They also eat sugar, but their main product is acid.

Here is the clever part: the two groups barely compete. F. sanfranciscensis loves maltose, a sugar released when flour enzymes chop up starch, while many sourdough yeasts can't use maltose at all and happily eat the glucose the bacteria leave behind. That division of labour is a big reason a healthy starter stays stable for years.

Where does the food come from if you only add flour and water?

Flour looks like dry powder, but it's a packed lunch. Wheat and rye contain their own enzymes, chiefly amylases, which wake up the moment water arrives. They snip long starch chains into maltose and glucose, the sugars microbes can actually eat.

Whole grain and rye flours carry more enzymes, more minerals and more wild microbes than white flour, which is why many bakers start a new culture with them. Once the starter is established, plain white bread flour works fine as everyday food.

Water matters too, but less than people fear. Chlorinated tap water is usually fine for an established starter. If a brand-new culture refuses to wake up, leaving water out overnight or using filtered water is a cheap thing to try.

Why does the starter rise, peak and fall?

After a feed, a starter follows a predictable curve:

1. Lag (first hour or two). Microbes adjust to the fresh food. Not much happens on the surface.
2. Growth. Yeast multiplies and pumps out carbon dioxide. The flour's proteins form a loose, stretchy network that traps the gas, so the paste domes up and fills with bubbles.
3. Peak. The starter reaches its highest point, often double its volume. It smells pleasantly yeasty and lightly sour, with a slightly domed top.
4. Collapse. Food runs low, acid builds up, and the bubble network weakens. The top flattens or sinks, leaving a streaky "high-water mark" on the glass. The smell turns sharper and more vinegary.

At around 24°C (75°F), a 1:1:1 feed (equal weights of starter, flour and water) usually peaks in 4 to 8 hours. A bigger feed such as 1:5:5 dilutes the microbes and slows everything down, which is handy if you want it to peak overnight or while you're at work. Baking with starter at or just before peak gives the strongest rise, because the yeast population is at its liveliest.

If you ignore it long enough, a layer of grey or brown liquid appears on top. That's hooch, mostly water and alcohol, and it's a hunger signal rather than a sign of death. Pour it off for a milder flavour or stir it back in for a sharper one, then feed.

What makes sourdough sour, and can you control it?

Lactic acid bacteria make two main acids, and they taste different:

Some of the bacteria are heterofermentative, which means they make lactic acid plus acetic acid, ethanol and carbon dioxide. Others are homofermentative and make mostly lactic acid. The balance between them, and how long they work, sets the flavour of your loaf.

So if you want a tangier bread, keep your starter a bit stiffer (around 50 to 60% water to flour), use it slightly past peak, or give the shaped dough a long, cold rest in the fridge. For a milder loaf, keep the starter warm and runny and use it right at peak. A healthy starter typically settles at a pH of roughly 3.5 to 4, about as acidic as orange juice or slightly more.

Chef Fizz says: Your starter isn't moody. It's running on a clock, and temperature is the hand on the dial.

That acidity does more than add flavour. It also weakens gluten over time, slows staling, and is a major reason long-fermented sourdough keeps well. Our guide to how yeast makes bread rise covers the gas side of the story in more depth.

Want to practise reading peaks and feeding ratios? The Cooking Scientist app has a quick lesson on feeding a sourdough starter, with a mini game where you time the perfect bake.

Why does a new starter bubble wildly and then go quiet?

This trips up almost everyone. On day two or three, a brand-new starter often foams up dramatically and smells odd, like cheese, socks or sour milk. Then it goes flat for several days.

The early burst usually isn't yeast. Flour carries a crowd of bacteria, including types such as Leuconostoc and some enteric bacteria, that grow fast in a neutral, sugar-rich paste and make plenty of gas. As the culture becomes more acidic, those early arrivals can't cope and die back. Acid-loving lactic acid bacteria and tolerant yeasts take over, but it takes them a few days to build their numbers.

This is ecological succession, the same process you see in a forest after a fire, played out over a week in a jar. The quiet phase is normal. Keep feeding on schedule, keep the jar warm, and the real rise usually returns between day 5 and day 10. A starter is ready for bread when it reliably doubles within 4 to 8 hours of a feed, two or three days in a row.

Why do you have to throw some starter away?

Discarding feels wasteful, but it's maths. Say you keep 100 g of starter and feed it 50 g flour and 50 g water. The next day you have 200 g, then 400 g, then 800 g. Within a week you'd need a bucket and a bag of flour a day.

Removing most of the old starter before each feed keeps the ratio of food to microbes high, so every feed is a proper meal. It also removes accumulated acid and waste, keeping the culture lively. Sourdough discard is still perfectly good to eat in pancakes, crackers or flatbreads, where you don't need it to raise anything on its own.

How do you keep a starter healthy long term?

A few habits cover almost everything:

• Feed by weight, not by cup. Flour volume varies a lot depending on how it's scooped. A kitchen scale makes your starter predictable.
• Keep it warm while it's young. Around 24 to 27°C (75 to 80°F) speeds things up. A spot near (not on) the oven or on top of the fridge often works.
• Use the fridge for slow weeks. At fridge temperatures of 4°C (40°F) or below, activity drops right off. An established starter can sit for a week or two between feeds. Give it one or two room-temperature feeds before baking to wake it up.
• Use clean jars and spoons. Not sterile, just clean.
• Know when to toss it. A healthy starter can smell sour, yeasty, fruity or even like nail-polish remover (from ethyl acetate when it's hungry). Pink, orange or red streaks or fuzzy mould are different: throw the whole thing out and start again rather than scraping it off. A strong, low pH protects starters well, but it isn't a guarantee.

Try it at home

Run a temperature race with your own starter. After a normal feed, split it into two identical small jars, each with the same amount. Put a rubber band around each jar to mark the starting level.

Leave one jar at room temperature and put the other somewhere cooler, such as a cold windowsill or a room around 18°C (64°F). Check both every hour and mark the top of the starter with a dry-erase marker.

What you'll see: the warmer jar peaks hours earlier, often in 4 to 6 hours, while the cool jar may take 8 to 12. Smell both at peak. The cool one usually smells noticeably sharper and more vinegary. That's the acetic-acid shift in action, and it shows you why bakers adjust temperature to steer both timing and flavour. Combine the two jars at the next feed and carry on.

Common mistakes

• Giving up during the quiet phase. Days 3 to 6 often look dead because the early gas-making bacteria are dying off. Keep feeding on schedule and the real yeast activity usually arrives.
• Measuring with cups. A heavy-handed scoop of flour can be 30% more than a light one, so the starter's thickness and timing drift. Weigh starter, flour and water.
• Baking with a starter long past peak. Once it has collapsed, much of the yeast is starving and the acid is high, so the dough rises slowly and tastes harsh. Feed it and use it near peak instead.
• Keeping it too cold while it's young. A new culture in a chilly kitchen can take weeks to establish. Find a spot around 24 to 27°C (75 to 80°F) until it's reliable.
• Sealing the jar airtight. Gas pressure can build up, and lids can pop. Rest the lid on loosely or use a cloth secured with a band.
• Scraping off mould and carrying on. Fuzzy mould or pink and orange streaks mean the culture has been compromised. Discard it and begin a fresh starter, ideally with some whole grain flour for a stronger launch. You can read more about how flour and gluten behave in our guide to what kneading does to dough.