A sourdough starter is a living culture of wild yeast and lactobacillus bacteria, kept alive in a jar with nothing more than flour and water. It is the engine that makes a sourdough loaf rise, the chemistry that gives it that mild, yoghurty tang, and — once you have one — a small, surprisingly characterful kitchen companion. This guide explains what's actually in the jar, what it does, and why so many of us end up giving ours a name.
A simple definition
The one-sentence version
A sourdough starter is a living culture of wild yeast and lactobacillus bacteria, kept alive in a jar of flour and water. Think of it as a small, slow-breathing ecosystem on your worktop — bubbly when it's eaten recently, sleepy when it hasn't.
Why it's called a "starter"
The etymology is dull but useful: it's what starts the rising of a sourdough loaf. Before commercial yeast existed (industrially in the 1870s, in home kitchens from the 1920s), every loaf was leavened this way — a piece of fermented dough held back from yesterday's bake, used to leaven today's. The word "starter" survives from that practice. The UK Real Bread Campaign — the food charity Sustain's standards body for genuine sourdough — still defines a "true sourdough" loaf as one leavened only with a starter, no commercial yeast added.
What it's actually made of
What is a sourdough starter made of?
A sourdough starter is made of just two ingredients: flour and water. The magic isn't in what you add — it's in what arrives. Wild yeast and lactobacillus bacteria, present naturally in the flour and floating in the air of your kitchen, colonise the mixture within a few days. A mature starter contains roughly a billion lactobacillus and ten million yeast cells per gram. That's the whole recipe.
The yeast: wild, airborne, not the Tesco sachet
The yeast in a starter is not added — it arrives. It's already on the flour, in the air, in your kitchen. The dominant strains are Saccharomyces cerevisiae (the same species as commercial baker's yeast, but a wild strain that behaves quite differently — slower, more acid-tolerant) and Candida humilis, which thrives in acidic conditions that would shut down commercial yeast. Gänzle and Vogel's review of sourdough microbiology identifies these as the workhorse species in mature wheat and rye starters.
The bacteria: meet Lactobacillus sanfranciscensis
The bacteria are what make sourdough sour. The dominant species in a mature wheat starter is Lactobacillus sanfranciscensis — first identified, as the name suggests, in San Francisco starters in the 1970s, but now known to dominate mature starters worldwide, including British ones. It produces both lactic acid (mild, yoghurty) and acetic acid (sharper, vinegary). It thrives at the acidic pH (3.5–4.0) the starter itself creates — a small chemical fortress against intruder microbes.
How many microbes are in a single gram
1,000,000,000 + 10,000,000
In a single gram of mature, well-fed sourdough starter there are roughly one billion lactobacillus bacteria and ten million wild yeast cells. The bacteria outnumber the yeast by about a hundred to one — and that ratio is what makes sourdough sourdough, rather than just slow bread.
What's actually in the jar
- Wild yeast
- Saccharomyces cerevisiae and Candida humilis — produce the CO₂ that makes the bread rise.
- Lactobacillus bacteria
- Mostly L. sanfranciscensis — produce the lactic and acetic acids that give sourdough its tang.
- Gluten matrix
- The webbed structure of hydrated wheat proteins that traps the gas and gives bread its crumb.
- Gas bubbles (CO₂)
- By-product of yeast fermentation. Bigger and more visible near the top of a peaking starter.
- Acid layer / hooch
- The thin liquid that pools on top of a hungry starter. Alcohol and water. Harmless.
- Jar wall
- Glass with a loose-fitting lid — so gas can escape and the starter can breathe.
How a starter is different from commercial yeast
How is sourdough starter different from yeast?
A sourdough starter is a community of wild yeast and bacteria, kept alive with flour and water. Commercial yeast is a single domesticated strain, Saccharomyces cerevisiae, dried into sachets. Starter is slower, more flavourful, and produces lactic acid alongside carbon dioxide. Commercial yeast is fast, neutral in flavour, and produces only carbon dioxide. One is a slow-cooked stew; the other is a microwave meal. Both make bread; only one makes sourdough.
Speed, flavour, and what each is good at
| Attribute | Sourdough starter | Commercial yeast (dried sachet) |
|---|---|---|
| Leavening speed | 8–12 hours bulk ferment | 1–2 hours bulk ferment |
| Flavour | Mildly tangy, complex, yoghurty | Neutral, faintly bready |
| Gluten effect | Partially broken down by long ferment | Largely intact |
| Digestibility | Often reported easier on the stomach | Standard |
| Microbiome effect | Live bacterial culture; small but positive evidence | None |
| Shelf life | Indefinite if fed; years if dried | ~2 years sealed; weeks once opened |
Why bakers choose one over the other
Commercial yeast is brilliant if you want a loaf in three hours and a neutral, soft sandwich crumb. Starter is the right tool if you want flavour, slow fermentation, and the texture of a proper open crumb. Neither is morally superior; both are useful. The choice is a flavour and lifestyle decision, not a virtue one.
What it actually does in a loaf of bread
What is the point of a sourdough starter?
The point of a sourdough starter is to leaven bread without using shop-bought yeast. The wild yeast inside it produces carbon dioxide, which makes the dough rise, while the lactobacillus produces lactic and acetic acid, which create sourdough's characteristic mild tang. A starter also pre-digests some of the flour's gluten and starches, which is one reason many people find sourdough easier on the stomach than supermarket bread.
Lift (the rise)
The yeast eats sugars in the flour and produces carbon dioxide. The CO₂ inflates the gluten network. That's the rise. Same mechanism as commercial yeast — different speed (8–12 hours vs 60 minutes) and different by-products (acid alongside the gas).
Flavour (the tang)
The bacteria produce lactic acid (mild) and acetic acid (sharp). Together they create the "sourdough" flavour — closer to mild yoghurt than vinegar in a well-balanced starter. The ratio between the two is largely set by temperature, which we'll come to in a moment.
Digestibility (the gut-friendly bit)
The long ferment pre-digests some of the flour's starches and breaks down some of the gluten. Many people who find shop bread heavy report finding properly fermented sourdough easier on the stomach. Tim Spector and the ZOE team have been collecting evidence on fermented foods and the gut microbiome in recent years — growing evidence, not proven medicine. We'd flag this carefully: the science isn't settled, but the lived experience of many bakers is consistent.
Why it tastes the way it does
Lactic acid vs acetic acid
Quick chemistry in plain English. Lactic acid = yoghurt, kefir, sauerkraut. Mild, slightly sweet. Acetic acid = vinegar. Sharp, pungent. A starter at a warmer temperature (24–28°C) produces more lactic acid (mild). A starter at a cooler temperature (16–20°C) — or kept in the fridge — produces more acetic acid (sharper). Most home kitchens land somewhere in the middle.
Britain's mild climate and a balanced ratio
The British kitchen, sitting most of the year between 16°C and 22°C, naturally produces a balanced lactic-to-acetic ratio. This is one reason many UK sourdough loaves have a softer, more rounded tang than the sharper San Francisco style or the more aggressive bakes of warmer Mediterranean countries. The climate is, quietly, doing some of the work for you. Vanessa Kimbell's UK fermentation tables in The Sourdough School map this out in more detail.
How old a starter can get
How old is the oldest sourdough starter?
The oldest documented working sourdough starter is held by a bakery in Belgium that traces its culture back over 130 years. Several UK artisan bakeries, including members of the Real Bread Campaign, maintain starters that are 40 to 80 years old. A starter can, in principle, live indefinitely — as long as it is fed regularly, kept clean, and protected from extreme heat. Many home bakers pass theirs down across generations.
Working starters that are decades old
The Puratos Sourdough Library in St Vith, Belgium, holds the world's oldest documented continuously-fed culture — a starter from a Belgian bakery dating to the 1890s. Closer to home, several UK Real Bread Campaign bakeries maintain starters of 40 to 80 years old. It's worth pausing on what that actually means: not the same yeast cells, of course (they replicate every few hours), but the same culture — an unbroken genetic line carried forward through endless feedings.
The Real Bread Campaign and the UK "true sourdough" tradition
The Real Bread Campaign, run by the food charity Sustain, defines a "true sourdough loaf" as one leavened only by a live culture — no commercial yeast added. Their Sourdough Loaf Mark certifies UK bakeries that follow the definition. It's a quiet but important part of the UK sourdough story, and it traces straight back to the kind of starter sitting in your jar.
Clara's own starter is called Gerald. He's eight years old, lives in a glass Kilner jar on the windowsill above the kitchen sink in Somerset, and has been on holiday with us exactly once. We don't recommend it.
How a starter is stored
Counter-top: daily feeding
Room temperature, fed once a day, ready to bake within 4–12 hours of a feed. This is the right choice if you bake regularly — twice a week or more. The full feeding schedule covers ratios and timing.
Fridge: weekly feeding
Around 4°C, fed once a week, takes a day or two on the counter to "wake up" before the next bake. The right choice for most home bakers. The cold suppresses the yeast but the lactobacilli keep ticking over, so the starter develops a slightly sharper, more acetic flavour over time.
Dried or frozen: long pause
Smear thin, air-dry over 24–48 hours, store in an airtight pouch — keeps for years. Frozen lumps keep for months. Both wake up with a few days of feeding once you're ready. This is how we ship our kits — and how Gerald survived his one holiday.
How a starter dies (and how to tell)
Mould, pink streaks, and the bin
Fuzzy mould (white, black, blue-green) on the surface = bin it. Pink or orange streaks running through the starter = bin it (that's Serratia, a bacterium you don't want). Both are signs that something other than your lactobacillus and yeast has taken hold, and neither is safe to bake with. Wash the jar with very hot water and washing-up liquid, and start again. Rare, but real.
"Hooch" is not death — it's hunger
The grey liquid that sometimes pools on top of a hungry starter is called hooch. It's alcohol — produced by the yeast when it has run out of food. Smells boozy. Looks alarming. Is harmless. Pour it off, or stir it in if you want a sharper flavour, then feed as normal. This is one of the most-Googled "is my starter dead" panics — and the answer is almost always no, it's just hungry.
What "fed" and "hungry" actually mean
A fed starter, in pictures
Bubbly, domed, almost doubled in size. Smells of ripe yoghurt with a faint sweet edge. Light when stirred — feels almost mousse-like. This is the moment to bake.
A hungry starter, in pictures
Is sourdough starter alive?
Yes — a sourdough starter is genuinely alive. It is a culture of wild yeast and lactobacillus bacteria, and like any living thing it needs feeding, warmth, and the occasional clear-out. A healthy starter doubles in size within hours of being fed, smells faintly of ripe yoghurt, and is visibly bubbly. Neglect it and it slows down; freeze or dry it and it pauses; feed it and it wakes back up.
A hungry starter looks flat, deflated, possibly with hooch on top. Smells boozy or sharply vinegary. Not dead — just due a feed. If yours has gone quiet, our guide to why a starter sometimes goes quiet walks through the ten most common causes.
What's next
If you've followed this far, you know what a starter is, what's in it, and what it does. The next step depends on where you are: build one from flour and tap water (we'll make your own from scratch in 7–14 days); read the bigger picture in our complete UK guide to sourdough starters; or learn the rhythm of feeding in the dedicated feeding schedule article.
If you'd rather skip the 10-day grow-from-scratch and start baking this weekend, one of our five starter kits ships a mature, lively starter ready to feed once and bake with the day after.
Frequently asked questions
What exactly is a sourdough starter?
A sourdough starter is a living culture of wild yeast and lactobacillus bacteria, kept alive in a jar with flour and water. It is what makes a sourdough loaf rise without any shop-bought yeast, and what gives it that mild, yoghurty tang.
Is a sourdough starter actually alive?
Yes. A starter is a real, living microbial culture — billions of bacteria and millions of wild yeast cells per gram. It eats (flour), produces waste (carbon dioxide and acid), and reproduces. Feed it regularly and it will live indefinitely.
How is a sourdough starter different from baker's yeast?
A starter is a community of wild yeast and bacteria; baker's yeast is a single domesticated strain (Saccharomyces cerevisiae) dried into a sachet. The starter is slower, more flavourful, and produces lactic acid as well as carbon dioxide. The sachet is faster and flavour-neutral.
How old can a sourdough starter get?
The oldest documented working starter is over 130 years old, held in a Belgian sourdough library. Several UK artisan bakeries, including Real Bread Campaign members, maintain starters 40–80 years old. A well-cared-for starter can in principle live indefinitely.
What makes a sourdough starter sour?
Lactic acid and acetic acid, produced by the Lactobacillus bacteria in the starter. Lactic acid is mild and yoghurty; acetic acid is sharper and more vinegary. The balance shifts with temperature — warmer kitchens make milder starters, cooler kitchens make sharper ones.
Can a sourdough starter die?
Yes — though less easily than people fear. The clearest signs of a dead-or-dangerous starter are coloured mould (fuzzy white, black, or blue-green) or pink/orange streaks. Bin it, wash the jar with very hot water, and start fresh. A flat, sleepy, slightly boozy-smelling starter is just hungry, not dead.
Has sourdough always been baked in the UK?
Yes. Until commercial bakers' yeast became widely available in the late 1800s, almost all British bread was made with a sourdough culture in one form or another. The Real Bread Campaign still defines a "true sourdough loaf" as one leavened only by a live starter — a direct line back to that older tradition.
