What You Are Actually Making

A sourdough starter is nothing more than flour and water left to get on with it, but the transformation is remarkable. Within a few days, wild yeasts and friendly bacteria that live on the surface of the flour and in the air around your kitchen begin to colonise the mixture. The yeasts produce carbon dioxide, which lifts your bread, while the bacteria create lactic and acetic acids, which give sourdough its distinctive tang. You are, in effect, farming a tiny ecosystem on your worktop.

The process takes between five and ten days from a standing start. It is not difficult, but it is not instant either. The main ingredients are patience and a regular routine; the flour and water are almost incidental.

What You Will Need

• A clean glass jar holding about a litre, ideally with a clip lid or a screw top you can leave loose. A large jam jar works perfectly well.
• Digital scales accurate to a gram. Eyeballing flour and water leads to a starter that is too wet or too stiff, and consistency matters enormously.
• Wholemeal or rye flour to begin with. Stoneground wholegrain flours carry more wild yeast and minerals, so they ferment faster than white flour alone.
• Strong white bread flour for later feedings, once things are underway.
• Warm water at roughly 28–30°C. If your tap water is heavily chlorinated, leave it in a jug overnight or use filtered water.
• A spatula and a tea towel, plus somewhere warm to keep the jar.

You do not need a proving box, a special jar or any expensive kit. A consistently warm corner of the kitchen, somewhere between 21°C and 24°C and away from draughts, is the single most useful thing you can provide.

The First Week, Day by Day

Day one. Weigh 50g of wholemeal flour and 50g of warm water into your jar. Stir vigorously with the spatula until you have a thick, sticky batter with no dry patches. Scrape down the sides, cover loosely and leave it be.

Day two. You may see a few bubbles, or nothing at all. Discard half the mixture — about 50g — and add a fresh 50g of flour and 50g of water. Stir, cover, leave.

Days three and four. This is when things get interesting, and often unpleasant. The starter may froth up dramatically and then smell of old socks, cheese or nail varnish. This is a normal bacterial scrap before the yeasts take charge. Keep discarding and feeding once a day and push through it.

Days five to seven. The smell settles into something yeasty, faintly sour and rather pleasant, like beer or yoghurt. The starter should rise after each feed, dome slightly, then fall back. If it is rising vigorously, feed it twice a day instead.

Days eight to ten. Most starters are ready to bake with by now. If yours is still sluggish, do not despair — cool kitchens slow everything down. Give it another few days of consistent feeding.

Reading the Signs

A starter is ready when it reliably doubles in volume within four to eight hours of feeding. Look for a domed, slightly glossy top, a network of bubbles through the mixture, and a spongy, springy texture when you dip a spoon in. The aroma should be clean and tangy, never sharp enough to make you wince.

The old float test — dropping a teaspoonful into a glass of water to see if it floats — is a reasonable indicator, though not infallible. A dark liquid layer on top, known as hooch, simply means the starter is hungry. Stir it back in or pour it off, then feed as normal. It is a sign of neglect, not death.

Keeping It Alive

Once established, a starter needs feeding roughly once a week if you keep it in the fridge, where the cold slows fermentation to a crawl. Take it out the day before you plan to bake, pour off most of it, and feed it at least twice at room temperature to wake it up properly.

A typical feed is equal weights of starter, flour and water, though you can use a larger ratio — one part starter to five parts flour and water — to build a milder, less sour flavour with a longer gap between feeds. If you keep it on the worktop, feed it daily. If you bake rarely, dry a thin layer of starter on greaseproof paper, crumble the flakes into a jar and store them in a cool cupboard as an insurance policy.

When Things Go Wrong

• No activity after five days: move it somewhere warmer, switch to rye flour for a few feeds, and check your water is not heavily treated.
• Pink, orange or fuzzy patches: this is mould, and it cannot be salvaged. Bin the whole lot, sterilise the jar and start again.
• A strong acetone or pear-drop smell: the starter is starving. Discard more and feed more often for a few days.
• A sulphur or eggy smell early on: entirely normal during the first week. Carry on.
• Bread that tastes aggressively sour: shorten the time between feeds and keep the starter warmer, which encourages gentler lactic acidity over the harsher acetic kind.

Be patient, be consistent, and within a fortnight you will have a jar of living leaven that improves with every loaf you bake.