How to Make a Sourdough Starter From Scratch

A sourdough starter is a mix of flour and water that's colonized by wild yeast and lactic acid bacteria. You make one by combining equal weights of flour and water, then discarding part and feeding it fresh flour and water every day until it doubles reliably after each feeding, which usually takes 7 to 14 days.

What's living in the jar

Flour isn't sterile. It carries spores and cells of wild yeasts and bacteria from the grain, the mill and your kitchen. Add water and they wake up and start eating the sugars in the flour.

Over the first two weeks, a fermentation succession plays out:

1. Days 1 to 3: the false start. Fast-growing bacteria multiply first and produce lots of gas. The starter may double on day 2 and smell odd, a bit like cheese or old socks. This isn't yeast yet.
2. Days 3 to 6: the quiet phase. These early bacteria make the jar more acidic, and the acidity kills them off. Activity drops, sometimes to nothing. This is where most people give up.
3. Days 6 to 14: the takeover. Acid-tolerant lactic acid bacteria (mostly Lactobacillus-type species) and wild yeast take over. The bacteria make lactic and acetic acid, which give sourdough its tang. The yeast makes carbon dioxide and alcohol, which make bread rise.

A mature starter has a pH of about 3.5 to 4. That acidity is what protects it from harmful microbes. See pH in cooking for more.

Choosing flour and water

Flour. Start with whole wheat or whole rye. The bran and germ carry more microbes and more nutrients than white flour, so the starter wakes up faster. Once it's established, you can switch to unbleached all-purpose or bread flour, or keep a blend.

Water. Most tap water is fine. If yours is heavily chlorinated, leave it out uncovered overnight or use filtered water. (Chloramine, used by some cities, doesn't evaporate; filter it out instead.)

Ratio. Equal weights of flour and water is called 100 percent hydration. It's the easiest to read and the most common. Use a scale; flour volume varies too much for this.

Steps

1. Mix 50 g whole wheat or rye flour with 50 g water at about 25 °C / 77 °F in a clean jar.
2. Cover loosely and leave at warm room temperature, ideally 24 to 26 °C / 75 to 78 °F, for 24 hours.
3. On day 2, add 50 g flour and 50 g water, stir well and wait another 24 hours.
4. From day 3, discard all but 50 g of starter each day, then feed it 50 g flour and 50 g water.
5. Mark the level with a rubber band after each feeding to track the rise.
6. Keep feeding daily through the quiet phase on days 4 to 6 without giving up.
7. Switch to twice-daily feedings once it starts rising noticeably.
8. Use it once it doubles within 4 to 8 hours of feeding for three days in a row and smells pleasantly sour.

Chef Fizz says: Name your starter. It sounds silly, but you'll be much less likely to forget to feed something called Bubbles.

Why you discard

Each feeding works like a refill. If you kept everything and only added 50 g of flour each day, the jar would soon hold a kilogram of acidic starter with too little fresh food per microbe. Discarding keeps the population at a manageable size and the food-to-starter ratio high, so every feeding brings a real boost.

The 1:1:1 feeding (equal weights of starter, flour and water) is a good default. If it peaks too fast in a warm kitchen, feed at 1:2:2 or 1:5:5 to slow it down.

How to tell it's ready

• It doubles or more within 4 to 8 hours of a feeding at 24 to 26 °C.
• It's domed and full of bubbles at its peak, with a web-like structure when you pull a spoon through it.
• It smells tangy and yeasty, like yogurt or beer, not like nail polish or cheese.
• It does this predictably for several days.

The float test (dropping a spoonful in water to see if it floats) is popular but unreliable, especially with wetter or whole grain starters. Trust the rise.

Keeping it going

On the counter. Feed once or twice a day at 1:1:1. Best if you bake several times a week.

In the fridge. Feed it, let it sit 1 to 2 hours, then refrigerate. Feed once a week. Take it out and give it one or two room-temperature feedings before baking. Cold slows fermentation, so a fridge starter can survive weeks of neglect.

Long breaks. Smear a thin layer on parchment, let it dry completely, crumble and store in a sealed jar. Revive with flour and water over a few days.

Common problems and fixes

No activity after day 3. Usually the quiet phase. Keep feeding. If it's still flat after day 10, move it somewhere warmer; temperature is the biggest factor.

Hooch on top. A grey or brown liquid means it's hungry. Feed more often or use more flour.

Acetone or vinegar smell. Also hunger. One or two regular feedings fix it.

Runny and won't hold bubbles. Common with white flour or very warm conditions. Use a little less water, or add some whole grain flour.

Mould, or pink or orange streaks. Throw it out and start over with a clean jar. Don't try to scrape it off.