Making a sourdough starter from scratch sounds like something that requires a chemistry degree. It does not. You need flour, water, a jar, and about a week of patience. The process is simple even if the science behind it is fascinating. Wild yeast and bacteria that are naturally present in flour and in the air colonize the mixture, creating a living culture that leavens bread without commercial yeast.
What You Need
A clean glass jar or container, at least one quart in size.
A kitchen scale for consistent measurements. All-purpose flour or whole wheat flour. Filtered or bottled water at room temperature. A rubber band to mark the level. Avoid using bleached flour for the initial creation phase.
Day 1: Getting Started
Combine 50 grams of whole wheat flour and 50 grams of room temperature water in your jar. Stir vigorously until there are no dry spots. The mixture should look like thick pancake batter.
Mark the level on the outside of the jar. Cover loosely and place somewhere at room temperature, ideally between 70 and 78 degrees Fahrenheit.
Days 2 Through 4: The Waiting Game
Every 24 hours, discard about half of the mixture and feed it with 50 grams of flour and 50 grams of water. During these early days, you will likely see some bubbling activity around day two. This is not your starter becoming active.
It is a different type of bacteria called leuconostoc that produces gas during the initial phase. This activity often peaks and then dies off completely. Do not worry. This is completely normal. Keep feeding on schedule.
Days 5 Through 7: Signs of Life
By the end of the first week, you should start seeing consistent bubbling after each feeding. The mixture will rise noticeably between feedings, sometimes doubling in size.
The smell should shift from funky and sharp to pleasantly tangy and slightly yeasty. When your starter reliably doubles in size within 4 to 8 hours of feeding, it is strong enough to leaven bread.
The Float Test
Drop a small spoonful of starter into a glass of room temperature water. If it floats, the starter has enough gas trapped inside to leaven dough. Perform the test when your starter is at its peak, which is the point of maximum rise after feeding.
Ongoing Feeding Schedule
For room temperature maintenance, discard all but 50 grams of starter and feed with 50 grams flour and 50 grams water every 12 to 24 hours.
For refrigerator storage, feed the starter, let it sit at room temperature for an hour, then refrigerate. The cold slows fermentation to a crawl. When you want to bake, pull it out, warm it up, and give it one or two feedings until bubbly and active.
Troubleshooting
If your starter is not rising, the most common reason is temperature. If your kitchen is below 68 degrees, try moving the jar to a warmer spot.
On top of the refrigerator or inside the oven with just the light on can provide the extra warmth needed.
A layer of dark liquid forming on top is called hooch. It signals that your starter is hungry. Pour it off or stir it back in and feed immediately. Fuzzy spots of pink, green, black, or white mold mean the starter has been contaminated and should be discarded. Start over with a clean jar.
Your First Bake
Once your starter is reliably doubling after each feeding, you are ready.
Combine 350 grams of bread flour, 250 grams of water, 75 grams of active starter, and 8 grams of salt. Mix until a shaggy dough forms, then let it rest for 30 minutes. Perform a series of stretch and folds every 30 minutes for the next two hours, then let the dough bulk ferment until it has grown by about 50 percent.
Shape the dough into a round, place it in a floured banneton, and refrigerate overnight.
The next morning, preheat your oven to 500 degrees with a Dutch oven inside. Score the top of the dough, drop it into the hot Dutch oven, cover, and bake for 20 minutes. Remove the lid and bake for another 20 to 25 minutes until deeply golden brown. Let the loaf cool completely before cutting. The interior structure sets as it cools, and cutting too early gives you gummy bread.