For the first few millennia of bread's existence, it was flat. The dough was made of stone-ground grains and seeds, mixed into a porridge, and baked on a stone heated by fire. Historians disagree about when exactly humans began to eat leavened bread, but the first use of wild yeasts in bread dough likely occurred in Egypt and the Levant around 4000 years ago. The Greeks began using "pre-ferments" or bits of dough leftover from the previous day to inoculate and ferment each new dough batch.
A sourdough starter is built on this same concept; it is essentially a jar of dough that is continually fed and cultured to kick-start the fermentation of each new loaf. Much like the SCOBY (symbiotic culture of bacteria and yeast) or "mother" that turns sugar and tea into bubbling kombucha, a sourdough starter turns flour, water, and salt into a fluffy loaf of bread.
What Happens During Fermentation The process of fermentation begins when tepid water and flour are mixed. Yeast, lactic acid bacteria, and other microbes in the air, flour, and water start consuming sugars and starches, creating carbon dioxide in the process. Lactic acid bacteria create the signature sour taste of sourdough as they digest carbohydrates in the flour and produce lactic acid as a byproduct. The production of lactic acid also creates an environment within the starter or bread dough that is inhospitable to harmful bacteria.
The role of yeast is essentially to eat, fart, and die. The gas produced by the digestion of sugars and the subsequent death of yeast cells creates bubbles trapped within the dough. As the dough is kneaded, the dough's gluten structure becomes stronger, and more gas is trapped. Gluten is the combination of two proteins—glutenin and gliadin—and only occurs when flour encounters water. The more water there is in bread dough, the stronger the gluten structure. It is possible that many of the gluten-related health problems people deal with today are a product of both the industrialized growing process of modern wheat as well as the decreased fermentation time of much of the bread we eat. Long fermentations break down gluten structure, making sourdough breads and pastries easier for the body to digest.
When the dough is cooked, the remaining yeast cells die as the internal temperature of the dough reaches 140 degrees Fahrenheit. The resulting gas is trapped in the dough's gluten structure and builds the loaf's final shape.
Wild Yeast vs. Commercial Yeast Wild yeast is everywhere. You can see it on the skins of grapes and plums, and it's the reason your dog's paws smell like corn chips. Wild yeast leavened those first loaves of bread thousands of years ago. It is also what makes your sourdough starter bubble as it sits on the counter in your warm kitchen. Wild yeast, along with the microbes essential for fermentation, live on every surface of your home and body. Because of this, it is best to refrain from using bleach or anti-microbial soaps in the kitchen if you want to build a robust and biodiverse sourdough starter.
Baker's yeast, which you can find in jars or packets at the grocery store, is a specific strain of yeast called saccharomyces cerevisiae. This yeast strain also occurs in the wild but has been singled out for particular use in commercial baked goods and bread that need to be leavened quickly. Pre-packaged, commercial baker's yeast gained popularity after World War II when the granulated, temperature tolerant yeast developed for use by the US Army became widely available to the public. Granulated or caked baker's yeast can be used in everything from quick-rise breads to long-ferment sourdoughs.
Different Types of Starters A traditional sourdough starter is a mixture of bread and water usually fed once daily with a smaller mixture of flour and water. The ratio of flour to water in both the starter and the feeding mix will vary depending on the type of flour used and the type of bread made from it. A rye starter, for instance, will have a much higher ratio of water to flour because rye absorbs so much water. Different bread dough formulas call for different hydration percentages, and the proportions of the starter should reflect that. Flour-to-water ratios can be changed a few days prior to baking to match the final dough's formula. The part of the sourdough starter used in a bread formula is often referred to as the levain.
Pre-ferments, on the other hand, are usually made with baker's yeast and are used for very specific purposes. Several culinary traditions across the globe have pre-ferments for each type of bread that is being baked.
In France, a pâte fermentée, translated as "old dough,” is a piece of dough reserved from the previous day's bake and added to the next day's dough. However, it’s also possible to make a pâte fermentée in just 12 hours using flour, water, salt, and commercial yeast.
A Poolish is a 50/50 mix of water and flour with just a tiny amount of yeast added (about 1% of the total mixture). The Poolish mixture will be batter-like due to the high hydration. Initially used for pastry production in Poland, it is now used as a pre-ferment for many high-hydration bread doughs.
A Biga is an Italian pre-ferment, usually used in ciabatta and pizza doughs. Bigas can range anywhere from 50% hydration to 100% hydration like a Poolish.
How to Build and Use a Starter Building a sourdough starter is much simpler than one might imagine. Remember that people have been building and maintaining sourdough starters for thousands of years, most of that time without the precision of scales or thermometers. Keep in mind that your sourdough starter will be unique to the microbes and yeasts living in your kitchen, your water, and even on your hands. Using freshly-milled flour is best, as it contains the whole wheat berry as well as the most active microbes, both of which factor into robust sourdough fermentation.
To build the starter, combine 100 grams of white bread flour and 100 grams of whole wheat flour. You may even want to add a few grams of rye flour to the mix as it usually retains the most fiber and nutrients of any flour, making it perfect for microbes and yeasts to snack on. Then, with your hand, mix the flour with 100 grams of warm water. Cover the bowl or jar with a cloth held in place by a rubber band or string. After two days, remove about 80% of the mixture (throw it away or compost it) and add 25 grams of white flour, 25 grams of wheat, and 50 grams of water. Mix it and leave it alone. Leave the starter in a warm place, out of direct sunlight. Continue this mixing process every day for ten days or until the starter is consistently bubbly and smells pleasantly sour. You can now use the starter for baking sourdough.
Once you have a basic recipe, you can experiment with different hydration ratios and types of flour. You might not always make the perfect loaf, but even a flat, dense, over-proofed bread loaf makes the house smell nice.