How to Ferment Sourdough Properly

One of the key concepts to master for baking great sourdough bread is proper fermentation: This starts with a strong starter and continues through to proofing and baking.

The levain used for sourdough bread rises in a glass jar, about triple its starting volume.

Serious Eats / Tim Chin

So far in our series on sourdough bread, we've discussed the science of sourdough, explained how to get a starter up and running, compared starters made from different flours, and offered a solid recipe for baking a sourdough loaf. But if you want to master this craft, it helps to understand two foundational concepts in more detail: proper fermentation and proper dough handling. As the sourdough luminary Trevor Wilson writes in Open Crumb Mastery, 80 percent of an open crumb comes from those two concepts alone. Here we’ll talk about fermentation and how you can apply it to your sourdough baking.

We’ve already covered and defined fermentation as a general concept in sourdough starters: Microorganisms (yeast and lactic acid bacteria) break down starches to produce carbon dioxide gas and sour flavors. For breads, proper fermentation maximizes the potential for effective expansion and trapping of gases in a well-developed gluten matrix. It is also the key to developing flavor in your bread. We can break fermentation down further into two parts: 1) making a strong and active starter and levain and 2) ensuring proper bulk fermentation.

Making a Strong and Active Starter and Levain

Up front, we need to make the distinction between a starter and a levain, since they're similar enough to cause confusion. In short, a starter is the home base of your microbial culture that you maintain indefinitely, whereas the levain is starter that is destined to be mixed into a bread dough (where it will leaven bread and then perish in the heat of the oven). Think of it this way: A starter is like your savings account, a precious microbial resource that you foster and care for daily—you may not need it right away, but you always want to maintain a balance for a day when you do (i.e., a day when you decide to make bread). When you want to bake bread, you borrow some of that culture and move it to your checking account: The levain, which, when mature, will be ready to raise bread.

Some bakers build their levain with their starter and then reserve a portion of that levain to continue the starter culture (this would be like transferring the entire balance of your savings account into your checking account to cover any needed expenses, then transferring the unused portion back to your savings account after). Others are more judicious and like to keep a separate starter constantly running even when a levain is being built (this would be like transferring only as much money as you know you'll need into checking while keeping the rest of your funds in your savings account). While the difference between a starter and a levain may be slight, it’s still a useful concept toward understanding the overall baking process.

Starter Strength

An active sourdough starter in a glass jar: small air bubbles are visible and the starter is roughly three times higher than where it started, as marked by a rubber band.

Serious Eats / Tim Chin

Good sourdough bread hinges on having a robust, active starter. If you’ve been following along with our guide to making a sourdough starter, you should have a culture that rises, peaks, and falls predictably. It should triple or even quadruple in volume as it peaks.

While some bakers use starters that only double in volume at peak activity, I’ll tell you straight up: Don’t settle. A starter that doubles in volume will work, but the overall fermentation and rising of dough might be slower, and the resulting bread potentially flatter. Instead, look for signs of tripling, or even quadrupling in volume. A more vigorous and active starter improves the chances of producing bread that is tall, light, and open-crumbed. In other words, the stronger your starter is the happier you will be with your final loaf. And conversely, if your starter sucks, then your bread will suck.

Your starter should not only show signs of strong activity but also have a relatively thick viscosity. Bakers add starter in amounts upward of 20 percent of the total flour weight in a dough, so you don't want an overly thin starter going into the mix. Putting aside hydration levels (the amount of water relative to flour), the thickness of a starter is largely determined by its maturity. A starter is a pre-fermented mix of flour and water, and over time, acids build up in that mixture; at first, the texture may be stiff, but as a starter matures, the buildup of acids coupled with enzymatic activity begin to break down gluten, resulting in a runnier viscosity. So depending on timing, this mixture can have a texture ranging from relatively stiff to runny, or even soupy. Using a starter at peak activity—before the mixture collapses and thins—is a helpful way to ensure that you aren’t adding excessively proteolytic, gluten-degraded soup to your dough.

Levain and Maturity

Collage of before and after mixing flour, water, and starter to create a levain for sourdough bread
Before and after: A levain grows to more than triple its original volume.

Serious Eats / Tim Chin

The levain is whatever portion of mature starter you use to mix into your dough. This distinction between a starter and a levain is slight, but bakers refer to “building” a levain prior to mixing, a concept which underscores both the scaling necessary to turn a relatively small stash of starter into a large enough quantity to leaven one or more loaves of bread, as well as the scheduling required to pull it all off, so that the bread is ready to be baked at the desired time. For instance, you may keep a relatively small total starter amount (50 grams or less) on a day-to-day basis. But 50 grams of starter won’t take you very far—doubly so if you intend on baking multiple loaves. To produce enough starter for baking, we take some portion of the original culture (say, 30 grams) and feed it at least 1:1:1 (starter to flour to water), which gives 90 grams of mature starter to work with. At this feeding ratio, it takes about four to five hours for the levain to reach peak activity at 78-80°F.*

*In addition to scaling up the amount of available starter, using a levain allows you to customize its composition for specific bread doughs. For example, if you want to bake a whole wheat bread, you might build your levain with whole wheat flour instead of whatever flour or flour blend you typically use to feed your starter.

But what if waiting four or five hours for the levain to peak isn't going to work with your schedule? Here you can manipulate variables like the feeding ratio (starter to flour to water) and temperature to adjust the timing of mixing. For example, you can build your levain the night before mixing: By lowering the temperature and increasing the feeding ratio, the yeast and bacteria have a much larger reservoir of food to metabolize before the culture peaks, resulting in much slower fermentation. “Right before I go to bed, I’ll feed 1:10:10,” says sourdough expert Kristen Dennis. “So a massive ratio switch. I put it in a pretty cool spot, in my dining room (about 72[°F] overnight). And I’ll let it go very slowly, and by morning I have just enough time to do a quick autolyse and throw the levain on. [The levain] peaks probably about 10 or 11 hours later at that ratio.”

Finally, bakers make the distinction between using a young and mature levain. Degree of maturity refers to the point at which you choose to incorporate your levain into a dough. A young levain is used at or just before peak activity; it smells mild, faintly sour, and has a thick consistency. A mature levain smells more acidic and pungent, and it might be runnier in texture. In general, I opt for using a young levain because it doesn’t compromise gluten structure, facilitating a tall loaf. According to Kristen, young levains are a bit more “foolproof” for beginning bakers. Why would you bother to use a mature levain? A mature levain contains more acid than a young levain, resulting in a sharper, more pungent flavor; a young levain, on the other hand, results in loaves with milder flavor. If you’re a fan of funkier, tangier flavors in your bread, then a mature levain is the way to go: It’s teeming with fermentative byproducts (alcohols, ketones, aldehydes, and other volatiles) that contribute to a more robust flavor. Greater “acid load” from a mature starter also lowers the pH of your dough, making it more elastic and less extensible early on. But in the long term, acidic conditions tend to degrade gluten, making a dough slack, weak, and, in the worst cases, a soupy mess. Given these characteristics, mature levains are better suited to lower hydration doughs. But there’s no hard-and-fast rule.

Ensuring Proper Bulk Fermentation (or Bulk Proof)

A flour-and-water dough sits in a metal mixing bowl before the levain is mixed in.

Serious Eats / Tim Chin

According to Trevor, bulk fermentation starts as soon as you add levain to a dough and ends when you divide or shape the dough. This stage is usually the longest period of fermentation, in which the majority of your dough’s flavor develops, gases accumulate in the dough, and—crucially—the gluten structure fully develops to trap those gases effectively.* In general, the majority of a dough’s fermentation occurs during bulk fermentation. Subsequent steps like the final rise and retarding represent a small fraction of the overall microbial fermentation and gluten development.

*This idea assumes that you haven’t fully developed the gluten initially through machine mixing or aggressive, prolonged kneading. Instead, bakers often rely on time to fully develop a gluten matrix. Over time, enzymes, such as proteases, begin snipping strands of gluten into smaller pieces that are able to bond with one another. Chains of gluten elongate, and the network strengthens as more and more molecules stick together.

Why is it called “bulk”? The answer is primarily a matter of scale. In a high-volume bakery setting, bakers mix enormous amounts of dough in order to produce dozens of loaves. Handling individual portions of dough over the lifetime of the baking cycle is inefficient, time-consuming, and takes up too much space in the kitchen. Instead, bakers let an entire mass of dough ferment before dividing and shaping into individual loves. This process ensures consistent and predictable fermentation for several reasons: Chiefly, a larger mass of dough is more temperature-stable, and the baker only has to pay attention to one mass of dough as opposed to several dozen at a time. For home bakers baking one loaf at a time, bulk fermentation in this manner is not necessary. But even if you’re scaling up a batch of dough to bake two or more of the same loaf, it can be helpful to bulk en masse prior to dividing and shaping.

Some bakers refer to this stage as the ‘proof’ stage. Others, like Trevor Wilson, distinguish bulk fermentation from bulk proof. Bulk fermentation starts when the levain is mixed into the dough (which can occur before the dough is fully mixed); the proof stage starts when all ingredients (such as salt) are incorporated. Proof highlights the point at which the dough begins to rise—underscoring the transition from aggressive kneading or mixing to more gentle dough handling and folding as the dough slowly rises.

The raw dough for Sourdough bread sits in a square glass baking dish; it looks soft and tender with some visible pockets of air under the surface

Serious Eats / Tim Chin

Proper bulk fermentation means giving dough enough time (at a given temperature) to ferment. Under- or over-proofing leads to suboptimal results: An under-proofed dough can be dense and have an uneven crumb while an over-proofed dough can be hard to handle and feature an overly tight crumb structure.

How do you know when bulk fermentation is complete? It’s difficult to answer definitively. Remember, you’re going to be baking your bread in a ripping hot oven. That blast of heat encourages rapid metabolism of starches—a feeding frenzy—and expansion of the dough. If you vastly overproof your dough, those trapped gas bubbles coalesce into a tight network and stick to each other, resulting in an overly tight crumb structure; at worst, the dough collapses. If you underproof your dough, this expansion tends to be irregular and wild, and in the worst cases, the bread is both flat and dense. Generally, Kristen aims for a 60 percent increase in size. But a 60 percent increase is difficult to track visually. When you factor in folding and de-gassing of the dough, visual identification becomes even harder. Instead, she recommends bulk fermenting for a set amount of time, and adjusting that time as needed over successive bakes.*

*If you want to be more precise, there is a way to track 60 percent rise. Kristen employs a small jar and feeds a small amount of starter into that jar as soon as she incorporates the levain into her dough. Left undisturbed at the same fermenting temperature as her dough, this “aliquot jar” provides a reasonably reliable method to measure a 60 percent rise in volume.

“New bakers aren’t going to know what to look for,” says Kristen. “Say I had the same starter, I had the same temperature, and I bulked for five hours. [The loaf] didn’t turn out how I wanted it to. Next time, I bulked for six hours, and now it looks a little better. Same thing for seven hours. It’s not something you can do over one bake. It has to be done over many bakes, [using] side-by-side comparisons.”

Still, it’s helpful to provide some visual cues. A properly “bulked” dough will be slightly rounded at the top and edges; it will be bubbly on the surface and jiggle slightly when shaken; finally, it should release from the bowl or container relatively easily before shaping. Some bakers use the “finger poke test” to roughly determine level of proof: If you poke a dough, and the indentation springs back quickly or too fast, the dough is under-proofed; if the indentation stays or doesn’t change at all, the dough is over-proofed; and if the indentation slowly springs back to its original shape, the dough is just right.

The Effect of Temperature on Bulk Fermentation

Temperature has a profound effect on the rate and quality of fermentation. In our sourdough starter guide, we even broke down the complex relationship between temperature and development of differing strains of yeast and lactic acid bacteria. You can bulk ferment at practically any temperature between 65°F and 90°F with acceptable results. But in general, a lower fermenting temperature will prolong bulk fermentation while a higher temperature speeds up bulk fermentation. The trick is to find a temperature at which your dough rises sufficiently, but gluten also develops enough for the dough to hold shape. Typically, the “goldilocks” temperature zone lies somewhere between 72 and 78°F—not so slow to impede rise or for gluten to break down but not so fast that gluten is underdeveloped by the time bulk fermentation is complete.*

*Again, there are always exceptions in sourdough baking. For example, a machine-mixed dough lends itself to bulk fermentation at high temperatures. Machine mixing develops gluten fully up front, so you don’t need to rely on time to develop a strong dough that holds its shape.

It Takes Two

Proper fermentation will take you a long way toward baking good sourdough bread. But even if you nail every aspect of it—even if your starter is popping off and you’ve bulked your dough sufficiently—you still have to touch and handle your dough. From a baker’s perspective, fermentation is passive: It depends on calculated judgments of time and temperature. Dough handling is active: It involves physical actions that alter the physical characteristics of your dough. And somewhere, at the intersection of these two concepts, you can produce amazing sourdough bread. In our next post, we’ll lay out several of those strategies for handling your dough.

July 2020