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The Science of Sourdough Starters

These are wild times. Many are stuck at home, locked into a relentless repetition of time and place in which weekends mean nothing, and distance means everything. The crisis outside our windows and walls rages on. And it seems that everyone, from professional bakers suddenly out of work to first-time dabblers, is making sourdough bread. My Instagram feed has become an endless flood of blistered, perfectly imperfect boules and batards, and is peppered with snapshots of fledgling baby starters bubbling away.

When using hand sanitizer and fanatical handwashing have become ethical and civic responsibilities, cultivating a starter seems to fly in the face of what we’re supposed to be doing right now. And yet here we are, baking up a collective storm and comparing crumb shots on social media. Making sourdough also taps into that primal drive to survive and be self-sufficient in times of duress: While murder hornets and a deadly pandemic threaten your very existence, at least you can bake a nice loaf for yourself. All you need is some flour, water, salt, and your own two hands.

Now—perhaps more so than any other time in your life thus far—you have the time to make a starter. You’re stuck at home. You can afford to tend to something, to give it both your mental and physical attention.

But before making a starter, it helps to understand what it is. There’s an entire microbial universe at work that leads to that crackling crust, that creamy, honeycombed crumb with an impossibly complex flavor, and that absurdly photogenic loaf of your dreams.

Let’s take a look at what’s really going on under a sourdough starter’s bubbling hood.

Wild Beginnings

A sourdough starter—or levain, if you’re French or fancy—is a complex community of microbes used to leaven breads, imparting a distinct sour flavor and light texture along the way. Like many ferments, starters have been around for thousands of years, with the earliest known leavened bread dating back to 3700 BCE in Lausanne, Switzerland. In fact, it’s only in the last 150 years or so that commercial baker’s yeast has come into fashion, while the slow, plodding, sometimes mercurial process of natural leavening has faded away, only to be found in artisanal bakeries, restaurants, and enthusiasts’ homes. Commercial yeast has its merits: It works fast, it’s convenient, shelf stable, and, until now, it’s been readily available. Since the onset of the pandemic, yeast has all but disappeared from store shelves, as manufacturers race to keep up with demand. Sales of baking yeast skyrocketed 647.3 percent in March 2020 compared to the year prior, according to Nielsen. But sourdough has always been, and will always be there, as a reliable way to make bread.

Sourdough Starters Versus Commercial Baker’s Yeast

Take a bite out of a slice of sourdough and another from a loaf made from industrial baker’s yeast and you’ll notice a difference right away. Sourdough breads just taste better—they’re more complex, more aromatic, and more adaptable to a wider range of flavors than commercial yeast. On the other hand, breads made with commercial yeast have a distinct calling card: A monotone, sweet, beer-like aroma that often dominates in breads like a brioche or a white pullman loaf. Baking with a sourdough starter can bring other flavors to the fore, such as the caramel, earthy notes of whole wheat or the subtle sweetness of dairy. This improved flavor comes from a sourdough starter’s microbial diversity, a feature that commercial yeast lacks.

Sourdough breads are also arguably more easily digestible for most people, with a greater bioavailability of nutrients, and are well-tolerated by those with certain sensitivities to commercial baker’s yeasts, sugars, or other additives.

That doesn’t mean breads made with conventional yeast are bad. They have their place in the world of baking, too. But sourdoughs are their own beast, and there’s a lot going on that makes them so special.

How Sourdough Starters Work

The abridged version of the process goes something like this: mix equal parts flour and water in a jar and wait. Take some of that pasty sludge out and discard it; stir in more flour and water, and keep waiting. After some period of time repeating this process over and over, you produce a bubbling, doughy-gooey mass that rises and falls with some predictability. Over time, this mixture contains the proper collection of yeast and bacteria that can leaven bread and bestow that distinctive tangy, creamy flavor and light texture that we know and love—it becomes a sourdough starter. In exact terms, we say a starter has fermentative power—the ability to convert sugars into products like ethanol, carbon dioxide, and organic acids.

Microbial Power

Here’s the long version: A sourdough starter is a culture of microorganisms. Where do those microbes come from? They’re everywhere: In the flour you use, in the air, on your hands, in the jar, maybe even on the spatula or spoon you use to stir. Common belief holds that the majority of microbes come largely from flour and, to a much lesser extent, the surrounding air. But there’s evidence that yeast and bacteria come from less obvious places: Pulling data from bakers across the globe, this study suggests that some of the diversity of microbes and flavor differences between starters comes from the microbes that live on the hands of those bakers (known as the skin microbiome).

Starters rely on one of the fundamental forces of evolution: natural selection. You’re honing a microbial ecosystem and harnessing it for bread making. How do these microbes grow? When flour and water mix, enzymes (amylases) in flour convert long starch molecules into simple sugars, providing the perfect fuel for microbial reproduction.

Putting it all Together: A Story of Symbiosis

How are yeasts and lactic acid bacteria able to coexist peacefully in a starter? Like any bustling city, there are limited resources in a sourdough culture. Let’s call those resources simple sugars, of which there are several: glucose, fructose, and maltose, to name a few. Yeasts like C. milleri and S. cerevisiae prefer to feed on glucose and fructose. Meanwhile, LAB such as L. sanfranciscensis thrive off of maltose. A stable starter features a balance of microbes that don’t compete much for each others’ food.

Both yeasts and LAB work to make their surroundings inhospitable to most other microbes. Yeasts give off ethanol, but oddly enough, the LAB can tolerate ethanol quite well. On the flip side, LAB secrete acids, but wild yeasts are also tolerant of the increasingly acidic conditions. To top it all off, yeast cells produce additional amylase enzymes as they reproduce, which convert additional starch to simple sugars to help feed the whole gang. These two microbes survive, thrive, and outcompete others in a stable starter culture—in perfect symbiosis. That’s the kind of elegant, seamless teamwork that would make NBA Hall of Famer Phil Jackson’s legendary triangle offense look like a game of 4th grade pickup basketball.

That was a lot of hard microbiology. Fortunately, you don’t need to retain a lick of it to successfully make your own starter. But like most granular topics in cooking and baking, it helps to understand what’s actually happening. While science is a useful tool, making a starter ultimately involves your senses—your hands, your eyes, your nose, and yes, even your palate—and there’s no better way to learn that process than to start making your own.