The Science of Sourdough Starters

close up of ap flour sourdough starter on day 11

[Photographs: Tim Chin unless otherwise noted]

Editor's note: This article is the first in a series exploring sourdough bread in depth. Stay tuned for our starter recipe, bread recipe, and more.

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.

Why the sudden interest in sourdough, and baking in general? There’s plenty of bread on the store shelves. That’s not the problem. Maybe it’s the relief of a time-intensive, all-consuming endeavor. Or maybe making sourdough plays a more abstract role. “I think [sourdough] bread is a symbol for home, for comfort and community,” says Daniela Galarza, Features Editor at Serious Eats. The concept of taking raw ingredients and microorganisms—ones that aren't dangerous pathogens like the novel coronavirus—and making something that’s nourishing provides solace. “It gives people a sense of control that they otherwise don’t have in other parts of their lives right now.”

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

Years ago, I used to work at a neighborhood restaurant in the East Village. My pastry chef at the time maintained a modest sourdough program, churning out several loaves and baguettes daily. She had affectionately named her starter ‘The Bitch.’ It lived in a crusty, red-lidded, 12-quart Cambro container with a weathered tape name tag in big, defiant upper-case letters. Every day, it was the first item on my prep list: Feed The Bitch. Sometimes I fed it rye flour. Most times I fed it white wheat flour. Other times, I treated it to cider or beer. And, occasionally, I would arrive late on a warm summer morning to find it spilling out onto the tile floor, angrily gurgling away from the speed rack. The Bitch was a fickle—albeit essential—coworker.

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

[Vicky Wasik]

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

Sourdough starter tests day 1: rye, 50/50 rye, whole wheat, 50/50 whole wheat, and ap flours

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.

Simple, right? Not so fast.

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.

In the world of sourdough starters, the two most important microbes are yeasts and lactic acid bacteria. Let’s break those down in detail.

Yeasts

50/50 rye flour sourdough starter full jar

Yeasts are a diverse set of single-celled microorganisms that make up approximately 1 percent of the entire fungus kingdom. There are more than 1500 known species of yeasts. The species we know best is Saccharomyces cerevisiae—or common baker’s yeast—which is used in both baking and the production of alcoholic beverages like beer. But there are many more yeasts that are useful in food production.

Yeasts contribute mainly the leavening power of a dough, and somewhat to the flavor and aroma. How does yeast do that? In order to reproduce, most yeasts like S. cerevisiae convert simple carbohydrates (sugars) to carbon dioxide and ethanol. This process is known as alcoholic fermentation. As the yeasts continue to feast on available sugars, they multiply. This reproduction rapidly occurs at warm temperatures (between 86–95°F, or 30–35°C), but also occurs at lower temperatures, although at a slower rate. The production of carbon dioxide creates gas bubbles in dough, which, when trapped in a well-developed gluten matrix, expand the dough. When baked at a high temperature, these bubbles expand further as more and more carbon dioxide is produced until the yeasts die off, resulting in that airy, spongy loaf we call bread.

As you might expect, given the sheer diversity of yeasts, S. cerevisiae isn’t the only species living in a sourdough starter. The reality is much more complex. In studies of starters from around the world, DNA sequencing from varying samples has revealed the presence of a wide array of wild yeasts: Saccharomyces servazzii, a funky-smelling prolific producer of carbon dioxide with profound leavening power (it’s so powerful that it’s even a bane to industrial food production, where it causes packaging to explode); or Saccharomyces unisporus, found more commonly in liquid and warm starters; Pichia anomala, which produces isoamyl acetate, which smells like artificial banana; and no less than seven other species of yeasts, all with varying characteristics and functions. The most commonly occurring yeasts include S. exiguus, S. cerevisiae, and Candida milleri (or humilis).

The differing ratios of these yeast populations alone is enough to explain the degree of variability between sourdough starters. But yeasts are just one side of the microbial coin.

Lactic Acid Bacteria

Lactic acid bacteria (LAB) are rod-shaped or spherical, and primarily produce lactic acid. Much smaller than yeasts, they are found in decomposing plants, dairy products, on the skins of vegetables, fruits, and even on your own fingers. In a typical starter, LAB outnumber yeasts by as much as 100 to 1. Like certain yeasts, LAB digest simple carbohydrates, but instead of the alcohol created by yeast, LAB mostly produce sour lactic acid as a byproduct.

Why are LAB important in sourdough? First, the production of lactic acid (as well as acetic acid) lowers the pH of your starter to around 3.5 (and as high as 5). This lowering of pH results in that characteristic sour flavor of sourdough. Second, a low pH eliminates unwanted pathogens like enterobacteria or Staphylococcus. Simply put, microbial baddies can’t survive in an acidic environment. This feature alone is the driving force behind lacto-fermentation, the age-old technique of preservation that has produced foods like kimchi, sauerkraut, and kosher dill pickles. A low pH also gives sourdough a longer shelf life than other breads by inhibiting mold growth. Finally, LAB release protease enzymes that break down gluten over time, resulting in a softer, lighter texture.

They are generally classified into two groups: homofermentative and heterofermentative strains.

  • Homofermentative (or homolactic) LAB only produce lactic acid. They prefer temperatures between 86 to 95°F (30–35°C), though they grow at lower temperatures as well. They produce flavors characterized by dairy, cream, or yogurt notes. Bacteria in this category include Lactococcus, Enterococcus, Streptococcus, Pediococcus, and L. acidophilus.
  • Heterofermentative LAB produce lactic acid, but also acetic acid, ethanol, and even carbon dioxide (thus providing some leavening power). These bacteria thrive at temperatures between 59 and 72°F (15–22°C), but can grow over a much wider range as well. They impart a sharper, more vinegar-like tang to foods, likely due to the extra production of acetic acid. The most relevant species are L. plantarum and L. fermentum, among others.*

* Within heterofermentative LAB, there exist two subcategories: Facultatively heterofermentative LAB metabolize certain sugars to lactic acid, and other sugars to lactic acid and acetic acid, as well as acetate in the presence of oxygen; obligately heterofermentative LAB always metabolize sugars to produce lactic acid, acetic acid, and carbon dioxide. But for the purposes of this article, you don’t need to get too worked up about the difference.

As with yeasts, a single sourdough starter will likely contain several species of LAB over the course of its lifetime. For instance, there’s L. sanfranciscensis, the bacteria for which the San Francisco-style sourdough is named, which produces a distinctly tangy flavor. Early on in development, species like homofermentative Pediococcus, Enterococcus, Streptococcus, and Weisella bacteria have been shown to predominate. But evidence suggests that over time, stable sourdough cultures contain mostly heterofermentative LAB such as L. fermentum and L. plantarum, which outcompete less adaptable homofermentative lactobacilli. (In other words, a stable starter tends to have a more sour aroma, and imparts more sourness to breads, than a young, one-week-old starter due to the additional production of acetic acid from heterofermentative LAB.)

Microbe Type of Fermentation Function Examples
Yeast Alcoholic Fermentation: sugars converted to carbon dioxide (CO2) and ethanol Leavening power, aroma, and flavor S. Cerevisiae, S. exiguus, C. milleri (humilis)
Lactic Acid Bacteria (Homofermentative) Homofermentative reactions: sugars converted to lactic acid Produce complex sour flavors, improve shelf life, improve texture L. Acidophilus, Lactococcus lactis, Enterococcus
Lactic Acid Bacteria (Heterofermentative) Heterofermentative reactions: sugars converted to lactic acid, acetic acid, and CO2 Produce complex sour flavors, improve shelf life, improve texture, leavening power Provide acetic "vinegar" flavor L. Plantarum, L. fermentum, L. sanfranciscensis

Putting it all Together: A Story of Symbiosis

bread loaf cross section with AP flour sourdough starter

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. Stay tuned for our step-by-step guide later this week!