Baking is applied microbiology. That may seem like an odd way to look at it, but it’s only a modest exaggeration. All yeast-leavened breads owe their shapes and textures to the actions of microbes. The yeast used to create bread can be commercially derived (baker’s yeast), or it can be cultivated from the environment around us in the form of a levain (sourdough starter). There are many reasons to use this popular preferment. Levains produce breads that have a depth of flavor that commercial yeast-based breads don’t and are more forgiving thanks to the longer fermentation time. Starting a levain takes time, though, and when you create a preferment using microorganisms from the environment, you must maintain the culture.
A variety of myths and legends surround sourdough starters, and many of them date far back in the long history of yeast and bread. Before it was possible to observe fermentation through a microscope, no one could have imagined—much less explained—how dough could leaven itself, as if by divine intervention. We’ve come a long way since then, and useful information about the science of levain and sourdough breads abounds today. And that’s important, because having a basic understanding of how the microbes in levain behave can make working with this preferment more straightforward.
Getting Cultured: Yeast and Lactic Acid Bacteria
A levain is a preferment used to make sourdough bread, composed of a mix of water and flour that is fermented by lactic acid bacteria (LAB) and wild yeast. By themselves, the raw ingredients that go into a sourdough are essentially flavorless. The sweet-and-sour flavors we love in these breads are by-products of the microbes’ mutually beneficial fight to survive and grow in a complex microscopic ecosystem. And the makeup of that ecosystem evolves over hours or days of fermentation.
Unlike commercial baker’s yeast, which are strains of yeast within the species Saccharomyces cerevisiae, the yeasts in levain are varied, including not only S. cerevisiae but also a mix of other species, such as S. exiguus, Hanensula anomala, and Candida tropicalis. This particular mix of yeasts makes each levain unique flavor-wise—and most importantly, gives the dough rise.
While many people think that their sourdough starter is made up primarily of wild yeast, it is far outnumbered by the lactic acid bacteria in the culture— LAB outnumber yeast cells in a mature sourdough starter by roughly 100 to one. In fact, a levain isn’t stable without the lactic acid bacteria that symbiotically live with the wild yeast.
Like yeast, many kinds of bacteria also engage in fermentation. Smaller than yeasts, most of these bacteria are members of the genus Lactobacillus, so named because the 200-odd species in this group produce lactic acid as they digest sugars. The fermentative power of an individual bacterium is far less than that of a yeast cell, which contains about 20 times the volume of a lactic acid bacterium such as Lactobacillus brevis. San Francisco–style sourdough bread, as well as many other sourdoughs from around the world, derives its characteristic tangy flavor from L. sanfranciscensis. Bacterial species from the genera Leuconostoc, Pediococcus, Enterococcus, Streptococcus, Weissella, and Lactococcus are also common in levain.
Yeasts and LAB coexist so well because each can grow alongside the other and tolerate, to a certain extent, the other’s defense mechanisms. Lactic acid bacteria, like yeasts, are greedy when it comes to resources. The two work together to poison their surroundings—the toxic cocktail they create is full of alcohol and acids that are made during fermentation. It’s a less than warm welcome for other microbes.
Lactic acid bacteria aren’t much inhibited by the ethanol that the yeasts give off. In fact, some strains of lactobacilli are more tolerant of ethanol than yeasts are. The LAB, meanwhile, secrete acids—notably, lactic acid and acetic acid—that lower the pH of the levain. (Scientists who have compared the pH of commercial yeast-based breads and sourdough breads have found that the pH of sourdoughs is much lower: 3.8 to 4.6 versus 5.3 to 5.8 typical of commercial yeast-bread breads.)
But the wild yeast species in levain are able to survive in the increasingly acidic mixture. Without each other, pure cultures of yeasts and LAB can be invaded by other microbes, and if left unchecked, both yeasts and LAB will produce more alcohol and acid than even they can tolerate.
When it comes to peaceful coexistence, it helps that sourdough yeasts and LAB like different foods. Yeasts are better able to make use of a wide range of sugars and starches. C. milleri and other yeasts are happiest eating glucose and fructose (and sucrose, which enzymes quickly break down into these two simpler sugars). L. sanfranciscensis and other LAB, in contrast, prefer maltose. Another display of teamwork is that yeast cells also produce amylase, an enzyme that splits the complex starches and polysaccharides in flour into sugars that are more digestible to the yeasts and their bacterial neighbors.
The Evolution of a Levain
When bakers create levain, they exploit one of the principal forces of evolution— natural selection—as they shape a microbial ecosystem into a tightly controlled tool for bread making. The process illuminates the remarkable ability of yeasts and LAB to adapt to specific environmental conditions.
The growth of yeast and bacteria depend on three key factors: availability of nutrients, acidity, and temperature. Because growth can happen exceptionally fast, species and strains that aren’t adapted to a specific diet (like flour) can quickly be overwhelmed and die out. This is precisely why the inoculants, such as raisin water, that some bakers use to jump-start their levain don’t make a difference. (We think flour, which is chock-full of microbes, and water work just fine.)
Additional factors, including hydration, also influence how a sourdough starter matures. Levain can vary in hydration. If you mix together equal parts water and flour, you’ll produce a levain that is fluid—that is, highly hydrated. We refer to this as a liquid levain (pictured on the right in the image below). If you add more flour to the mixture, say 120% flour to 100% water, the result will be stiff (left). In our experiments, we noticed perceptible differences in pH: the more liquid the starter, the more acidic it will be. (So if you like your sourdoughs good and sour, use a liquid levain.) Your culture can also be affected by contamination or invasion by dust particles, spores, and the like, which can introduce new microbes
Many bakers swear by their particular starter too. But from a microbiology standpoint, the makeup of a starter will be very different if the feeding schedule or temperature is inconsistent. If you aren’t careful, your special starter may be very different on day 1 than it is on day 20 (or even day 2). And different starters can create surprises, which isn’t a good thing if you’re trying to make consistent loaves.
A long-lived levain is almost certainly going to change in composition over time. Think of it like a city; a great city may be just as grand two centuries from now as it is today, but it will have different inhabitants—including some who are descended from the current residents and some who moved in later. A starter’s composition will stay the same only in a perfectly maintained sterile environment, more like a laboratory setting than a bakery. The community of microorganisms will fluctuate and adjust to whatever foods they are given and whatever living conditions they experience. If one strain finds the environment more welcoming than the others, it will quickly grow and crowd its neighbors.
But locking in a specific population of bacteria is not important. What matters is creating a hearty colony of yeasts and lactic acid bacteria that behaves predictably; in other words, as long as the levain is fed on the same schedule and kept at about the same temperature and hydration, it will ripen and mature as expected.