I’ve soaked figs and fermented them with flour, simply because I like figs. I’ve poured hot water over wheat bran, then used this bran tea to make sourdough starter. This works because arabinoxylan—the sugars that reside in bran cellulose—pump up the acidity in the medium and increase the metabolism of sourdough microbes. Minerals in bran also promote fermentation.I’ve sprouted barley in a three-day process in which you keep the grains damp until a little sprout appears, then dry them in a very low oven and grind them into barley malt; add a pinch to flour to energize a fermentation or darken a loaf. I’ve pounded whole wheat grains, then used this coarse flour to begin a starter, because the minerals and enzymes in freshly ground whole wheat flour make an especially active fermentation medium. I’ve also added sourdough to wort—the liquid that eventually is fermented into beer—and then made bread with this high-octane starter. I would recommend trying any and all of these methods. I found I’ve ended up with powerful leavens after each of these ferments matures—and also achieved subtle differences in the flavor profile.
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This multitude of methods shouldn’t be too surprising, considering how many foods are actually fermented—from soy sauce to sauerkraut. When someone asks me what’s the best way to make a sourdough starter, I think of Pliny the Elder, who wrote about various methods in his encyclopedic Natural History in the first century A.D. What he recorded aren’t “recipes” but descriptions of a multitude of ways to ferment grains common in the baking rooms of Rome at the dawn of recorded history. When it comes to sourdough, Pliny shows we have nothing on the Romans.
Pliny, for example, said that millet made the best sourdough when mixed with the three-day-old juice of freshly crushed grapes—known as “grape must.” This doesn’t seem too far off from Silverton’s approach, though she used wheat instead of millet. Millet is also promising since it has a rich history of fermentation: millet wine has been found in Chinese tombs of the Xia and Shang dynasties, predating Pliny by at least a millennium. Pliny also says to soak wheat bran with white wine must for three days, then dry it in the sun and shape it into small cakes. To use it as a leavening agent, soak the cakes in water, heat the mixture with spelt flour, then knead them into the dough. “It is generally thought that this is the best method of making bread,” he says. He also called for grapes “at the time of vintage,” a telling piece of advice, as we shall see.
He describes yet another leaven made with barley and water, formed into cakes and then baked “till they turn reddish brown. When this is done, the cakes are shut close in vessels, until they turn quite sour.” To use them as a leavening agent, they are steeped in water. But the most common method, he tells us, was to boil flour, water, and salt until it formed a porridge; it was then left to sit until it turned into sourdough. As a final aside, he notes that bakers “make use of a little of the dough that has been kept from the day before”—a common method still used today and known as pâte fermentée, or fermented dough. Bakers save a bit of dough for the next day’s mix to enhance the flavor of the bread.
When I discussed these methods with Andrew Ross, a cereal scientist and avid baker at Oregon State University, he mentioned boiling was advantageous because it breaks open the starch in the grain, freeing sugars for yeast and bacteria to feast upon. Pliny’s instructions “should make a particularly active starter,” he said.
But Pliny’s description raises another question, too, about the presence of grapes in so many of these recipes. As it turns out, Saccharomyces cerevisiae, the ubiquitous species of yeast used in baking, brewing, and wine making, is naturally found on grapes. But not just any grapes. Pristine grapes tend to have very little wild yeast on them, yet on about a quarter of grapes with ruptured skins the yeast shows up. One recently investigated vector for this yeast was wasps, especially queens, which harbor yeast in their intestines during the dormant winter and then spread them to their progeny the following year. The wasps peck at the ripe fruit in the summer, transferring yeast to the grapes when they pierce the fruit’s skin. One study, which investigated wasps in Tuscany, found that their intestinal yeast varied seasonally, but Saccharomyces cerevisiae was most prevalent right at the time of the grape harvest. This gives a new meaning to seasonality, for not only was the fruit ripe for wine making, but so, too, were the populations of wasp gut yeast that could infect the grapes. The yeast found in the wasps’ innards also matched the strains found in Tuscan wine and in baker’s yeast. So, the wasps, grapes, bread, and wine of Tuscany were part of a singular yeast ecology that varied seasonally. No wonder Pliny specified “grapes at the time of harvest.” The key, though, is to seek out ripe grapes with punctured or split skins, because each fruit can add between 10 million and 100 million organisms to a starter.
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At its most basic level, what baking does—what all grain fermentation does—is to rely on single-cell organisms to hack the seed’s built-in process of germination and growth. By doing so, the seed is transformed and becomes food for us, rather than a plant.
Here’s how it works. The seed has three components: the starchy endosperm; the oil- and vitamin-rich germ, which is where the embryo of the plant springs to life; and the nutritious and fibrous bran coating, which protects the seed. When the kernel becomes wet, hormones awaken and trigger the process of growth. Amylase enzymes located in the outermost layer of the endosperm, called the aleurone, come alive. They flow through the endosperm and begin to break down starch, a polysaccharide, made of long chains of sugar molecules, into shorter chains that can feed the plant embryo. Protease enzymes do the same thing to proteins, creating smaller chains of amino acids, so they, too, can feed the germinating plant. In this way, the seed offers a complete life-giving package. It contains its own protective casing (the bran), a backpack of high-energy food (the endosperm), an enzyme-rich food processing facility (the aleurone), and the plant embryo which springs to life (the germ). All this processing facility needs to get going is water and warmth. If the seed has all of these things, you get a wheat plant.
Bread baking subverts, or hijacks, this entire life process. Because when bakers make dough, they trigger the same chain of events: water activates amylase enzymes in the flour, which then flow through the viscous substance to reach the starch, breaking it into smaller chains of sugars. These sugars then feed the yeast or sourdough that the baker also adds to the mix. Malted barley, which is rich in amylase, helps this process along, which is why flours are often spiked with a judicious amount of the substance. Saliva, which contains ptyalin, an amylase enzyme, does the same thing, which is why native peoples from Asia to Africa to the Americas masticated grains and spit them into a bowl to spur fermentation.These days, however, malted barley is a sufficient source of amylase, so there’s no need to chew and spit unless you’re so inclined.
With sugar levels rising in the dough, yeast release their own suite of enzymes to help convert the starch to sugar. The yeast initially consume oxygen in the dough, which is why a dough whipped up in a high-speed mixer will ferment so quickly. After taking in oxygen, the yeast cells will exhale carbon dioxide gas and sweat out water in a process of aerobic respiration, not unlike what you do when you go for a jog. But when the oxygen runs out, as it soon does, the yeast do not die as you would on your oxygen-deprived run. Instead, the yeast begin a process of anaerobic fermentation, continuing to process sugar without the fuel of oxygen. This fermentation process causes the yeast to burp carbon dioxide gas and emit ethanol alcohol, which not only allows the dough to rise but creates unique flavor compounds in the bread. Ethanol is also toxic to many species of bacteria that might compete with the yeast for these sugars.
Saccharomyces cerevisiae—the same yeast species found in the guts of Tuscan wasps—is particularly adept at this sugar-eating-carbon-dioxide-belching activity, which is why it has been employed as a fermenting agent for so long. Chinese wine making dates back more than nine thousand years; evidence of fermented beverages appears in Iran a
bout six thousand years ago. Over many millennia, this yeast species was selected by humans and domesticated for fermentation. Recently, 651 variants of Saccharomyces cerevisiae were genetically mapped, revealing their relationship on a vast family tree (the parentage of French champagne yeast wasn’t too distant from Sicilian bread yeast). Aside from these domesticated strains, Saccharomyces cerevisiae also live in the wild, found in everything from rotting grapes to the bark of oak trees. Yeasts—a fungi—are actually ubiquitous, with more than 1,500 species identified thus far with more genetic diversity than all the vertebrate species in the world. At least 23 yeast species have been found in various sourdough starters. Saccharomyces cerevisiae is the most common, though usually one or two others, such as Candida humilis, take up residence as well.
The reliance on these yeasts to ferment foods dates back to prehistoric—even to prehuman—times, since animals ranging from birds to elephants have been spotted eating fermented fruit and then getting drunk. Hence, the “drunken monkey hypothesis,” which posits that we got our taste for alcohol from ancestral chimps who have been observed imbibing the fermented-fruit equivalent of two bottles of wine in twenty minutes and then staggering around. With this deep biological bias for drink, it’s a short jump from chimps and rotten fruit to wine making, and from there to beer, which has the advantage of using a dried store of grains rather than ripe seasonal fruit. A long-lasting stash of dried barley berries meant our ancestors living in the Middle East could make an alcoholic beverage any time of the year, not just when grapes were ripe. Beer as opposed to wine, though, has a number of disadvantages: grains, specifically barley, must be sprouted and malted, or at least masticated and spit to stimulate enzymes that will turn carbohydrates into simple sugars. And that’s much harder than picking ripe fruit, dropping it in an earthen vessel, mashing it with honey (which also contains wild yeast), and waiting, which is why many archeologists think that wine came first.
But if wine came before beer, the anthropologist Sol Katz, professor emeritus at the University of Pennsylvania, told me he thought beer came before bread. “A sprouted grain is easy to discover,” Katz told me. “The grains get wet, they sprout, you eat them, and they taste sweet. The moment you detect sweetness, you connect it with the fermentation of fruit.” Making the leap to beer would probably have been easier than taking the additional steps of grinding the grain, fermenting it, and then cooking it over fire, in a form of bread. Plus the grain in question was barley, which was among the earliest wild cereals gathered at the dawn of agriculture. Although much of the evidence has been lost to time, archeologists have found chemical traces of barley beer in a jug from a site in western Iran dating from six thousand years ago. Another religious site in southwestern Turkey where wild grain was consumed might push the beer-making date back to more than eleven thousand years ago. But while the Middle East is often thought of as the birthplace of viniculture, it wasn’t likely the first. Rice and honey mead, along with wild grapes and hawthorn fruit, were fermented in China around nine thousand years ago in the Yellow River valley.
Bread made with a beer starter and “spent grains” from the beer-making process
Beer versus bread does become something of a chicken-and-egg problem, but whatever the progression, fermentation was one of the oldest food-processing technologies. As the archeologist Patrick McGovern tells us in Uncorking the Past, yeast was employed for making wine since before the dawn of agriculture. Beer and bread probably weren’t too far behind.
With sourdough, though, the story gets more complex because yeast don’t work alone but rather in tandem with lactic acid bacteria (Lactobacilli), a family of organisms that can survive and even thrive in the alcohol-rich and highly acidic environment that the yeast produces. These bacteria create a range of sharp and mild acids that influence the taste of the loaf, the texture of the interior crumb, the appearance of the crust, and the loaf’s longevity, since they delay staling. In a marvelous relationship, the yeast and bacteria work together, for the yeast converts starch into the type of sugars that the bacteria can digest. The bacteria also expel carbon dioxide gas, helping the loaf to rise. Debra Wink, a home baker in Pittsburgh with a background in microbiology, explained to me that these organisms don’t appear all of a sudden. The starter’s ecology evolves, with one set of organisms appearing only to be supplanted by another as the culture matures and acidifies. To create an acidic niche for the most beneficial organisms in a sourdough starter, she uses pineapple juice to help the process along, though any citrus or even crushed vitamin C will lower the pH enough to achieve the effect. But, as with grapes, I’ve found this isn’t absolutely necessary.
While yeast may be spread by wasps, lactic acid bacteria are also found in many places: in the mouth, in the digestive tract, on fruits and vegetables, in feces, and in compost piles. But they are not floating around “in the air,” as I’ve also heard so often from bakers, and which I believed for years. Neither yeast nor bacteria survive long in open air, which is why it’s unlikely that a traipsing bacteria or yeast cell will blow into your kitchen and inoculate your infant batch of sourdough starter. These organisms need to travel on some kind of agent—saliva, juice, insects, skin, fruit—but eventually they can become established in the microclimate of a bakery, or even brewery, and then, yes, they can be found in the air. But the science is clear: they aren’t in your kitchen at the outset.
Scientists are just starting to tease out the origins of these microscopic cells, and they are finding strong evidence that they originate in the bowels of various animals. From there, they migrate into flour or dough. One study found that Lactobacillus reuteri in a rye sourdough matched a strain found in the entrails of rodents. Another species in wheat sourdough was identical to one found in the human intestine. Still other strains mirrored those found in the vagina. (This gives new context to the role of the seventeenth-century French geindre, mounting the dough in a fetid basement baking room as he kneaded the mass, sweating and sputtering and inoculating the substance with his various effusions. Maybe this made for a lively fermentation.) But lactobacilli are far more prevalent in the viscera of pigs, mice, chickens, and rats than in humans. Michael Gänzle, an authority on the science of sourdough at the University of Alberta in Canada, who coauthored the groundbreaking study on the intestinal origin of lactobacilli, told me that rodents likely infect the grain at the farm, mill, or bakery. Or they might arise, as one paper delicately put it, from “fecal contamination in the sourdough production environment.”
Insects may also play a key role. In a South African study, thirty species of lactic acid bacteria were isolated from the guts of fruit flies, including those bacteria commonly found in sourdough starters. When I first heard about this study, it suddenly hit me that this may be the reason that fruit flies congregate above the starter bubbling away on my kitchen counter. They love the stuff. I actually created a fruit fly trap by putting ripe sourdough in a plastic container, covering it with plastic wrap and punching holes in the top. The fruit flies dive in like kamikazes and then perish. The reason: fruit flies lay their eggs when they smell ethanol from a fermented substance, for it signals that their larvae will have access to a complete diet of sugars, high-protein yeast, and even alcohol, for survival. Maybe they were the source of the bacteria in my sourdough. But when I put it to Gänzle, he told me that this hasn’t been studied enough to get a definitive answer.
Sadly, these microbiologists destroyed a myth that I and so many bakers had accepted for so long: the idea that sourdough reflects a particular region. Lactobacillus sanfranciscensis, first discovered in 1971, for example, was long thought to be unique to San Francisco and gave the city’s sourdough bread its tangy flavor. It’s a nice story but the species has now been found globally and is among the most prevalent type of Lactobacillus in sourdough. Interestingly, it hasn’t yet been found anywhere else in nature, aside from sourdough and the guts of South African fruit flies.
Ultimately, the bacteria that do
take up residence in sourdough reflect the ecology of the substance. This, in turn, is determined by the amount of water in the starter, the frequency of refreshment, the ambient temperature, the type of flour, and how much starter is used to rebuild a new batch. Change these factors and the microbiota within the mix might change as well. Feed the leaven at a temperature between, say, 68˚F and 80˚F (20˚C to 27˚C), once or twice daily, and odds are about one in four that you will eventually get Lactobacillus sanfranciscensis in your starter. But if, like industrial makers of sourdough, you keep the temperature of your leaven at 100˚F (38˚C) and feed it every few days, far more acid-tolerant bacteria will arrive, such as Lactobacillus reuteri.
I’ve learned to manipulate sourdoughs to work with various flours, and I now believe that the starter has more influence on the flavor of the bread than the flour itself. That said, the type of flour used to feed the starter will affect the bread’s taste as well. If you mix the sourdough with rye, it will taste more earthy and less grassy than wheat. If you mix the starter so it resembles a stiff ball, and keep it at a rather cool temperature of around 67˚F (19˚C), it will develop a tangy edge. That’s because the bacteria producing the sour-tasting acetic acid will dominate in a cooler, stiffer culture. Produce a leaven that resembles pancake batter and ferment it at 80˚F (27˚C) and you will instead favor the production of lactic acid, which resembles the rounder notes of yogurt. The most extreme example I’ve encountered is a German Detmolder rye sourdough described in Jeffrey Hamelman’s book Bread. The sourdough is fermented and refreshed at three different temperatures for varied lengths of time to influence the taste of the final loaf. There’s no need to go to these extremes—you’d need a temperature-controlled cabinet to achieve such Prussian precision in making a sourdough culture—but it does help illuminate what a bit of flour, water, organisms, and fermentation can achieve. Whatever the method, if the starter is fed regularly and maintained at a consistent temperature, the microbes will likely remain consistent, too. That is why scientists have found highly stable bacterial cultures when looking at bakeries’ sourdough starters.
In Search of the Perfect Loaf: A Home Baker's Odyssey Page 8