Photograph by Catherine Hoggins, Alamy
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The bubbles in this sourdough starter are signs of life—and the chaotic interplay underway among yeast cells and natural bacteria.

Photograph by Catherine Hoggins, Alamy

The funky science of yeast, the gassy microbe behind your pandemic bread

Baker’s yeast has transformed into a prized stay-at-home commodity. Here’s how it works—and how you can grow your own.

Here is a story, for quarantined times, about extremely tiny organisms that do some of their best work by burping into uncooked dough. In the end, if things go well, there is good bread. If things go poorly, there is bad bread, or a mass of gluck you heave out so you can try again. This is the nature of yeast, which in its most familiar packaged version started vanishing from markets sometime in March, right after toilet paper and hand sanitizer. “Blowing out of here,” says Kyle Oney, who owns a grocery in Bishop, California. “Gone within 20 minutes of being put on the shelf.”

The run on home bakers’ yeast—not the hefty bulk packages commercial bakeries need, but those jars and quarter-ounce envelopes you used to find in the aisle with the sugar and cake mixes—has staggered manufacturers. “I’ve been with this company for 23 years and I’ve never seen anything like this,” says Bill Hanes, a vice president at the Milwaukee-based Lesaffre Yeast Corporation, which is the world’s largest yeast producer and makes the popular Red Star brand. “Our plants are running 24/7, but as soon as people can see it, it’s gone. I get pictures every day, from people around the country, of empty grocery shelves.” (This the science behind panic-buying.)

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A vintage image of Red Star yeast, still one of the world’s most popular brands. Although humans have been fermenting with yeast for at least 10,000 years, commercial bakers’ yeast production didn’t take off until the end of the 19th century.

Neither Hanes nor his flour business counterparts, who are confronting their own supply chain shortages, are perplexed as to what’s going on. Sheltering-in-place people, those lucky enough to have ovens inside their shelters, are making a lot of bread. “This is like nothing anyone has ever seen,” says Martin Philip, the head baker for King Arthur Flour. “Some people have said ‘tsunami.’” Phone calls are overwhelming the hotline at the company headquarters in Vermont, which brings Philip a certain comfort: all those bakers, novice and experienced, seeking counsel on their loaves.

“But at the same time my heart bubble sort of gets popped,” Philip says. “We’re not here because of something we want to celebrate. These are tough days. But hopefully we’re supporting people who are baking to soothe, or baking as an act of love, or baking as a way to do something in a day, and do it well, and enjoy it.”

Also it is perhaps useful to find a single small-scale pandemic anxiety, MUST OBTAIN YEAST, taking up brain space that might otherwise be occupied by the bigger ones. In Atlanta, where a National Geographic editor named Hyatt Mamoun is doing the shopping for more COVID-19-vulnerable family members, every supermarket visit now involves a yeast hunt. (So far, no luck.) In San Francisco, an artist named Katie Sellergren is hoarding a yeast stash mailed to her by a sympathetic professional baker friend in Iowa; after reverently opening the padded envelope, Sellergren decanted most of the yeast for herself and set a little aside so she could barter for flour. She figured one teaspoon of yeast per cup of flour was a fair exchange, although in her first transaction she buckled. “I was too generous,” Sellergren says. “I gave her tablespoons instead of teaspoons. But she’d never made anything with it before, and she brought four cups of flour and a piece of banana bread.”

And in Boston, as a scientist named Sudeep Agarwala video-attended a birthday party last month, another guest complained that she had just bought a breadmaker but for the life of her could find no yeast. Agarwala broke up laughing. He told her what to do. The next day, on a whim, he got on Twitter and typed out more or less what he’d said.

“Friends,” that opening tweet began. “I learned last night over Zoom drinks that y’all’re baking so much that that there’s a shortage of yeast?! I, your local frumpy yeast geneticist, have come to tell you this: THERE IS NEVER A SHORTAGE OF YEAST.”

In his little Twitter portrait he’s wearing a red cardigan sweater, like Mr. Rogers. “Here’s where I’m a viking,” Agarwala wrote. “Instructions below.”

Somebody retweeted. The retweetees retweeted. Total number of times people Twitter users shared Agarwala’s 3/29/20 yeast-harvesting instructions on Twitter, as the numbers were finally slowing down last week: 27,900. “It just wouldn’t stop,” he says. (Turns out "Zoom fatigue" is real. Here's how taxes the brain.)

Sudeep Agarwala is not, or wasn’t until 3/29/20, a big Twitter guy. He works for a bioengineering company called Ginkgo, where part of his job—his passion, he says—is researching yeast. “My heart belongs to yeast,” Agarwala says. He truly talks like this, and if you listened to him for a while your heart would belong to yeast too. “I apologize for being a nerd here,” Agarwala says. “But it is actually incredible to see this dust you put into warm water, with a little sugar, and all of a sudden you see this … bloom. To this day it makes me smile. It’s a bit of a miracle.”

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An electron microscope captures the budding cell growth of Saccharomyces cerevisiae, the yeast most commonly used for many kinds of commercial fermentation. Thousands more varieties also live around us.

What Agarwala and his fellow yeast enthusiasts understand (he has plenty of company, it turns out) is that yeasts are single-celled creatures so complex and diverse that scientists have named only some of the many thousands of varieties living all around us. The kinds known to work most reliably for fermenting—think of bread, beer, wine, kombucha, cheese—include one with a Latin name in which you can discern cerveza, the Spanish word for beer: Saccharomyces cerevisiae. That’s what Lesaffre and other commercial manufacturers grow in their facilities. It’s a predictable yeast. It packages well. It can be compressed into cakes or dried into granules.

The commercialization of S. cerevisiae dates back only to the late 1800s; humans have been fermenting drinks and baking risen breads for at least 10,000 years. Yeasts live and reproduce in our homes, in our compost and sidewalk weeds, in our air, on our produce, on our skin. All sourdough starter, whether purchased or made at home or handed down from your grandmother, begins with a kind of yeast-wrangling that anybody can replicate with some flour and a collection jar. If you are hunting for yeast, or have a restless kid in your household, you might want to try it yourself. The burping part will appeal to the restless kid. You are also on safe scientific grounds if you describe it as an experiment in watching microbes fart.

A compressed version of Agarwala’s yeast-harvesting method:

  1. Find some dried fruit in your kitchen—raisins, prunes, etc. You don’t need much.
  2. Put tablespoon or so of the fruit (whole is fine) in a jar with about three tablespoons of water. Stir it around. The water’s getting cloudy, right? That’s yeast. It’s coming off the fruit. You’re just harvesting it now for your own purposes.
  3. Mush in three or four tablespoons of flour, doesn’t matter what kind, Agarwala likes using white. As he says in his tweets: “DON’T GET FANCY.” You want something the consistency of wet dough.
  4. Cover the jar loosely with a lid—you’re leaving room for gas exchange—and find a warmish place to put it. Around 70 degrees is right, but don’t get fancy about this, either; your stove top should work, near burners that are turned on frequently.

The dried fruit is an Agarwala flourish, by the way; had you looked up “sourdough starter recipe” online last week, you would have hit 9,800,000 Google results, some advising just water and flour, which inherently contains its own yeast cells that with patience can probably be prodded out of dormancy. There are other suggested additions as well—potato, fruit juice, and so on—but the principle remains the same: For a week or so you keep tending to your jar, adding a little flour and water every day, watching for the bubbles that tell you your starter is coming to life.

An advisory from this writer: Do not decide that because your Northern California house is rarely warmish it’s a clever idea to turn your slow cooker to its lowest setting and put the jar in there. Too warm. You will massacre your harvested yeast before it can do its job, which is to wake up and make more of itself. When yeast is happy, a word the enthusiasts actually use in this context, its one-cell body grows buds; those buds grow so fast that they quickly break off, each becoming a new body.

And all the multiplying yeast bodies are eating sugar molecules they encounter in their surroundings. Yeasts like sugar. As they digest sugar, though, they discharge gases pungent enough to lift your bread dough, ferment your drink, and allow the kid in your household to examine a jarful of belching and flatulence. (Although as a Lesaffre technical director decorously points out, for conceptual purposes we should probably settle on one or the other. “There aren’t two ends to a yeast cell,” he says.)

Inside a production facility this sequence is sterile and controlled, but in a starter-maker’s kitchen it’s a gorgeous molecular riot. The yeasts might include S. cerevisiae, if it happens to be present in the ingredients or the air; wilder cousin yeasts lurking in the vicinity will find their way in there too, as will natural bacteria. All these organisms are mingling and wrestling inside the jar, a process that creates acids; that’s the “sour” in sourdough, tart or mild, depending. Different combinations of yeast and bacteria, flora and insect life of the local geography, air temperature, kitchen humidity, the chemistry of the baker’s own body—there are so many possible variations that every starter develops its own characteristics, which is why people like Agarwala take such delight in the microbes that create them. (Meet a scientist brewing beer with wasp yeast.)

“It’s this wonderful living thing you’re working with,” says Anne Madden, a North Carolina State University adjunct biologist who studies microbes. She and partner scientists showed recently that when bakers in different locales use exactly the same ingredients for both starter and bread, their loaves come out smelling and tasting different. “Which I think is fantastic,” she says. “It’s evidence of the unseen. And as a microbiologist, you so rarely get to measure things about microbes with your nose and your taste buds.”

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Karl de Smedt, a Belgian yeast lover who describes himself as “the world’s only sourdough librarian,” maintains this collection of live starters bakers sent in from around the world. The jars of “mother doughs,” as de Smedt calls them, are kept in cool storage inside the Center for Bread Flavour outside Brussels. DeSmedt, sheltering at home this month, says he’ll need to visit the library for a while this week—the mothers must be fed.

With NCSU ecology professor Rob Dunn and a research team, Madden has helped run an international study now in the writeup stage; they hope to publish scientifically next year, and in the meantime have read scores of heartfelt personal tales about cooks, their starters, and their starters’ lineages. Calling their undertaking the Global Sourdough Project, the researchers sent questionnaires to bakers around the world, inviting some respondents to send samples of their starters to a study lab at Tufts University. “And we left a little box saying, ‘Tell us about your starter,’” Dunn says. “Within a week we had 80,000 words.”

Because this is another miraculous thing about yeast and starter, as the aficionados know: You can keep it alive, as long as you regularly “feed” it flour and water, for generations. You can inherit it, pass it on, scoop some out to give it away to your neighbors. Perhaps you could trade it for toilet paper. In any case your starter will offer a story to tell; if you take after the most ardent keepers of sourdough tradition, it may end up having a name, too. People especially attached to their starters place them somewhere on the emotional spectrum that includes pets, and for reasons too obscure to go into here, variations on “Herman” are popular. That Iowa baker who mailed some to Katie Sellergren named her own homemade starter—not the commercial product she uses for work, but the kitchen versions she’s cultivating during her shelter-in-place time—PeeWheat Herman.

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If Seamus Blackley and researchers prove correct, this bread is the product of Egyptian yeast cells tweaked back to life after 4,500 years of dormancy. Blackley, a physicist, is also a longtime baker and amateur Egyptologist. To see whether ancient yeast could be resurrected, he and his team drew cells from the insides of baking and brewing pottery in museum collections. While microbiologists study the cells, Blackley tried using some to bake. Whatever was in those pots, the original cells or natural yeast collected over time—it worked. “We don’t know if we got the ancient stuff,” he says. “But if we did, it’s this great moment, because these guys—“ he means the yeast cells—“are eating for the first time in 4500 years.”

Consider this, too, in case your restless kid could use one additional nudge. That flour you’re mixing in, to keep the starter alive, contains yeast cells. So: cannibalism.

In the lab, Agarwala says, he sometimes grows new yeast by feeding it Marmite, the salty food spread based on … yeast.

“Yes,” Agarwala says cheerfully. “Yeasts are cannibals.” He’s working from home this month, trying between video meetings to manage the new barrage of thanks and questions on his Twitter account. He reads and answers as many as he can. A couple of weeks ago, a suitable number of days after his original “frumpy yeast geneticist” instructions, finished-product photos started showing up.

They are beautiful. In a terrible, scary time these photos, portraits of food that strangers made using Agarwala’s yeast-harvesting advice, are a thing to behold. Cinnamon bread, pancakes, focaccia, English muffins, challah. Just-browned sour loaves, crusty and jagged, resting on their oven racks as though posing for the cover of a baking cookbook. “It is so rare that I get to feel this useful in my life,” Agarwala says. “People are making the most spectacular loaves of bread.”