Often called mothers, sourdough starters are more like babies, demanding care and feeding. The reward for adoring bakers: indescribably delicious bread.

Every year on his birthday, no matter where in the world he was, the prolific New York restaurateur Jason Denton used to get a package from his grandmother Harriet containing a loaf of freshly baked sourdough and a jar of raspberry jam. Harriet was a small-town country gourmet—her spicy mustard made it into Denton's Simple Italian Sandwiches cookbook—and her sourdough starter had a pedigree. It had been her mother's, lovingly coddled and fed through two World Wars and the Great Depression, the source of Sunday pancakes for three generations. Denton let it be known he had designs on the starter, but sometime toward the end of her life, Harriet confronted him with an awful truth: She had accidentally thrown it out. He was crushed—he had dreamed of using it to open a bakery, just Pullman loaves and silver-dollar pancakes—but stoic in the face of the immutable. The starter was gone, and with it a hundred-year-old family tradition.

When Harriet died last summer at the age of 96, Denton went home to Idaho for the funeral. There, one of his grandmother's friends, a woman of similar vintage, approached him. "This was my mother's," she said, handing him a small glass jar full of sandy sludge, with a thin layer of darker liquid at the top—her starter. "It's 125 years old. Would you take care of it for me?" Now he makes pancakes for his boys every weekend, in the hopes of nurturing a successor. "Normally, pancakes are creamy golden yellow," he says. "These tend to be darker. The smell is musky, in the best of ways. You know the starter has been around for awhile. It's alive."

In an era where the world's best chefs are hacking industrial food and the growing ranks of curious amateurs embrace anything locavore and DIY, artisanal baking is staging a comeback. Commercial yeast? Too generic, too easy, too processed, too 20th century. Starter-born breads have personality; they show the maker's hand. In the most romantic light, an artisanal baker is a nano-forager, gathering from the wild the tiny elements that transform flour and water into loaves.

The mother, madre, starter, seed, chef, levain—by any name, it is the bubbling, breathing slick of wild yeast and Lactobacillus bacteria that feed on flour and water, creating the biochemical conditions to make bread rise. Simply put, the yeasts break down the flour into sugars and carbon dioxide; the bacteria ferment those sugars to make acid, creating a complex, pleasantly sour flavor. Starters can range from dough-like in texture to batteresque and are the color of whatever flour the baker uses.

Being the custodian of a starter is like being both the last name bearer in a line and the baby nurse to a billions-strong, highly cooperative microscopic family. "There was a period of time when I was too immature to have a starter," Angela Sherry, a passionate home cook, told me. "I would be crashing on a friend's couch and leave a jar of starter in the fridge. And then my friend's husband would say, 'It looked like it was growing mold. I threw it away.' Ahhhhhhh!" These days, Sherry tends a starter that's been kept alive since at least 1954, when her newly married grandparents bought it from the general store in Glasgow, Montana. Throughout her childhood—and even at her wedding brunch—her grandfather, Papa Leo, made sourdough pancakes, which he served with homemade chokecherry syrup and cheese. Fourteen years ago, Sherry moved from Seattle to Brooklyn by train, bringing Papa Leo's starter with her. When Vinegar Hill House opened down the street from her, she gave the chef a cup of starter to use for pancakes—which now have a cult following. As a houseguest, she always brings a cup of starter for her hosts; she wakes up early to make pancakes and knows there's one more person she can call if her power goes out and she loses her levain.

"It's a sense of responsibility, or guilt almost," says Floyd Mann, who runs the baking site The Fresh Loaf, about caring for a starter. (Users of his site give their starters names like Carl and Bubbles.) "It's not a sentient being, but people do treat it almost as a pet. If you neglect it and it dies, you feel very bad. I've had to remind myself, 'It doesn't feel any pain!'"

Making bread the old-fashioned way requires loverly ministration. The more serious and frequent the baker, the more lavish the attention: Starters in constant use require a steady supply of fresh flour and water to perform consistently. Chad Robertson, the breadmaker at Tartine Bakery in San Francisco and a leader of the wild-fermentation movement, used to bring his starter to the movies with him so that he could feed it on time. He has swaddled it in blankets to keep it warm. "I feed it three times a day," he says. "It has to be kept at the temperature we like—between 68 and 82 degrees—or it's not comfortable." He once packed it in a hard case with its preferred flour and took it on vacation to Europe. Many professional bakers also have stories about feeding their starters on trans-Atlantic flights. In Stockholm, a new center of artisan breadmaking, there is a "hotel"—actually some dedicated fridge space at a deli—where you can park your starter when you go out of town. For an extra fee, the cooks who look after it will bake your bread for you.

Before becoming a baker, Michael London, the co-proprietor at the renowned Mrs. London's bakery in Saratoga Springs, New York, devoted himself to poetry for 15 years. Bread occupies a mystical space for him. He says, "The bread is already there in the formative, etheric forces that surround us. It's like frost on the window, how you see leaf patterns. All I ever do is lasso the wild spores from the air and gather that into a culture. It really has to do with an inner culture." London guards his starter jealously. Once, he says, he came around the corner from the cooler and caught an employee skulking away with a bucket of it. "I told him I could help him with that, and I poured bleach in it," he told me. "I don't know what his intentions were." He paused and added, darkly, "He has since started a bakery."

In the early 1970s, two USDA researchers workingat a lab in Northern California began to publish the first scientific papers on the yeasts and bacteria that make up what they (regrettably) called the "mother sponge." Analyzing the starters from five bakeries in San Francisco—a city famous for its sourdough—the researchers identified a new species of bacteria, which they called Lactobacillus sanfranciscensis. Here, at last, was a scientific explanation for the excellence of San Francisco bakers' bread: They had the right bacteria.

In the years since, however, L. sanfranciscensis has been found in bakeries all over the world, and the understanding of the properties of starter is shifting. Rachel Dutton, a Harvard microbiologist who has examined miso from Momofuku in New York City and homemade yogurt from food scientist Harold McGee, as well as the starters from Tartine Bakery and Manhattan's Sullivan Street Bakery, says, "The old model was that everyone's starter was unique, and that it was constant." In reality, the community of microorganisms is much less diverse and far more adaptive. "The microbes have an incredible ability to evolve and can easily pick up characteristics," Dutton says. A starter born in San Francisco and moved to Boston will soon start dropping its Rs; it is reincarnated, as in Hindu cosmology, rather than eternal, as in Christian. The differences between starters, she believes, have much more to do with other variables, like the temperature at which they're kept and the kind of flour with which they're fed. And, by the way, the flour itself—not the air—is the most likely source for the wild yeast and bacteria. "You're 'lassoing' things that are on your grains," she says. "They're already in the bowl."

Starter demystified, though, is not starter without ceremony. Matt McDonald, who was the gruffly unsentimental head baker at Napa Valley's Bouchon Bakery for six years, says, "Idolizing 300-year-old starters seems like when-the-world-was-flat mentality. We just didn't know any better." But he has a starter ritual of his own. Whenever he begins working in a new kitchen, he, exorcist-like, amazes his superstitious underlings by killing the starter and mixing a new one. Where other bakers make a fetish of using grape must or bacteria-laden cabbage leaves to get a starter going, McDonald simply mixes flour and water. And, like Robertson, he freely gives his starter away. "I would have guests at the bakery all the time who'd say, 'Dude, I need your starter.' I'd be like, 'Have it, but I guarantee you that when you take it home, it'll become your starter.'"

Not long ago, I went to visit a small-scale Los Angeles baker named Mark Stambler, who last year helped pass California's Cottage Food Law, which allows home cooks to sell their products to the public. It was early afternoon at Stambler's house in Los Feliz, and the temperature in his kitchen was in the mid-80s. He was wearing flip-flops, and the lenses of his glasses were dusted with flour. On his bench sat an open Tupperware container of starter, for making batards of pain au levain. I studied it: A worn khaki color, it heaved gently, the chest of a deep-asleep child. Occasionally, a small bubble formed and popped, giving off a nice beery scent. The lid was marked April, 2013. "I believe in, once or twice a year, chucking it out and starting over again," he said.

I watched him do the "pre-ferments," taking 100 grams of starter, chopping it up in distilled water, and adding precise amounts of hand-milled flour. Early the following morning, he added more flour and water and let the dough rise in a big plastic bin on top of the stove. By the time I arrived, the masonry-brick oven he'd built in the back was at 600 degrees. Stambler opened the bin and scraped the dough onto his bench. Huge, gassy bubbles yawned, and long strands of gluten hung like streamers; cooked, they'd trap the gas and give the loaf its internal architecture. Over the next couple of hours, he shaped the loaves, let them rest, pounded them into batards, slashed them with a razor blade and eased them into the oven on big wooden peels. They came out 20 minutes later, caramel-colored and gently blistered. As soon as I was in my car and out of view, I ripped off a piece, burning my fingertips. The bread was mild, buoyant and sweet; I finished half the loaf before I got it home.

When I arrived at my house, there was a box on the porch. Inside was a cooler, and in the cooler was a bag filled with a tacky yellow-white mixture: Papa Leo's starter. Angela Sherry had frozen it and overnighted it to me from Brooklyn so I could have a mother of my own. In my email in-box was a picture of a beautifully mottled old recipe card, Germanisms and shorthand intact. "Saur Doe Pancakes," it said. "Put starter in 2 cups milk + 2 1/2 cups flour, stir well cover + let stand on cupboard over nite."

I checked it in the morning. With the milk and flour feeding, it had grown fizzy and lightly sharp, like the cidery smell of trees on a New England campus in fall. I added the rest of the ingredients—eggs, baking soda, sugar, oil, salt—and wrestled dabs of the sticky stuff into a skillet. In the midst of the chaos in my kitchen—my children, their friends, everyone wanting to crack an egg and control the whisk—I overlooked the second line of Leo's recipe, which Sherry had admonished me to heed: "Take out 1 cup starter + put in fridge." I was halfway through eating my pancakes—hefty but airy and wonderfully tart, perfectly pan-marked as only the last of a batch can be—when I realized what I had done. I had turned all of the starter to batter.

Sherry had encouraged me to experiment with the starter and pass it around to my friends. Her only request was that I not open a pancake house. When I called her and told her I had something to confess, her first wary question was, "Are you opening a restaurant?" On hearing my tale of woe, though, she said she'd be only too happy to send me some more. Secretly, I suspect she wants a stash of it in California as a hedge. I am just happy I'll be able to make Papa Leo's pancakes again. Though by the time I've fed the middle-aged, transcontinental microorganisms local milk and my favorite flour, and neglected them in my own inimitable way, it may be more accurate to claim the resulting pancakes as my own.

Dana Goodyear is a staff writer at the New Yorker and the author of the new book Anything that Moves.