Would-be bread bakers, you're not mistaken. The spike in demand for flour and yeast that you're witnessing in supermarkets is real.

"At the beginning of the COVID-19 crisis, people worried about staying at home, and so they stocked up, a lot," said Mike Oase, chief operating officer of Kowalski's Markets, which has 11 Twin Cities locations. "People bought everything in sight. It was bigger than a Christmas week."

Oase said that flour supply lines are returning to normal. "And yeast will be back in stock in the next few weeks," he said.

The phenomenon of empty baking-aisle shelves did not go unnoticed at America's Test Kitchen, the Boston-based publisher of Cook's Illustrated and Cook's Country magazines.

The difficulty of finding yeast at the supermarket came up during an online staff meeting. That's when senior editor Andrew Janjigian — known to his fellow staffers as "the Bread Guy" and/or "the Bread Whisperer" — offered a solution.

"It's the perfect opportunity to start a sourdough starter," he said.

He'd tackled the subject in Cook's Illustrated about four years ago, but that formula called for two pounds of flour.

"Given how little flour there is out there right now, I decided to try it on a much smaller scale, and see how it works," it said.

Turns out, it works just fine. Janjigian began to document the process on his Instagram account (@wordloaf), and the experiment — which he dubbed #quarantiny-starter — went viral. "Or, as I like to say, 'fungal,' " he said with a laugh, mentioning that yeast is a fungus.

To date, he's playing a kind of sourdough Pied Piper to about 600 fellow bakers, following along from as far away as Australia and Malaysia. "The list of names that people are calling their starters is really good," he said with a laugh. They include Clint Yeastwood, Courtney Loave, Carrie Breadshaw, Bread Astaire, Holly Doughlightly and Quentin Quarantino, and no, Janjigian hasn't christened his.

(See the starter formula below.)

Because Janjigian has so many bakers following along in real time, he has fielded a lot of questions. Here he is on ...

Flour: "Ideally, you want to start with a mix of white with whole wheat or rye, because that extra nutrition will make things happen faster," he said. "But I've done it using only all-purpose. It has to be wheat flour and unbleached. The organisms that you get in sourdough don't come from the air or from your hands. It's from the flour itself. Ideally, it would be organic flour so that there are no fungicides, and you'd be using filtered or bottled water for that same reason. But this experiment is showing that it can work without ideal conditions." Note: Once you've hit the twice-a-day refreshing period, switch to using white flour only. The starter no longer requires the nutrients in whole grain flours.

Temperature: "The ideal temperature for sourdough is 78 degrees; that's when both bacteria and yeast are happiest," he said. "Too much higher, and it will kill all that, and too much lower and it slows everything down." If it's too cool, he suggests placing the container in an insulated cooler or freezer bag, or an unheated oven ("Don't forget it's in there, and turn on the oven," he said), or near a light bulb or a working appliance. "Something to keep the chill away," he said. "The only way to screw it up is to get it so warm as to encourage mold. That's about 90 degrees or higher."

Container: "It doesn't really matter, as long as it keeps the starter from drying out," he said. "Glass, plastic, Mason jars, deli containers. It's nice to have a clear, straight-sided container, so you can see when it's doubling and tripling."

"Feeding" vs. "refreshing" the starter: "Think about pets," he said. "You put food in a bowl, and they eat it. That's feeding. But here, you're moving your culture through fresh food and water each time. Rather than continually adding water and flour to the same container, it's the idea of taking a portion of the old culture and using it to inoculate a fresh container — you want the acids that they produce to be relatively dilute — so they can propagate in that new environment."

The schedule: "Those first two to three days are to get it alive," he said. "After that, you start refreshing it daily. After about seven days, it should be going enough so that you can do it twice a day. That's the typical schedule of a starter at a bakery: once in the morning and once in the evening. By that point, it should grow two to three times in volume every 12 hours or so."

His most frequently shared advice: "Just keep going," he said. "I think I've said that more times than anything else. After the first three days, you'll get a lot of activity. It's funky and bubbly, because the initial organisms wake up quickly, but then they die off. Then other things start growing, but they're not as obvious. That's when everyone thinks their starter is dead. That's when I say, 'Keep going, it'll come back.' And of course, it does."

Learning from his Instagram audience: "When I teach, it's mostly to people who have a little experience," he said. "I can make assumptions about what people know and don't know. But in the world of Instagram instruction, I have to assume that people know nothing. It's humbling, and it's been a good teaching tool and an exercise in patience."

Preserving leftovers: During the build-a-starter period, Janjigian advises against discarding the extra culture. "Save it," he said, reserving it for a day (in the refrigerator) as backup insurance; once you've refreshed the following day, and the need for backup has passed, the backup can be scraped into a collective "discard" jar in the refrigerator. "There's a whole world of cooking using sourdough discard," he said. "You can't use it to leaven bread, but you can use it for pancakes, waffles, quick breads, pasta dough, all kinds of things. I'm going to try some Korean-style pancakes."

Storage: Once it has hit its bread-making stride, the starter can be stored in the refrigerator, untouched, for a few months. "But remember, the longer it goes without being refreshed, the longer it takes to get back to its full vigor," he said. "Bring it to room temperature and refresh it a few times before using it. Bakeability and storeability are the two goals, so don't put it in the refrigerator until it's healthy. A three-week starter isn't as vigorous as a six-month starter. Just keep refreshing it on this small scale to keep it getting healthier and healthier. If you get to the point where you're using it once or twice every one to two weeks, and you refresh it before you put it back in the refrigerator, it will keep going forever."

Anyone with a passing interest in bread baking should follow this self-described "breadhead" online, and subscribe to his newsletter; sign up at wordloaf.substack.com. And, of course, read him in Cook's Illustrated, where he's been working for a decade.

"The first thing I learned to cook was pizza, and I became obsessed with it," said Janjigian. "But I got the bread bug right after the no-knead recipes came out in about 2006. That's when I realized how good homemade bread is, and what I'd been missing. That's when I wanted to master it."

Naturally, there's a Minnesota connection in all of this: Janjigian's spouse, Melissa Rivard, is a Twin Cities native. In their Cambridge, Mass., kitchen, he's been averaging two loaves of a bread a day during this shelter-in-place period. He's looking forward to warmer weather, when he will fire up their backyard pizza oven.

"It needs to be put to use," he said. "I may even bake some bread in it."