The Science of Making a Wild Sourdough Starter

Wild Sourdough
A recently-fed sourdough starter bubbles with activity from natural yeast. (Credit: Wild Sourdough Project)

By now, you’ve  almost certainly heard the news: Homemade sourdough is the  greatest thing since, well, sliced bread.  Being stuck in quarantine gives many of us more time to do things around the house, like baking. And stores are short on household staples, including bread, so, sure, might as well give breadmaking a try. But why sourdough and not brioche, or a French baguette?

In terms of ingredients, it rarely gets any simpler than bread. Flour, water, salt and, unless  you’re  making flatbread, yeast.  For many of us, yeast is just something that comes from the grocery store in a little packet  or squat brown  jar, and  looks  unsettlingly like fish food  when you open it. Along with self-rising flour,  prepackaged baker’s  yeast disappeared  from most grocery store shelves  long ago.


Take Part: Make Your Own Sourdough Starter for Science

wild sourdough loaf
(Credit: Lauren Nichols)

Sourdough Starter to the Rescue

This is where sourdough makes its entrance into the quarantine zeitgeist.

Sourdough  doesn’t  need baker’s yeast. Instead, it’s made with sourdough starter, which promotes the growth of wild, naturally occurring yeasts from its environment. This process may seem fantastical, but there’s no magic involved. Sourdough starters simply expose the mundane  and  abundant presence of yeast in our world, and,  in particular, in our homes.

 This flour and water slurry  is an incubator,  providing  a small home where, over the course of a couple of weeks,  specific microbes  like bacteria and yeasts can effectively set up camp and exclude other microbes like molds.  Generally speaking, leaving out a petri dish for microbes to colonize  is a recipe for a health hazard. So how is sourdough different?

It isn’t, at first.

Young sourdough starters contain many opportunistic generalists that grow equally well on plants, animals, and soil. But once naturally-occurring yeast and lactic acid bacteria set up shop, they produce alcohol, lactic acid, and acetic acid. These substances prevent the growth of other life forms that can be harmful to human health, ultimately creating a little haven of bread-making specialists.


sourdough sliced open

(Credit: Lauren Nichols)

The Origins of Bread

If baking your own bread feels  nostalgic,  that’s  because it  does harken back to an earlier time.

Scientists are still  following  literal  breadcrumbs  back  to the origin of  bread baking, but a recent  archaeological discovery of burnt bread fragments  near  an ancient  hearth  in Jordan  dates the earliest known  bread  to  around 14,000 years ago.  These leftovers from a tribe of hunter-gatherers predate the invention of agriculture by about  4,000 years, suggesting that the practice of baking bread is  even  older than  the practice of farming grains.

We’ve  been fermenting for so long that, in a way, the microbes have domesticated us, too.

Have you ever experienced that  seemingly Pavlovian  response when you think about bread? Saliva is our body’s first wave of digestion. Spit  contains  amylase, an enzyme that breaks starch into sugars. So your “I’m hungry” drool response means that your body recognizes a food as, well, food.

Saltine crackers and many store-bought breads lack the acids produced in sourdough. As a result, they tend to dry your mouth out, instead of inducing salivation. But the moment you take a bite of sourdough bread, your flavor receptors light up and your body says “time to digest this delightful meal,”  starting with a rush of saliva.

Our ancestors  couldn’t  source  their  yeast from a grocery store.  They  needed the wild yeasts in their environment to ferment  their dough,  just like the quarantine bakers of the present.

So, our relationship with yeast is ancient.  But even though our  forebearers were scientists in their own right — harnessing  the natural processes of  fermentative  microbes  to  create  all kinds of novel food sources, from bread to kimchi, beer and wine, pickles, yogurt, and more — there is still so much to learn about this  microbial  world.  Over the millennia,  humanity has had ample time to observe  the  effects  of  these  microbial  communities in our baking (think  tangy  flavor and bread rise).


sourdough science
(Credit: Neil McCoy)

A Mystery at the Heart of Bread Baking

But now we  have the opportunity to  find out which types of  microbes  help cause these effects.

Over the last couple of  years,  people  all around the world  have  joined  the Science of Sourdough projects created by Rob Dunn’s lab in the department of Applied Ecology at North Carolina State University, to contribute data  and  learn about  yeasts  and  bacteria so diverse  that  scientists  haven’t  even named  them all yet.  What better time for us to  crowdsource  new  data from sourdough starters  than now, when sourdough is having such a moment?

That’s why Erin McKenney, Lauren Nichols, Anne Madden, and Rob Dunn have launched a new citizen science effort called the Wild Sourdough Project. The experiment’s goal is to reveal how sourdough starter communities form over time, and to understand how factors like geography and the kind of flour used impact these communities.

You’ll learn how to make a sourdough starter from flour and water, and we’ll help you bake with your sourdough starter using our basic sourdough bread recipe.

In a time when a lot of the press about microbes is negative,  we’re  excited to focus on the bacteria and yeasts that help us thrive.

The yeast species living in sourdough starter are fungi, which are more closely related to us  humans  than they are to their bacterial neighbors, who we also need in some combination to get our delicious bread. The relationship between yeast and  lactic acid bacteria  — much like the relationship between humans and our sourdough starters — transcends species.   So, much of this experiment  is focused  around getting to know the life of sourdough better.  But your starter’s story is also about you. How has your life with your sourdough starter changed you — and,  in particular, your attitude  toward  microbes?

As we all search for life together through this experiment,  remember that  the vast majority of  microbes, those living in  us, or on us, or around  us, are  not harmful.  In fact, many  of  the  microbes  around us  are  absolutely necessary  for human life to exist — microbes help, and are necessary  for,  our survival. And  some, of course, do some  pretty cool  stuff for us too — like helping us to make  bread.

Find more citizen science projects using our Project Finder. 

Erin McKenney is director of undergraduate programs in the Department of Applied Ecology at North Carolina State University. Peregrine Bratschi and Max Cawley work in the Innovation and Learning Department at the Museum of Life & Science in Durham, North Carolina.

Categories: Biology, Citizen Science, Other

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