HILLSBOROUGH — A gloppy mixture bubbles to life in Emily Buehler's makeshift chemistry classroom, which is actually her kitchen in Hillsborough.

It's a cold, drizzly weekend in January, and Buehler, who holds a Ph.D. in chemistry, is putting her know-how to work with flour, water and yeast. The result will be two batches of bread - English muffins and a French baguette.

The exercise is a dress rehearsal for a weeklong course, "The Science of Bread," Buehler is teaching this month at the John C. Campbell Folk School in Brasstown, in Western North Carolina.

Have you ever wondered why bread rises? What makes crust crispy? Why sourdough bread tastes different from French?

From the molecular structure of gluten to shaping the perfect baguette, Buehler's course aims to demystify the science and craft behind one of our most familiar foods.

Bread science isn't entirely new. Buehler and her students are part of a growing fleet of artisan bakers and kitchen chemists, working to revive small-batch baking techniques that are hundreds of years old.

Buehler first discovered these techniques 10 years ago, after finishing her doctorate atUNC-Chapel Hill.

Looking for a change of pace after earning her degree - a dissertation on reaction dynamics on semiconductor surfaces - Buehler started working as a bread baker at Weaver Street Market in Carrboro.

Before long she was teaching bread-making classes, and eventually wrote a book on "Bread Science: The Chemistry and Craft of Making Bread" (Two Blue Books, 2006).

Dough, deconstructed

After mixing in more flour, water, yeast and salt, she transfers the sticky mass to the countertop and begins to knead: pressing the heels of her hands into the dough, folding it over, and pressing again with a rocking motion.

Invisible to the eye, an elastic network begins to form within the dough. Protein molecules in the flour called glutenin and gliadin link up during kneading to create a larger complex called gluten, Buehler explained.

Gluten is what makes bread chewy. Think bagels, or pizza crust.

During the kneading process the coiled, kinky gluten molecules cross-link into a stretchy 3-D latticework, Buehler said.

As she continues to knead, the dough is transformed from a sticky, shaggy mess into a smooth, satiny ball that is delightfully malleable; when fully kneaded and pinched between the fingers it has the consistency of ear lobes.

Yeast 101

Buehler transfers the dough to a bowl and covers it with a towel. An hour later, the dough has doubled in size.

The powerhouses behind this transformation, she explained, are tiny microorganisms - single-celled fungi called yeasts. Too small to see with the naked eye, yeasts take in oxygen and expel carbon dioxide, just as we do when we breathe.

Enzymes in the flour break down starches into simpler sugars, which yeast and bacteria eat, Buehler said.

Given enough food, time, and a comfortable temperature, the yeasts grow and multiply, producing gas and alcohol as a byproduct through theprocess of fermentation.

"Bread is essentially yeast burps and sweat," said Peter Reinhart, baking instructor and faculty member at Johnson & Wales University in Charlotte.

The same basic process is what makes champagne bubbly and beer foamy. Although there are hundreds of different yeasts, we use the same species, Saccharomyces cerevisiae, for both baking and brewing.

"Beer is basically liquid bread; they were invented around the same time," Reinhart said.

On the wild side

Long before we could buy yeast in the grocery store, people made bread by letting mixtures of flour and water spoil naturally.

Wild yeasts in the air and on the surface of grains make themselves at home in the flour-water mixture, producing lactic and acetic acids that give breads such as sourdough their distinctive tangy taste.

"When we use baking powder or baking soda, we're mixing alkaline and acid to make carbon dioxide, but in a chemical way rather than a biological way," he said.

But in yeast breads, as the dough ferments, it fills with gas and inflates from within. Millions of tiny bubbles get trapped in the gluten and expand, creating the characteristic air holes in a slice of bread.

"Some people think of it like tiny balloons," Buehler said.

Gluten gives dough its strength and stretchability, but is also a source of gastrointestinal distress for people with celiac disease or other gluten insensitivities, Reinhart said.

Reinhart, author of "Whole Grain Breads" (Ten Speed Press, 2007) and five other books on bread-making, is now hard at work on a gluten-free baking book.

The challenge for gluten-free bakers, he explained, is to mimic the spongy texture of bread without using wheat flour.

"Without the gluten, other grains don't provide a lot of rise," Reinhart said.

Buehler punches the dough to get the gas out, folds it over and lets it sit for an hour to rise again.

Bread-making is not for the hurried.

For artisan bakers such as Reinhart and Buehler, the secret to great flavor is not sugar or shortening, but time. Longer fermentation gives yeast more opportunity to do its work.

"Grains don't have much flavor when you just eat them plain," Reinhart said. "Long, slow fermentation releases the flavor that's trapped in the starch."

From four simple ingredients - flour, water, yeast and salt - the magic of fermentation produces hundreds of organic molecules. The longer the dough ferments, the more flavor it has.

"Scientists have identified nearly 300 different flavor molecules in bread dough," Buehler said.

Into the oven

An hour later, Buehler rolls the dough into a log, lets it rest for a few minutes, then carefully folds and stretches the dough into a baguette shape for the final rise.

When the dough is ready, she scores the top with a knife, then puts it in the oven to bake.

Once in the oven, the heat kills the yeast and burns off the alcohol.

"The yeast dies as soon as the interior temperature of the dough crosses 140 degrees. We call it the thermal death point," Reinhart said.

The outside of the dough reaches a higher temperature than the inside, initiating browning reactions that give bread its characteristic crust.

"Sugars and amino acids harden and darken on the outside of the dough to give a beautiful brown crust," Reinhart said.

The same reactions are responsible for the color and flavor of roasted coffee, seared meat, caramel and maple syrup.

Before long, the aroma of fresh-baked bread fills the kitchen. Buehler pulls a hot, crispy baguette from the oven, lets it cool, then tears off a piece and pops it in her mouth.

The best part of this science lesson, she said, is eating the results.

ras10@duke.edu