What do flour and water have to do with yeast? (Photo by Jared Soares for The Washington Post)

Next week, Jews all over the world will be backing out of brunch.

“I can’t eat bread,” they (or you) will say.

Did we all hop on the gluten-free train at once? Nope, it’s Passover, and for a week the only bread observant Jews are allowed is matzah. While the reasoning is “God said so,” the science is basic biochemistry – thanks to the single-celled microorganism known as yeast.

Matzah, for the uninitiated, is a bland, cardboard-thin, cracker-like, constipation-inducing flatbread. On top of the matzah requirement, observant Jews must also avoid chametz: any wheat, rye, barley, oats or spelt that’s been in contact with any water at all for more than 18 minutes.

That rules out lots of foods, like French toast, pizza, cookies, pretzels, pasta and croissants (although it does not necessarily rule out chicken tenders). But it seems like a weird rule at face value. Why hate on wet wheat?

First, the "God said so" version: Jewish people have these traditions because, as the story goes, they were held as slaves by the ancient Egyptians. The slaves found an opportunity to run away, but had to leave really quickly and didn’t have time to let their bread rise before they baked it, according to my elementary Sunday Hebrew school teacher. It might just be a story, but stories have a way of turning into, you know, stuff your Sunday Hebrew school teacher teaches.

Another reasoning for the ban is that Egyptian captors are thought to have discovered the fermentation process that allows bread to rise. The Jews stopped eating leavened bread as a way of rejecting Egyptian culture, according to the blog of Tzvi Pittinsky, the director of educational technology at the Frisch School, a Modern Orthodox yeshiva high school in New Jersey. While it's hard to know when humans first accidentally started leavening their bread, it's true that the first evidence of fluffy baked goods comes from ancient Egyptian hieroglyphics. So even if Egyptians weren't the first group to rely on yeast in their bread making, it was certainly a big part of their local cuisine.

But the tradition of eschewing chewy bread wouldn’t exist without Passover’s petite pariah, yeast, and its secret biochemical weapon, the fermentation process. And that's where the larger ban on chametz comes in.

Yeasts are a family of single-celled organisms – tiny blob-beings – far more complex than any bacterium. Like cars burning gasoline, yeasts are powered by sugars (the gasoline) and spit out an exhaust of carbon dioxide and alcohol.

When a yeast cell meets dough, it munches up some of the carbohydrates, which are made up of sugar, and spits out lots of carbon dioxide bubbles. When dough rises, it's because it's literally being lifted up by all those bubbles. The little air bubbles increase the size of the dough and the resulting baked good, but that extra space is full of air. The resulting texture is what makes leavened bread chewy instead of crispy.

That explains why bread with yeast in it is off limits, but the water-flour combo goes a bit further – and the science behind it is pretty fascinating.

When the ancient Egyptians were baking bread, they didn’t know anything about microorganisms; they just knew that if dough sat around for a while, it would rise. That's because yeast is everywhere. Like, literally everywhere. It's true that modern bakers have a lot more success when they open up a packet of store-bought baker's yeast and pour it on in, but you can actually attract the stuff to your dough just by leaving it out in the open.

Together, water and flour make a nice environment – and a nice meal – for the vagrant yeast cells wandering through the air. In fact, sourdough bread gets its unique flavor from natural yeast. To make it, you create a home for vagrant yeast (flour and water) and hope that the ones hanging out in your hood are going to make nice bread. Then feed them (with more flour and water) until you've got a nice, yeasty glob going.

That's why bakers covet the "starters" of particularly tasty sourdoughs. All it takes is a bit of dough full of the right microbes to replicate San Francisco sourdough in South Carolina.

I won't pretend to know what the folks who started the matzah tradition were thinking, but it's true that if you want to avoid leavening bread and don’t know what yeast is, one way is simply to avoid mixing flour and water.

Ryan Mandelbaum is serious about science, pizza, Guy Fieri and not much else. He enjoys writing about the environment, physics and the macabre. 

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