This NBC Learn video, one in a 6-part "Cheeseburger Chemistry" series, uses bread-making to illustrate and explain how yeast works to convert starches and sugars in flour to CO2 gas (fermentation); effects of heat on gas; and gluten protein structures.
The Chemistry of Bread
AL ROKER, reporting: It doesn’t usually get much attention, even though it’s the foundation of every cheeseburger and what tops it off.
MARY WIRTH, Analytical Chemist, Purdue University: When people think about cheeseburgers or any kind of sandwich, they usually focus on what’s inside of it. But really, another important part of it is the bun, the bread.
ROKER: Making bread may not seem like much of a chemistry experiment – but it is.
Julie Yu is a scientist at The Exploratorium in San Francisco, who’s funded by the National Science Foundation.
JULIE YU, The Exploratorium When you go to the store these days and you go to the baking section, there’s a huge variety of flours.
ROKER: Flour is grain, most commonly wheat, that’s harvested, winnowed, then ground into powder – basically, pulverized starches, also known as complex carbohydrates, or, as chemists call them, polysaccharides: chains of sugar molecules. By whatever name, they are the main ingredient in bread, along with a little salt, some yeast, plus water.
YU: I have some warm water here and I’m going to add two tablespoons of water to each of these bowls and this is to activate the yeast.
ROKER: More on yeast in a minute. First: what happens when these ingredients mix.
YU: I’m going to add the flour. This is mixed together and you can see that the flour kind of gums up, it soaks up all the water. As I mix this, the water is getting in touch with all of the proteins in the flour. The proteins in the flour actually create a protein matrix called gluten.
ROKER: Gluten proteins are elastic; a gluten matrix is like a web of rubber bands, and is what holds bread dough together, gives baked bread its structure.
WIRTH: Gluten is actually based on the Latin word for glue. So gluten is a protein that acts like glue.
ROKER: To develop gluten for bread that’s ideal in texture and taste, bread-makers do this: knead the dough. Mary Wirth is an NSF-funded analytical chemist at Purdue University – and a practiced bread-maker.
WIRTH: As you’re kneading the dough what you find is it gets stretchier and stretchier. Kneading the bread helps mix things together well, so that the yeast are constantly finding new starch and it also helps the glutens form.
ROKER: Yeast – we said we’d get back to that. It’s been a key part of the chemical reactions going on since the dry ingredients met the wet.
WIRTH: You mixed the flour with water and yeast. And the yeast start to eat the starch. Yeast is a living thing, a single-celled creature that likes to eat a carbohydrate – it likes sugar and it likes starch.
ROKER: Think of yeast like this, doing this to sugar and starch molecules.
WIRTH: What the yeast does is, it consumes starch in the bread and produces carbon dioxide as a byproduct.
ROKER: Carbon dioxide, or CO2…a gas. Yes, the yeast expel gas after eating, like burping.
YU: We’re going to put these in a warm place.
ROKER: Gas, one of the common states of matter, expands when it’s heated, even just warmed for an hour.
YU: This dough has risen and you can even see bubbles inside of the dough. That’s the carbon dioxide made by the yeast.
ROKER: Just so you know, that process, microbes converting starches and sugars to gas, is called fermentation. The more CO2 gas is released, the more air bubbles there are; the warmer that gas, the larger the air bubbles. Just the way helium gas expands and inflates a balloon; the CO2 gas expands and inflates the elastic dough. Remember all those stretchy gluten proteins? It expands even more when heat is increased, when the dough is put into a hot oven to bake, usually at 400 degrees Fahrenheit or higher.
WIRTH: Now it’s hotter and the yeasts become even more active until they’re consuming the starch in the bread faster, producing carbon dioxide faster.
ROKER: But yeast microbes, like almost all micro-organisms, cannot survive high heat for long and many breads bake in hot ovens for almost an hour.
WIRTH: Then it’s too hot and the yeasts are less active and eventually killed.
ROKER: With no more gas produced, expansion stops; air pockets set, giving well-made bread, slices or buns, pillowy texture and light taste. So that’s it: a top-to-bottom look at the top and bottom of a cheeseburger.
Newsela Editor's Note: French scientist Louis Pasteur found ways to preserve beer and wine, and to fight diseases such as rabies and anthrax. This article originally appeared in the November 1884 issue of Scientific American.
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