The Magical Transformation of Bread

You may eat your PB&J sandwich without even thinking about what’s holding it together. But without bread, you won’t have a sandwich. You’ll just have a mess. To people from all around the world, bread is an important, familiar food. But it’s also an amazing transformer. To turn into those soft, chewy, and delicious slices of bread, your loaf had to go through an extraordinary series of changes.

Who Kneads Dough?

You only need four ingredients to make bread: water, flour, salt, and yeast. Yeast is a type of microbe, a living organism too small to see.

Flour is dry and powdery. It comes from ground-up grains of wheat. When you add water, the flour absorbs the liquid. The mixture forms clumps. If you continue mixing, you’ll get a big lumpy blob. As you knead it—stretch it, roll it, and flatten it—the blob soon becomes smooth. You can turn it to almost any shape. You have turned flour and water into dough.

This magical transformation requires proteins. Proteins are molecules found in all living things, from plants to people. There are two main types of proteins in flour: glutenins and gliadins. Together, gliadins and glutenins form a network of proteins called gluten, which gives bread its wonderful texture and shape.

Glutenins are why dough is stretchy. When flour absorbs water, the water allows the glutenins to move around. As you knead the dough, the glutenin and gliadin molecules start joining together. They form a stretchy network, like a net made out of rubber bands.  

The more you knead, the stiffer and more rubbery the dough becomes. “The classic example is to think about pizza dough,” says Andrew Ross, a scientist at Oregon State University. He studies wheat and is also a baker. “If you push it hard and try to spread it out into a disk, it fights back.” But if you leave the dough alone, those networks relax.

The other kind of protein, the gliadins, helps dough flow and soften. Without gliadins, your dough would be like a rubber ball. And the resulting bread would be dense and hard. But without glutenins, your dough won’t be able to hold its shape. It would spread out like a pancake.

Let It Rise

After kneading your dough, you must let it rest. It’s time for the yeast to do its job. These tiny critters eat the sugar in the flour. As they eat, they produce gas called carbon dioxide. It’s similar to how you exhale carbon dioxide when breathing. The gas gets trapped inside the stretchy gluten network of the dough. This forms air bubbles inside bread. The bubbles create a sponge-like structure that makes bread light and fluffy.

Yeasts also make different chemicals that give bread tasty flavors and smells. In sourdough bread, there are also microbes called bacteria. The bacteria produce interesting flavors, including a type of chemical called an acid, which can make the bread a little sour. Bread needs salt too. Salt strengthens dough and makes your bread taste good. It also helps prevent the yeast from producing too much carbon dioxide too fast.  

Into the Oven

When your dough has the perfect balance of stretchy gluten and air bubbles from the yeast, it’s time to bake. “Baking is one of the most fascinating transformations in cooking,” Ross says. “You put a lump of dough in an oven and a loaf of bread comes out.”

During baking, a lot of chemical reactions are happening. As the dough heats up, the yeast becomes more active and makes more carbon dioxide. The gas bubbles also expand like lots of tiny balloons, forcing the dough to rise even more. But when the dough gets hot enough, the yeast and bacteria die. Their job is done.

Flour contains starch. Starch is made up of sugars, which provide energy for people who eat the bread. In the oven, as the starch cooks, it pulls water from the gluten. Without water, the gluten becomes stiff, forming the structure of the bread. The starch absorbs the water. It expands, becoming soft and moist.

On the outside of the dough, the crust starts to form. Heat causes a series of chemical reactions with the sugars and other compounds in the dough. They’re known as Maillard reactions, and they are the same reactions that turn food brown during grilling, frying, and roasting. The dough hardens and becomes crunchy and crackly. It turns brown and creates toasty flavors that taste and smell delicious.  

Staleness Happens

Even after it comes out of the oven, a loaf of bread keeps changing. As soon as it starts to cool, it starts getting stale. It will slowly harden and won’t taste as good. The bread seems like it is drying out. But what causes staleness isn’t water loss.

One part of starch is a molecule called amylopectin. It has a lot of branches, like a tree. When you take the bread out of the oven, the amylopectin molecules start to tie together, forming crystals. Water gets trapped inside the crystals. After a few days, so much of the amylopectin has turned into crystals that the bread becomes hard and seems dry.

To make stale bread more appealing, it’s best to put it in the oven or toaster. The heat unwinds the amylopectin, letting out the water. It’s no longer a crystal. The bread becomes softer and moister again.

Over time, though, the bread loses flavor and aromas. So even if you can soften it again, it may not be as tasty as it once was. Still, it’s probably good enough for another PB&J.