Wheat becomes bread through a chain of transformations: the grain is milled into flour, mixed with water to form an elastic dough, fermented by yeast to create gas bubbles, and baked at high heat to set the structure and brown the crust. Each step involves its own chemistry, and understanding them reveals why bread has the texture, flavor, and appearance it does.
What’s Inside a Wheat Kernel
Every wheat kernel has three parts. The endosperm, the largest section, is packed with carbohydrates and protein. The bran is a fiber-rich outer shell containing B vitamins, iron, zinc, and magnesium. The germ is the tiny embryo at the kernel’s core, rich in healthy fats, vitamin E, and antioxidants.
For white flour, the bran and germ are stripped away during milling, leaving only the endosperm. Whole wheat flour keeps all three parts intact, either by grinding them together in a single pass or by separating and recombining them in their original proportions. This distinction matters for bread: the bran and germ add fiber and nutrients but also interfere with gluten development, which is why whole wheat loaves tend to be denser.
Milling: Turning Grain Into Flour
Before anything else, the raw wheat is cleaned. Scalpers pull out sticks and stones, screens sort by size, magnetic separators catch metal fragments, and disc separators reject kernels that are too large or small. A scourer then throws each kernel against a rough surface, buffing it and loosening the outermost bran layer.
Next comes tempering, a step that might surprise people. The cleaned wheat is soaked with a controlled amount of water and held in bins for 8 to 24 hours, depending on whether it’s a soft or hard wheat variety. This added moisture toughens the brittle bran so it peels away in larger flakes instead of shattering into tiny fragments that contaminate the flour.
The actual grinding happens through a series of corrugated steel rollers. The first set, called the “first break” rolls, cracks kernels into coarse pieces. Those pieces pass through a sifter with as many as 27 progressively finer screens. Larger chunks move on to a second set of break rolls spaced more tightly together, and the cycle repeats through four to six stages. At each stage, some fine flour sifts to the bottom while coarser fragments called middlings are sent to purifiers, where air currents blow away bran particles and additional screens sort by size and quality. The result is a fine, uniform powder.
Why Protein Content Matters
Not all flour works equally well for bread. The key variable is protein content. Bread flour contains about 12.7% protein, all-purpose flour about 11.7%, and pastry flour around 8%. Higher protein means more raw material for gluten, the stretchy network that lets dough trap gas and rise. Pastry flour makes tender biscuits precisely because it forms less gluten. Bread flour makes chewy, well-risen loaves for the opposite reason.
Mixing: Building the Gluten Network
Flour contains two families of proteins that are essentially dormant until water arrives. When flour and water combine, these proteins hydrate and begin linking together. The larger proteins form three-dimensional networks held together by strong chemical bonds between sulfur atoms on neighboring protein chains. The smaller proteins nestle into these networks through weaker attractions, particularly hydrogen bonds, adding plasticity.
Kneading accelerates the process. Inside each grain of flour, proteins are compartmentalized within the remnants of the plant’s cell walls. Mechanical mixing tears those compartments open and stretches the emerging protein strands into a continuous, elastic sheet. You can test this yourself: a freshly mixed dough tears easily, but after 8 to 10 minutes of kneading, you can stretch a small piece thin enough to see light through it. That “windowpane” is the gluten network at work.
Fermentation: How Yeast Makes Dough Rise
Yeast is a single-celled fungus that eats sugar and exhales carbon dioxide and a small amount of alcohol. In bread dough, the sugar comes from the flour itself. Wheat contains a starch-digesting enzyme called alpha-amylase that breaks long starch chains into smaller sugars yeast can consume. This conversion begins as soon as flour meets water and continues throughout fermentation.
When yeast cells metabolize those sugars, they release carbon dioxide gas. The gluten network acts like a balloon: stretchy enough to expand around the gas bubbles but strong enough not to pop. Over the course of one to two hours at room temperature (or much longer in a cold fermentation), millions of tiny gas cells inflate throughout the dough, roughly doubling its volume. Yeast also generates flavor compounds during this time, which is why a longer, slower rise generally produces bread with more complex taste.
What Happens Inside the Oven
Baking transforms soft, sticky dough into a rigid loaf through several overlapping physical and chemical changes, all driven by rising temperature.
In the first few minutes, the gas cells that formed during fermentation expand rapidly as they heat up. This burst of expansion, called oven spring, can increase the loaf’s volume by 30% or more. It occurs because carbon dioxide and alcohol vapor expand with rising temperature, and because the remaining yeast cells enter a final frenzy of activity before the heat kills them. Doughs with well-developed gluten networks handle this expansion best, stretching more than 100% without rupturing.
Between about 60°C and 80°C (140°F to 176°F), the starch granules in the dough absorb surrounding water and swell irreversibly, a process called gelatinization. At the same time, the gluten proteins coagulate and stiffen, the way egg whites firm up when cooked. Together, these two changes convert the stretchy dough into a solid crumb. This is why underbaked bread is gummy in the middle: the starch hasn’t fully set.
How the Crust Gets Its Color and Flavor
The interior of a baking loaf never rises much above 100°C (212°F) because it’s full of moisture. But the surface dries out quickly, and once it passes roughly 160°C (320°F), a reaction between amino acids from the flour’s proteins and reducing sugars kicks into high gear. This reaction produces hundreds of new flavor and aroma compounds along with brown pigments called melanoidins. It’s the same chemistry responsible for the color of seared steak and roasted coffee.
The higher the oven temperature and the longer the bake, the darker and more intensely flavored the crust becomes. A pale, soft crust baked at lower heat tastes relatively bland by comparison, because fewer of those flavor compounds have had time to form. Steam injected in the first minutes of baking keeps the surface moist a little longer, allowing more oven spring before the crust hardens, and also promotes a glossier, crackly finish on artisan-style loaves.
From Wheat Field to Finished Loaf
The entire journey relies on a chain of precise conditions. Milling must separate the endosperm cleanly. Hydration and kneading must build a strong gluten network. Fermentation must generate enough gas at the right pace. And baking must set the structure and develop the crust before the loaf dries out. Change any one variable, like protein content, fermentation time, or oven temperature, and you get a noticeably different bread. That sensitivity is exactly what makes bread baking both a science and a craft.