Baking bread is applied chemistry. The transformation of flour and water into a risen loaf involves dozens of simultaneous chemical reactions, each with its own optimal conditions and failure modes. Understanding these reactions doesn't just satisfy intellectual curiosity — it helps you diagnose problems when things go wrong and make decisions when recipes don't quite fit your conditions.
The Gluten Network
When flour meets water, two proteins — glutenin and gliadin — combine and form gluten. Glutenin molecules link together into long, elastic chains. Gliadin adds viscosity and extensibility. Together they create a protein network that traps the carbon dioxide produced by yeast, allowing dough to rise and maintain its structure.
The development of this network is what "kneading" accomplishes. Without kneading, the gluten strands are short and disorganized, producing a dense, flat crumb. With kneading, the strands align and strengthen. The windowpane test — stretching a piece of dough until it's thin enough to see light through — indicates properly developed gluten.
But there's a limit: over-kneaded gluten becomes brittle and tears, producing a dense, tough crumb rather than an open, elastic one. The point of optimal development varies by flour and hydration, and you learn to recognize it through feel rather than time.
Yeast and Fermentation
Yeast is a single-celled fungus that consumes sugars (both naturally occurring in flour and produced by enzymes during fermentation) and produces carbon dioxide and alcohol as waste products. The carbon dioxide is what makes dough rise. The alcohol evaporates during baking, contributing to flavor and the "bread smell."
Yeast activity is temperature-dependent: between 26-32°C, yeast is most active without being stressed. Below 15°C, activity slows dramatically. Above 40°C, yeast begins to die. This is why bread doughs are typically kept in this temperature range during fermentation — cold fermentation (in the refrigerator at 4-8°C) dramatically slows fermentation, which can be beneficial for flavor development in sourdough.
Enzymatic Activity
Flour contains enzymes — primarily amylase — that break down starch into sugars. This is essential: the sugars feed the yeast, producing the carbon dioxide that makes dough rise. But if amylase activity is too high (common in flour stored in warm conditions), it can over-proof the dough, producing a sour, alcoholic smell and a collapsed structure.
Acidic conditions (from sourdough's lactic acid bacteria) inhibit amylase, which is part of why sourdough has a more predictable fermentation. This is also why adding a small amount of acidity (like a tablespoon of vinegar) to white bread dough can improve its keeping qualities.
Starch Gelatinization
When bread bakes, the starch in the flour absorbs water and gelatinizes — the starch granules swell and burst, releasing amylose and amylopectin that form a gel. This gel sets as the bread cools, providing the structure that keeps the loaf from collapsing. If bread is cut while still hot, the starch hasn't fully set, and the crumb can become gummy as the starch continues to gelatinize and absorb water.
This is why cooling bread fully before slicing is essential. The internal temperature should reach at least 93°C before you cut — at this point, the starch has fully gelatinized and the bread is structurally stable.