How Bread Works
Gluten, fermentation and the alchemy of a crust
Bread is water, flour, salt and time — four ingredients that have fed more people for more centuries than any other preparation. The apparent simplicity is deceptive. What actually happens between mixing and the finished loaf involves some of the most interesting chemistry and microbiology in the kitchen.
There are four ingredients in a basic bread: flour, water, salt and a leavening agent. Everything else — the crumb structure, the crust, the flavour, the shelf life — emerges from the interactions between these four things, the hands of the baker, and time.
Understanding what happens at each stage does not require a biochemistry degree. It requires knowing, essentially, two things: what gluten is and what yeast does.
Gluten: the structure
Wheat flour contains two proteins — glutenin and gliadin — which are present separately in the dry flour but which, when hydrated and worked, combine to form gluten: a stretchy, elastic network of protein chains that gives dough its characteristic feel.
Gluten development is primarily a function of time and mechanical work. Mixing flour and water activates the proteins and begins their alignment. Kneading extends and aligns the protein chains into a more organised network. Stretching and folding the dough over time (the technique used in many sourdough methods) achieves the same thing with less mechanical violence, preserving the gas bubbles produced by fermentation.
The strength of the gluten network determines the bread's final structure: a strong gluten network traps the CO₂ produced by yeast, allowing the dough to rise and hold its shape. A weak network — from under-working the dough, too much water, or over-fermentation — produces a flat, dense loaf.
Yeast: the fermentation
Commercial yeast (Saccharomyces cerevisiae) is a single-celled fungus that consumes the simple sugars in flour and produces CO₂ and ethanol as waste products. The CO₂ inflates the gluten network; the ethanol evaporates during baking. This is what makes dough rise.
Wild yeast (present in a sourdough starter, also primarily Saccharomyces cerevisiae along with other wild strains) works through the same mechanism, but with two important additions:
**Bacteria.** A sourdough starter is not simply wild yeast — it is a co-culture of wild yeast and lactic acid bacteria (primarily Lactobacillus species). The bacteria produce lactic acid and acetic acid as fermentation byproducts. These acids are responsible for the sour flavour of sourdough and also contribute to gluten development (acid strengthens gluten) and improved digestibility (acid breaks down some phytic acid, which inhibits mineral absorption).
**Time.** Sourdough fermentation is slower than commercial yeast fermentation. This extended time allows more enzymatic activity in the flour — amylases breaking down starches into sugars, proteases partially breaking down gluten — producing a more complex flavour and an improved crumb structure.
Why retarded dough tastes better
Retarding — slowing fermentation by chilling the shaped dough in the refrigerator overnight — is the technique responsible for the deeper, more complex flavour of many good bakery breads. The cold does not stop fermentation entirely but slows it dramatically. Over 8–16 hours in the refrigerator, the bacteria continue working at a reduced rate, producing more acetic acid (which requires longer fermentation at lower temperatures than lactic acid). The result is a more pronounced sour flavour in the finished loaf.
This is why a sourdough baked the same day tastes milder than one that was retarded for a cold overnight proof.
The crust: Maillard and steam
The crust develops in two stages in the oven.
The first stage — the first 15–20 minutes — requires steam. As the dough enters a hot oven, it undergoes "oven spring": a rapid final expansion as the yeast is briefly activated by warmth before being killed by heat above 60°C, and as the gases in the dough expand. If the oven environment is dry, the crust sets too quickly and cannot expand. Steam — from a tray of boiling water placed in the oven, or from baking in a closed vessel like a Dutch oven — keeps the crust pliable during this initial expansion.
The second stage — after the steam has dissipated — requires dry heat. The crust must dry out to develop the Maillard browning and caramelisation that produce its colour and flavour. The sugars in the dough surface react with the amino acids from the flour proteins, producing hundreds of new flavour compounds: the complex, slightly sweet, roasted character of a good crust.
The crumb
The open, irregular crumb associated with good sourdough results from several factors working together: a strong gluten network that holds large gas bubbles without collapsing; adequate hydration (a wetter dough produces a more open crumb but requires more skill to handle); and proper fermentation that produces the right amount of gas and the right timing of fermentation activity.
A dense, tight crumb in a sourdough loaf usually indicates: insufficient gluten development, inadequate or over-fermentation, or too little hydration. Each has a different fix.
The home oven problem
Home ovens cannot produce the bottom heat of a professional deck oven. The workaround is a preheated baking stone or steel (preheated for at least 45 minutes at the oven's maximum temperature), which provides a large thermal mass that transfers conductive heat directly to the base of the loaf.
Baking in a preheated Dutch oven (a cast-iron casserole) solves both problems simultaneously: it creates a steamy environment for the first 20 minutes (remove the lid for the final 20–25) and provides direct conductive heat from beneath.
Bread is unforgiving of shortcuts. But understanding the principles — gluten, fermentation, crust development — means you can diagnose failures and iterate intelligently rather than following recipes blindly.