The Science of Bread — Why Dough Rises, Crusts Brown & Crumbs Form

Bread Is Applied Chemistry

When you bake bread, you're performing a sequence of chemical and physical transformations so elegant that scientists have spent centuries studying them. Understanding the why behind each step transforms you from someone who follows recipes into someone who can invent them. You'll know when to break the rules because you'll understand what the rules are protecting.

Let's walk through the five key scientific processes that turn flour and water into bread.

1. Gluten: The Structural Network

Flour contains two proteins — glutenin and gliadin. On their own, they do nothing. But when hydrated (mixed with water) and subjected to mechanical force (kneading), they uncoil and link together into long, elastic chains. This interconnected protein network is what we call gluten.

Gluten does two critical things. First, it creates an elastic, stretchy structure that can trap gas bubbles produced by yeast — without gluten, CO₂ would simply escape and the bread would stay flat. Second, it gives bread its chewy texture — the more gluten development, the chewier the result.

Factors that increase gluten development: More kneading, higher protein flour, autolyse (resting flour + water before kneading), longer fermentation, folding during bulk rise.

Factors that decrease gluten development: Fat (butter, oil, eggs coats proteins), sugar (competes for water), acid (weakens bonds), bran particles (physically cuts strands), very high hydration (dilutes proteins).

2. Fermentation: Yeast at Work

Yeast (Saccharomyces cerevisiae) is a single-celled fungus that eats sugar and produces two byproducts: carbon dioxide (CO₂) and ethanol (alcohol). In bread, the CO₂ is what matters — it inflates the gluten network like a balloon, creating the light, airy crumb we associate with good bread.

The ethanol mostly evaporates during baking, but during fermentation it contributes to flavour development. Yeast also produces dozens of other flavour compounds — organic acids, esters, and aldehydes — which is why slow-fermented bread tastes more complex than quick-rise bread.

3. Sourdough: Wild Fermentation

Sourdough uses wild yeast and lactic acid bacteria (LAB) instead of commercial yeast. These organisms live naturally on flour and in the air. A sourdough starter is essentially a managed colony of these microorganisms.

The LAB produce lactic acid and acetic acid — these are what give sourdough its characteristic tang. Lactic acid (milder, yoghurt-like) dominates in warmer, wetter starters. Acetic acid (sharper, vinegar-like) dominates in cooler, stiffer starters. By controlling your starter's hydration and temperature, you control the flavour profile.

The acids also act as natural preservatives — sourdough bread resists mould for significantly longer than commercial yeast bread. And the long fermentation breaks down phytic acid in flour, making minerals more bioavailable.

4. The Maillard Reaction: Why Crusts Are Brown

The deep golden-brown colour of bread crust is not caramelisation — it's the Maillard reaction, a chemical interaction between amino acids (from proteins) and reducing sugars that occurs at temperatures above 140°C. This reaction produces hundreds of flavour compounds responsible for the "bready," toasty, complex flavour of a good crust.

Several factors intensify the Maillard reaction:

• Higher surface temperature — higher oven temp = darker crust
• Surface sugars — egg wash, milk wash, or malt syrup on the surface accelerate browning
• Alkaline environment — this is why pretzels are dipped in lye/baking soda solution, which darkens them dramatically
• Steam followed by dry heat — initial steam delays crust formation, giving bread more time to rise. Then dry heat maximises Maillard browning.

5. Oven Spring: The Final Rise

When bread enters a hot oven, it rises dramatically one last time — often increasing in volume by 30–50% in the first 10 minutes. This is called oven spring, and it happens because:

1. Yeast goes into overdrive. Between room temp and 50°C, yeast activity accelerates rapidly — this last burst of CO₂ production is significant.
2. Gas expansion. The CO₂ and ethanol already trapped in the dough expand as they heat up (gases expand when heated — basic physics).
3. Steam formation. The water in the dough turns to steam, adding additional internal pressure.

Oven spring stops when the crust sets and hardens (at about 75°C surface temperature) and when the yeast dies (at 50–55°C internal temperature). This is why steam is critical — it keeps the crust soft and flexible longer, allowing more oven spring before the crust traps everything in place.

6. Starch Gelatinisation: Setting the Crumb

Between 60–80°C internal temperature, starch granules in the flour absorb water and swell — this is called gelatinisation. The starch transforms from a granular powder into a smooth, set gel that becomes the structure of the bread crumb. This is why bread transitions from a stretchy dough into a firm, sliceable loaf.

This is also why you should never cut bread too early. At 88°C internal temp, the starch has gelatinised but hasn't finished setting. Carryover heat continues the process for 15–30 minutes after you remove the bread from the oven. Cut too early and you interrupt this process, resulting in a gummy interior.

Summary: The Journey of a Loaf