Oven spring

When bread goes into a hot oven it often grows quickly in the first ten minutes. Bakers call it oven spring. Then, by the end of baking, the soft, stretchy dough has turned into a firm loaf that keeps its shape. Both things happen in one oven. Here is the order of events, from the outside in.

1. The dough warms up, and the gas expands. Gas expands when it warms, nearly in proportion to its absolute temperature (at constant pressure, V ∝ T, with T in kelvin). From a warm dough at 30 °C (303 K) to 75 °C (348 K) the factor is 348 ÷ 303 ≈ 1.15, just 15 percent. That is not much, so there is more going on.

2. More gas appears. CO₂ that was dissolved in the dough comes out of solution as it gets warmer. Water evaporates into the bubbles: 1 g of liquid water makes about 1.7 litres of steam at 100 °C and normal pressure. Ethanol from fermentation boils at about 78 °C and also turns to vapour. All of that pushes the bubbles bigger.

3. The yeast stops. Yeast is briefly very active as the dough warms, and then dies at about 55 to 60 °C. From now on no new CO₂ is made.

4. Starch gelatinises. Between roughly 60 and 80 °C the starch granules in flour soak up water, swell and burst, which makes a stiffening gel. (The water they take comes from the dough.)

5. Proteins set. Gluten and other proteins heat up, change shape and cross-link into a firm structure (more on that in World 2). Now the walls around the bubbles are solid, and the loaf keeps its size. Too soon, and a loaf collapses when it leaves the oven. Right on time, and the structure locks the loaf in at maximum size.

6. The crust browns. The surface loses water and heats well above 100 °C, so the Maillard reaction (World 3) begins and the crust turns golden brown. The inside of a loaf stays near 100 °C while there is water in it, so it never browns.

Bakers of crusty bread add steam for the first minutes. Steam condenses on the cool dough and keeps the surface soft and stretchy, so the loaf can keep growing before the crust hardens. Home bakers often use a covered pot for the same reason.

A dough that rose for too long (over-proofed) has stretched its gluten thin and has used up its food. In the oven it may spring poorly or even sink. A dough that was not given enough time (under-proofed) may rise too much in the oven and tear.