E71 Levado - Bread

Topic

The rising of this bread is the expansion of the dough caused by the carbon dioxide produced as the yeast ferments the sugars present within it; this carbon dioxide first dissolves into the dough, then diffuses into pre-existing bubbles, causing them to grow and thereby creating the bread's characteristic aerated structure. When the yeast ferments the sugars in the dough more vigorously, carbon dioxide is produced more rapidly.

The rate at which the yeast produces carbon dioxide depends heavily on the dough's temperature, following an Arrhenius relationship: at moderately high temperatures, the yeast produces roughly twice as much carbon dioxide as it does at cooler temperatures; however, above a certain higher threshold, yeast activity declines, and at sufficiently high temperatures, the yeast dies completely. This temperature dependence determines both the optimal fermentation conditions for the dough and the thermal death point of the yeast, which is eventually reached during baking. As the dough's temperature approaches this thermal death point, yeast activity progressively declines.

The Rayleigh-Plesset equation describes the growth dynamics of individual bubbles within the dough: for a bubble to continue expanding, the internal carbon dioxide pressure must overcome the surrounding dough's hydrostatic pressure, the viscous resistance the dough exerts against expansion, and the bubble's own surface tension. Consequently, the dough's high viscosity dampens bubble growth, resulting in controlled bubble sizes rather than uncontrolled expansion. The higher viscosity of the dough increases the viscous resistance to the growth of each bubble; consequently, the bubbles grow more slowly and in a more controlled manner.

The diffusivity of carbon dioxide within the dough is several orders of magnitude lower than it would be in free water, due to the resistance posed by the gluten network (described in another model in this series); this implies that carbon dioxide produced in a region of the dough lacking nearby bubbles takes several minutes to diffuse to the nearest bubble it can join. A denser gluten network further reduces carbon dioxide diffusivity, meaning the gas takes longer to reach a nearby bubble.

Thus, the bubble growth and carbon dioxide diffusivity characteristics described here allow the dough to achieve a specific expansion factor relative to its initial volume during a well-controlled fermentation. This expansion continues during baking—driven by water evaporation (covered in another model in this series) and the thermal expansion of the carbon dioxide—until the heat causes the gluten to coagulate, thereby setting the bread's final structure. The steam injected at the start of baking (addressed in a separate model in this series) delays the formation of the crust, giving the dough more time to complete its expansion before the crust sets the bread's final outer structure.

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