Gas bubbles (or pore spaces) are a fundamental component of many earth materials, yet processes that control bubble formation and migration are rarely addressed in basic earth science texts. Understanding bubble formation and migration is particularly critical for understanding volcano behavior, where gas expansion provides the primary driving force for volcanic eruptions. However, bubble behavior also affects magma chamber processes and ore deposit formation. The physical properties of bubbles that make them such effective drivers of magma motion are their buoyancy, their volume sensitivity to pressure and temperature, and their deformability, properties that are easily explored in the kitchen. Here we start our exploration of cooking analogies with bread, where the revival of artisan breads provides a wide array of bread textures and techniques.
Bubbles and baking – a tutorial
Making and growing bubbles is a common baking technique; in fact, controlling the timing, rates and extent of bubble formation lies at the heart of most cake, cookie and bread recipes. In baking, bubbles are forced into the batter by beating or kneading; during baking, gas is added to those bubbles by CO2-producing reactions that involve either slow-acting yeast (for stiff bread doughs) or rapid chemical reactions (acid + base; used in quick breads and runny cake batters). Times prescribed for bread to rise, and times and oven temperatures used for baking, are designed to balance bubble growth with changes in the property of the surrounding dough to produce the desired texture.
Baking causes both chemical and physical changes to the dough. Heating initially causes the dough to become more fluid and expand by a combination of steam formation from water in the dough and CO2 migration from chemical reactions into existing bubbles. After the initial expansion, bubbles are trapped in place by stiffening of the dough and the formation of an outer rigid crust. Also important at this stage is rupture of bubble walls to form an interconnected (permeable) gas network that allows the gases to escape. If the gas bubbles remained isolated, steam condensation on cooling would cause the structure to collapse. The rate of heating is critical because it controls the relative rates of bubble expansion and crust formation – if heating is too slow, bubbles will expand, coarsen, interconnect and even collapse; if heating is too rapid, the crust will stiffen more rapidly than the interior expands, causing the crust to rupture.
An illustration of the role of controlled vesiculation in bread baking is provided by the contrasting surfaces and interior textures of two artisan bread types: ciabatta and a multigrain bread. Ciabatta has a smooth crunchy crust and large vesicles inside (see above). This combination of smooth outside and coarse bubbly inside texture reflects rapid bubble expansion in a sticky dough- this is best done in a wood-fired oven, which reaches higher temperatures than a conventional oven, and the early introduction of steam to the oven, which increases the rate of heat transfer to the dough while at the same time maintaining a flexible crust that can accommodate the rapid volume increase of the expanding dough. Note that the bubble size is much smaller in the outer crust than in the interior, because it was “quenched in as the crust formed”, and that the largest bubble is in the upper half of the bread as a consequence of the rise and accumulation of escaping gases.
In crusty sourdough bread, the original crust has been broken by expansion of the bubbly interior (see above). One could estimate the extent of volumetric expansion after crust formation by determining the surface area (and contained volume) of the unbroken crust relative to that of the finished (fully expanded) bread. Examination of the internal structure of the bread shows that the bubbles are quite a bit smaller than in the ciabatta (a reflection of stiffer dough). Additionally, there is clear evidence of bubble deformation (representing flow accompanying expansion) oriented upward toward the opening crack.
Bubbles and basalt
Analogies to the bread forming textures can be found pyroclasts (literally ‘broken fire’ –), the products of explosive volcanic eruptions. Volcanic eruptions obtain their energy from expanding bubbles. As in bread, the bubbles are formed primarily by water and carbon dioxide, although in the case of magma, these gas phases were dissolved within the magma at depth, and come out of solution as the magma approaches the Earth’s surface. The rate of magma ascent, the amount of gas dissolved in the magma, and the magma composition and temperature (which control the fluidity) all determine when and where bubble expansion begins and ends. Pyroclasts formed by many basaltic eruptions continue to expand freely, with little restraint by the thin and flexible crust.
These pyroclasts are analogous to ciabatta, in that they typically have a smooth crust and may have large interior holes. At Stromboli volcano (Italy), very frothy but smooth-surfaced bombs are informally known as potatoes, for their rounded shapes and smooth light brown surface.
In Hawaii, bubbly lava flows are transported through complex lava tube systems; when these flows travel slowly across the surface, they also form bubble-rich lavas with characteristically smooth surface crusts.
Bubbles and breadcrust bombs
It is no accident that an entire class of pyroclasts has the name of breadcrust bombs. Breadcrust bombs are characteristic of a specific type of volcanic eruption that is called Vulcanian after the volcano Vulcano in Italy (the original volcano!). These bombs form degassed magma that forms a plug on the conduit. When bubbles collect below this cap they exert pressure until the plug fails and an eruption happens.
Magma that formed the plug has enough residual gas dissolved in the magma to form bubbles, but only after the outside has cooled and solidified. Therefore, bubble formation and expansion causes the outer crust to crack. Although there are many different types of fracture patterns, some bombs look very similar to the crusty bread shown above. If we break open these bombs, we see larger bubbles in the middle, just as we saw in the bread.
 For more on cooking science, see On Food and Cooking: The Science and Lore of the Kitchen by Harold McGee; for bread specifics we suggest The Bread Baker’s Apprentice: Mastering the Art of Extraordinary Bread by Peter Reinhart
 Both breads featured here were made by Hideaway Bakery in Eugene OR, who employ much better bakers than we are!