No one has ever seen the battle between a sperm whale and a giant squid. But the stomachs of dead sperm whales contain collections of squid beaks, the only part of the squid that can’t be digested. So each whale carries its own internal tally of fights won. As a successful whale swims back toward the sunlight, its lungs gradually reinflate and reconnect with its blood supply. As the pressure decreases, the volume once again increases until it has reached its original starting point.
Oddly, the combination of complex molecular behavior with statistics (not usually associated with simplicity) produces a relatively straightforward outcome in practice. There are indeed lots of molecules and lots of collisions and lots of different speeds, but the only two important factors are the range of speeds that the molecules are moving at, and the average number of times they collide with the walls of their container. The number of collisions, and the strength of each collision (due to the speed and mass of that molecule) determine the pressure. The push made by all that compared with the push from the outside determines the volume. And then the temperature has a slightly different effect.
“WHO WOULD NORMALLY be worried at this point?” Our teacher, Adam, is wearing a white tunic stretched over a happily solid belly, exactly what central casting demands of a jolly baker. The strong cockney accent is just a bonus. He pokes at the sad splat of dough on the table in front of him, and it clings on as though it’s alive—which, of course, it is. “What we need for good bread,” he announces, “is air.” I’m at a bakery school being taught how to make focaccia, a traditional Italian bread. I’m pretty sure I haven’t worn an apron since I was ten. And although I’ve baked lots of bread, I’ve never seen dough that looked like the splat, so I’m learning already.
Following Adam’s instructions, we obediently start our own dough from scratch. Each of us mixes fresh yeast with water and then with the flour and salt, and works the dough with therapeutic vigor to develop the gluten, the protein that gives bread its elasticity. The whole time we’re stretching and tearing the physical structure, the living yeast that’s carried along in that structure is busy fermenting sugars and making carbon dioxide. This dough, just like all the others I’ve ever made, doesn’t have any air in it at all—it just has lots of carbon dioxide bubbles. It’s a stretchy sticky golden bioreactor, and the products of the life in it are trapped, so it rises. When this first stage is done, it gets a nice bath in olive oil and keeps rising, while we clean dough off our hands, the table, and a surprising amount of the surroundings. Each individual fermentation reaction produces two molecules of carbon dioxide which are expelled by the yeast. Carbon dioxide, or CO2—two oxygen atoms stuck to a carbon atom—is a small unreactive molecule, and at room temperature it has enough energy to float free as a gas. Once it’s found its way into a bubble with lots of other CO2 molecules, it will play bumper cars for hours. Each time it hits another molecule, there is likely to be some energy exchange, just like a cue ball hitting a snooker ball. Sometimes one will slow down almost completely and the other will take all that energy and zoom off at high speed. Sometimes the energy is shared between them. Every time a molecule bumps into the gluten-rich wall of the bubble, it pushes on the wall as it bounces off. At this stage, this is what makes the bubbles grow—as each one acquires more molecules on the inside, the push outward gets more and more insistent. So the bubble expands until the push back from the atmosphere balances the outward push of the CO2 molecules. Sometimes the CO2 molecules are traveling quickly when they hit the wall and sometimes they’re traveling slowly. Bread bakers, like physicists, don’t care which molecules hit which walls at particular speeds, because this is a game of statistics. At room temperature and atmospheric pressure, 29 percent of them are traveling between 1150 and 1650 feet per second, and it doesn’t matter which ones they are.
Adam claps his hands to get our attention, and uncovers the rising dough with a magician’s flourish. And then he does something that is new to me. He stretches out the oil-covered dough and folds it over on itself, one fold from each side. The aim is clearly to trap air between the folds. My initial unspoken response is “That’s cheating!,” because I had always assumed that all the “air” in bread was CO2 from the yeast. I once saw an origami master in Japan enthusiastically teaching his students about the correct application of Scotch tape to an angular paper horse, and I felt the same unreasonable outrage then as in the bakery. But if you want air, why not use air? Once it’s cooked, no one will know. I succumb to the knowledge of the expert and meekly fold my own dough. A couple of hours later, after more rising and folding and the incorporation of more olive oil than I had believed possible, my nascent focaccia and its bubbles were ready for the oven. The “air” of both types was about to have its moment.