Storm in a Teacup: The Physics of Everyday Life(6)

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.

Inside the oven, heat energy flowed into the bread. The pressure in the oven was still the same as the pressure outside, but the temperature in the bread had suddenly gone up from 68°F to 475°F. In absolute units, that’s from 293 Kelvin to 523 Kelvin, almost a doubling of temperature.* In a gas, that means that the molecules speed up. The bit that’s counter-intuitive is that no individual molecule has its own temperature. A gas—a cluster of molecules—can have a temperature, but an individual molecule within it can’t. Gas temperature is just a way of expressing how much movement energy the molecules have on average, but each individual molecule is constantly speeding up and slowing down, exchanging its energy with the others as they collide. Any individual molecule is just playing bumper cars with the energy it’s got right now. The faster they travel, the harder they bump into the sides of the bubbles, so the greater the pressure they generate. As the bread went into the oven, gas molecules suddenly gained lots more heat energy and so they sped up. The average speed shifted from 1500 feet per second to 2200 feet per second. So the outward push on the bubble walls got much harder and the outsides weren’t pushing back. Each bubble expanded in proportion to the temperature, pushing outward on the dough and forcing it to expand. And here’s the thing . . . the air bubbles (mostly nitrogen and oxygen) expanded in exactly the same way as the CO2 bubbles. This is the last piece of the puzzle. It turns out that it doesn’t matter what the molecules are. If you double the temperature you still double the volume (if you keep the pressure constant). Or, if you keep the volume constant and double the temperature, the pressure will double. The complication of having a mix of different atoms present is irrelevant, because the statistics are the same for any mixture. No one looking at the final bread could ever tell which bubbles had been CO2 and which ones had been air. And then the protein and carbohydrate matrix surrounding the bubbles cooked and solidified. The bubble size was fixed. Fluffy white focaccia was assured.

The way that gases behave is described by something called “the ideal gas law,” and the idealism is justified by the fact that it works. It works spectacularly well. It says that for a fixed mass of gas the pressure is inversely proportional to the volume (if you double the pressure, you halve the volume), the temperature is proportional to the pressure (if you double the temperature, you double the pressure), and that the volume is proportional to the temperature, at fixed pressure. It doesn’t matter what the gas is, only how many molecules of it there are. The ideal gas law is what drives the internal combustion engine, hot air balloons—and popcorn. And it applies not only when things heat up, but also when they cool down.