The Science of Refrigeration

The refrigerator is the new hearth, an intimate and personal space where countless many find themselves in the search for comfort, enlightenment, company, peace. The center of household activity, a refrigerator can tell you a great deal about the people who use it. Its front is covered with dinky travel magnets from places like Pensacola and Niagara Falls, tiny word magnets assembled in dark snatches of verse, report cards ca. 2002, the embarrassing school portraits, pizza coupons, thank-you notes, and calendars. Happy Holidays! cards make their annual circuits on the magnetic cliff face. A quick glance at the front of any refrigerator could tell you more about its owners than the owners themselves could in thirty seconds.

Refrigeration is a concept as ancient as human consumption itself; we have long been obsessed with building and controlling a chilly micro-climate within the domestic sphere. Even before refrigeration, people recognized and understood the important relationship between temperature and food safety. It was the difference between pleasant, sleepy, post-dining haze and hours of gastrointestinal suffering. Temperatures between 35 and 40 degrees Fahrenheit effectively immobilize most microscopic agents of chaos, violently throwing them into deep sleep, in cryo conditions. For lack of refrigeration, households depended largely upon food preservation, and their innovative fermentations are unparalleled even by today’s hip ferment-head resurgence and fixation with the microbiota.

Refrigerators create and maintain low temperatures in cycles. Each cycle consists of the vaporization (in which a liquid turns into a gas, as in the steam off a hot cup of tea) and condensation (in which a gas turns into a liquid, as in that dew on the car windshield on a rainy day) of a liquid “refrigerant.” When liquids enter their gas form, they absorb heat, and when gases turn back into their liquid form, they release it. In refrigerators, applied pressure compresses gas into its liquid phase. The goal is achieved by compartmentalization: the refrigerant removes heat from one place (the innards of the refrigerator) and deposits it somewhere else (in this case, your kitchen). This is why the side of the refrigerator is so pleasantly warm when you touch it.

At its purest form, refrigeration is the displacement of heat from one location to another. You could say the whole Northern hemisphere of this planet is refrigerated every winter when the warm air moves down south and cold air rushes in to take its place, just as well as you could say schoolchildren are refrigerated like little cuts of meat every time they walk into an air-conditioned classroom. And so we subject our food to refrigeration, because we don’t want heat. Heat is bad news for food. Outside the refrigerator, temperatures are prime for microbial banging and baby-making. These creatures don’t need much aside from some carbohydrates and water. When it’s a balmy 75 degrees out and they happen upon your loaf of sandwich bread (and they always manage to find it), the little guys go nuts. And I sympathize, really. I imagine the immense joy I myself would feel if I discovered a giant mountain of breakfast, lunch, and dinner upon which I could sit and eat and copulate all day. I would never leave.

Edible Foam

There is perhaps nothing quite so inconsequential to ingest as foam, and yet foam – gas trapped in a liquid matrix – inspires fiery passions. Baristas strive for the smoothest microfoam, and a creamy, long-lasting head is the stuff of brewers’ dreams. Meringues, whipped cream, and souffles are foams too; edible foam exists in numerous, disparate spaces.

And of course, it can be found in haute cuisine. The godlike Ferran Adrià, Spanish champion of three precious Michelin stars, is credited for sparking the foam revolution in the mid-1990s. His experimental style turned to foams, a refreshing way of delicately incorporating savory flavors into dishes, and he did so with a rainbow of foods – olive oil, mushrooms, cod, and beets.

Foams
Microscope image of foam. http://science.nasa.gov/science-news/science-at-nasa/2003/09jun_foam/

Foam construction requires two major components: a gaseous phase, and an aqueous phase. In the case of culinary foam, using an espuma or thermo whip, the two are forcefully combined with pressurized gas (often nitrous oxide) to generate a quivering protein film surrounding evanescent gas bubbles. Hundreds of bubbles settle into a dynamic structure stabilized by surface tension and the hydrophobic effect.

A key factor in edible foam is lifespan; a poorly constructed cod foam soon reduces to a piddly mess of cod-juice. Stability of the foam dictates whether the foam stands tall or melts away, and in 1873, Belgian physicist Joseph Plateau described three universal rules for the molecular organization of foam. Should a rogue bubble fail to adopt these rules, it pops.

The first rule is that when bubbles meet at an edge, it will be at the intersection of three surfaces. Second, each pair of surfaces meets at an angle of 120 degrees. Three surfaces therefore meet at a sum of 360 degrees, forming a full circle. Finally, the last rule is that when bubbles meet at a point, it will be at the intersection of four bubbles, at an angle of 109.5 degrees.

These rules reflect the adoption of the most stable conformation. The most stable bubbles will be polyhedral in nature, a three-dimensional space with straight edges. Liquids destabilize the structure, causing bubbles to round out and flow freely. In the image of foam structure above, water destabilizes the bottom of the foam, and the bubbles are rounded. The top half is dry, and the bubbles are clearly polyhedral as a result of their increased stability.

Foams are unique culinary phenomena, ethereal and fleeting structures capable of delivering heady flavors in light and unexpected ways. I read about foams and saw a photo of a thermo whip, imediately recognizing it as a gadget I saw a woman buy at a thrift store earlier this week. I hadn’t recognized it at the time, but she bought the strange two-necked bottle for just 50 cents, and the cashier told her it was a steal.

“Yeah,” the woman agreed. “You know what it is, right?” I didn’t.

“Oh yes! Go forth and culinate!”