Bread Is Lighter Than Whipped Cream

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The headline above is surprising but true, and you can test it yourself: put 1 L of whipped cream on the left pan of a balance scale and a 1 L brioche on the right. The scale will tip to the left. Whipped cream has a reputation for being light and airy, but it’s about twice as dense as brioche.

The demonstration is hard to believe because it violates our expectation that a foam should be lighter than a solid. But bread is also a foam—it is just a set foam. The brioche’s crust is solid enough, but the crumb inside is mostly air.

This simple experiment illustrates that the density of bread—that is, its mass divided by its volume—is less than that of almost any other kind of food. Ciabatta, baguette, brioche, sandwich bread, and other common yeast breads typically have a density of just 0.22–0.27 g/cm3. Whipped cream, by comparison, has a density of 0.49 g/cm3. A liter of whipped cream thus weighs twice as much as a brioche of equal volume.

Bread seems denser than it is in large part because its mass is not evenly distributed: a crunchy baguette crust, which resists cutting and chewing, is 50%–100% more dense than the crumb. The crust is about as dense as pinewood (and whipped cream), whereas the density of the crumb is more like that of cork.

But if the crust is as dense as whipped cream, why does crust feel heavier? The short answer is that the chemistry of these two foams differs. To bite through bread (a set foam), you have to tear apart strong chemical bonds among adjacent molecules. But to eat whipped cream (a colloidal foam), you merely have to push adjacent particles apart.

Intuitively, you might expect that airier breads, such as a baguette, are less dense than loaves that have a tighter crumb, such as pumpernickel and other rye breads. And, in fact, that’s true, as this chart shows.

As it turns out, brick-like rye breads are more dense than red pine—and less dense a kernel of wheat. Scientific insights like this are why we find bread endlessly fascinating and fun.

Why food goes from almost done to overdone so quickly on the grill

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BY W. WAYT GIBBS
Associated Press

Ever tried toasting hamburger buns on a grill? It takes uncanny timing to achieve an even medium brown across the buns. Typically, they remain white for what seems like far too long. Then it’s as if time accelerates, and they blow past toasted to burnt in the time it takes to flip the burgers.

Barbeque_Hamburger Cutaway_VQ6B8473 With LAYERS

The same phenomenon is at work when you toast a marshmallow over a campfire: wait and turn, wait and turn… then brown, black and — poof! — it’s aflame. The problem is perhaps most acute when cooking shiny-skinned fish on a grill or under a broiler. Once the skin turns from silver to brown, the heat pours into the fillet, and the window of opportunity for perfect doneness slams shut with amazing speed.

Anytime you cook light-colored food with high heat, inattention is a recipe for disaster. But the physics here is pretty simple, and once you understand it you can use several methods to improve your odds of making that perfectly toasted bun, golden half-melted marshmallow, or juicy grilled fillet.

At high temperatures — about 400 F (200 C) and up — a substantial part of the heat that reaches the food arrives in the form of infrared light waves rather than via hot air or steam. The higher the temperature, the bigger the part that radiant heat plays in cooking. But this form of heat interacts with color in a profound way.

The bottom of a hamburger bun looks white because it reflects most of the visible light that hits it, and the same is true for infrared heat rays. There is a reason that white cars are popular in Phoenix — they stay cooler in the sunshine, which is full of infrared radiation.

A silvery, mirror-like fish skin is even more reflective than a white car. About 90 percent of the radiant heat striking it simply bounces away. Because only around 10 percent of the energy sinks in and warms the fish, cooking initially creeps along slowly but steadily.

That changes rapidly, however, as soon as the food gets hot enough to brown. It’s like changing from a white shirt to a black shirt on a sunny summer day. As the food darkens, that 10 percent of energy absorbed rises by leaps and bounds, and the temperature at the surface of the food soars.

So browning accelerates, which increases heat absorption, which boosts the temperature; it’s a vicious circle. By the time you can get a spatula under the fillet to flip it over, it may be almost black, reflecting just 10 percent of the heat and sucking in 90 percent.

There are at least three ways around this problem. The simplest is to stare, hawk-like, at the food and lower or remove the heat as soon as browning starts. That works fine for marshmallows but is not always practical in the kitchen or backyard barbecue.

In some cases, you can darken the color of the food at the start, for example by slathering it with a dark sauce or searing it in a very hot skillet before putting it on the grill. This is a way to make a fish steak cook more like a beef steak, which is fairly dark even when raw and so doesn’t experience such a dramatic shift in heat absorption. This method generally shortens the cooking time.

Finally, try piling other ingredients, such as sliced onions or zucchini, between the food and the coals or the broiler element to moderate the intensity of the radiant heat. Cooking times will lengthen — and you may end up having to toss out the sacrificial buffer ingredients if they get charred — but that window of opportunity will stay open longer.

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Photo credit: Ryan Matthew Smith / Modernist Cuisine, LLC

The Physics of Coffee & Cream

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Every Seattleite has been in this situation: On a cold, rainy December morning, you get your coffee to go from Vivace, Stumptown, or Starbucks, and then watch out the window for your bus. The bus, you know, might be a minute or two late, and you’ll have to wait a few minutes. You want to keep your coffee as hot as possible during your wait so that it’s still piping hot when you step out the door. You grab a lid for your cup, pausing at the cream. Should you add the cream to your coffee now, or will that only cool your drink faster? Maybe you should add your cream at the last minute, before you dash out the door.

The basic physics of heat provides the answer: you should go ahead and add the cream to your coffee now. Coffee with cream cools about 20% slower than black coffee, for three reasons:

  1. Black coffee is darker. Dark colors absorb heat faster than light colors (just think about wearing a black T-shirt versus a white T-shirt on a hot, sunny day). But dark colors also emit heat faster than light colors—absorption and emission are essentially two sides of the same coin. So by lightening the color of your coffee, you slow its rate of heat loss slightly.
  2. Stefan-Boltzmann says so. The Stefan-Boltzmann law says that hotter surfaces radiate heat faster—specifically, the power of emission is proportional to the temperature (in kelvin) raised to the fourth power. So let’s say you have two cups of coffee that start at the same temperature. You pour cream in cup #1 and the coffee drops in temperature immediately. But the rate at which it loses heat also drops. Meanwhile, the hotter black coffee in cup #2 cools so rapidly that within five minutes the two coffees are at about the same temperature. But you still haven’t added the cream to coffee #2! When you do, it cools even more; cup #1 is now the hotter of the two.
  3. Viscosity versus evaporation. This is the clincher. Adding cream thickens the coffee (adds viscosity), so it evaporates slower. You’d be surprised just how much heat evaporation carries away. Slow the rate of evaporation and you avoid a lot of that heat loss. (This is also one big reason that coffee stays warm longer with a lid on the cup.)

So, next time you’re caught in the rain, put the cream in your coffee right away. Your fingers will thank you.

Watch our high-speed video above of cream being poured into coffee at 2,000 frames per second.

Torch Tastes

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In response to my recent post on “doneness,” reader Rusty Shackleford posted the following question: “When using my blow torch, sometimes I notice unpleasant propane tastes. Anything you can tell me about general blow torch cooking?”
Blowtorch Searing Short-Rib
This brought to mind a similar question that I was recently asked about the use of other flammable gases in cooking. As is often the case at The Cooking Lab, one question leads to another and before I knew it, my short answer had grown beyond the scope of the original question. We cover the topic more extensively in the book, but here is a brief description of how the use of a blow torch and the type of gas therein can affect the flavor.

Natural gas (methane) is a common fuel for ranges and stovetops, but most torches used for cooking are fueled by propane or butane. Fuels like oxyacetylene and MAPP gas, however, typically burn hotter and thus can impart a larger amount of heat to the food for a faster sear.

The type of gas that you choose isn’t as important as the completeness of its combustion. Propane, butane, MAPP, and acetylene are all great so long as you adjust the flame of the torch so that it is a fully oxidizing flame. This is a flame that is produced with an excess of oxygen, either from the surrounding air or supplemented with compressed oxygen. You can tell that you have an oxidizing flame when the torch is burning dark blue, is relatively short in length, and hisses and roars. Frequently, people have too large of a flame that is burning yellow at the tip. This is a reducing flame, also referred to as a carburizing flame because there are uncombusted hydrocarbons from the fuel in the flame that will end up in the food, imparting an unpleasant taste. In my experience, butane torches are especially prone to this, but it can happen with any torch that hasn’t been properly adjusted before aiming it at the food.

Too often, people aim the blow torch at the food before they have it appropriately adjusted. Not only do they often end up torching the food with a dirty flame, but there is also some raw fuel being blown onto the food before it ignites. Like an old, carbureted car (and for the same reason), it is best to light the torch and adjust the fuel-to-oxidizer ratio before getting underway.

Long story short, always light your torch facing away from the food. Then adjust the torch to produce a short, hissing dark blue flame and you won’t have a problem.