What Einstein Told His Cook (23 page)

REDUCING ISN’T EASY

The other day I was making a glaze by reducing veal stock down to a small fraction of its volume. But it seemed to take forever! Why is it so hard to reduce a stock?

E
vaporating water sounds like the simplest thing in the world. Why, just leave a puddle of water standing around and it evaporates all by itself. But that takes time, because the necessary calories won’t flow into the water very fast from the room’s relatively cool air. Even on the stove, where you’re feeding lots of calories into a stockpot from a hot burner, you might have to simmer for an hour or more to accomplish that maddeningly simple-sounding recipe instruction to “reduce by half.”

Reducing an excess amount of water can be every bit as frustrating as reducing an excess amount of body fat, in that it is much harder to get rid of than you’d expect. To boil off even a small amount of water requires a surprising amount of heat energy.

Here’s why.

Water molecules stick very tightly to one another. It therefore requires a lot of work, that is, the expenditure of a lot of energy, to separate them from the bulk of the liquid and send them flying off into the air as vapor. For example, in order to boil off a pint of water, that is, to convert it from liquid to vapor after it is already at the boiling point, your range burner must pump more than 250 calories of heat energy into it. That’s the amount of energy a 125-pound woman would use up in climbing stairs nonstop for 18 minutes. Just to boil off one pint of water.

You can, of course, turn up the burner to add heat more rapidly. The temperature of the liquid will never rise above its boiling point, but it will bubble more vigorously and more bubbles will carry off more steam. It’s unwise to do that to a stock, however, unless you have already strained and defatted it. Until then, boiling, as opposed to gentle simmering, will break up solids into tiny pieces and fat into tiny, suspended globules, both of which will muddy up the liquid. A better way to speed things up is to transfer the liquid to a wider, shallower pan. The more surface area the liquid has, the more of it is exposed to the air and the faster it can vaporize.

WHY YOU CAN’T COOK OVER A CANDLE

I’m shopping for a new range, and all the literature keeps talking about “Btu’s.” I know they have to do with how hot the burners will get, but exactly what should those Btu numbers mean to me?

A
Btu is an amount of energy, just as a calorie is an amount of energy. Both are most commonly used to measure amounts of heat.

The Btu, which stands for British thermal unit, was invented by engineers, so while it makes sense to the guys who design the stoves, it doesn’t mean much to us in the kitchen. But by sheer luck it turns out to be almost exactly one quarter of a nutritional calorie. So, for example, the 250 calories that it takes to boil off a pint of water is equal to 1,000 Btu.

Another example: The total amount of heat given off by the burning of an average candle is about 5,000 Btu. That’s the amount of chemical energy the wax inherently contained, and the combustion process converts that chemical energy into heat energy. But a candle releases its heat slowly over a period of several hours, so it’s no good for cooking. In case you’ve been wondering, that’s why you can’t sauté a hamburger over a candle.

For cooking, we need a lot of heat delivered in a short period of time. Range burners are therefore rated according to how fast they can pump out heat, expressed as
Btu per hour
at their top settings. The confusion comes when people neglect to say “Btu per hour” and just say “Btu.” But the burners’ Btu ratings are not
amounts
of heat; they are the maximum
rates
at which they can pump out heat.

Most home gas or electric range burners produce from 9,000 to 12,000 Btu per hour. The gas burners in restaurant kitchens are capable of putting out heat twice as fast, because for one thing their gas-supply pipes are bigger and can feed in more gas per minute. Also, restaurant ranges generally have several concentric burner rings instead of just one. Chinese restaurants that need to do high-temperature wok cooking have broad gas burners that spew out heat like a dragon with a mouthful of habanero peppers.

Remember that to boil off a pint of water from a stock requires 1,000 Btu of heat? Well, using your 12,000-Btu-per-hour burner, that should take one-twelfth of an hour or five minutes. But you know that it takes a lot longer than that. The reason is that most of the heat emitted by the burner is wasted. Rather than going directly into the liquid in the pan, most of it goes into heating up the pan itself and the surrounding air. Put two different pots of food on two identical burners set at identical levels and they will heat and cook quite differently depending on their shapes and sizes, what materials they’re made of, how much and what kinds of foods they contain, and so on. That’s why you have to keep your eye on the pot and continually adjust the burner for every specific situation.

WINE, OR WINE NOT?

When I cook with wine or beer, does all the alcohol burn off, or does some remain, which could be a problem for a strict teetotaler, such as a recovering alcoholic?

Many cookbooks assert that all or virtually all of the alcohol “burns off” during cooking (what they mean is that it evaporates; it won’t burn unless you light it). The standard “explanation,” when there is one, is that alcohol boils at 173°F, while water doesn’t boil until 212°F, and therefore the alcohol will boil off before the water does.

Well, that’s just not the way it works.

It’s true that pure alcohol boils at 173ºF and pure water boils at 212ºF. But that doesn’t mean that they behave independently when mixed; each affects the boiling temperature of the other. A mixture of alcohol and water will boil at a temperature that’s somewhere between 173 and 212 degrees—closer to 212 if it’s mostly water, closer to 173 if it’s mostly alcohol, which I certainly hope is not the case in your cooking.

When a mixture of water and alcohol simmers or boils, the vapors are a mixture of water vapor and alcohol vapor; they evaporate together. But because alcohol evaporates more readily than water, the proportion of alcohol in the vapors is somewhat higher than it was in the liquid. The vapors are still very far from pure alcohol, however, and as they waft away from the pan, they’re not carrying off very much of the alcohol. The alcohol-loss process is much less efficient than people think.

Exactly how much alcohol will remain in your pan depends on so many factors that a general answer for all recipes is impossible. But the results of some tests may surprise you.

In 1992 a group of nutritionists at the University of Idaho, Washington State University, and the USDA measured the amounts of alcohol before and after cooking two Burgundy-laden dishes similar to
boeuf bourguignon
and
coq au vin
, plus a casserole of scalloped oysters made with sherry. They found that anywhere from 4 to 49 percent of the original alcohol remained in the finished dishes, depending on the type of food and the cooking method.

Higher temperatures, longer cooking times, uncovered pans, wider pans, top-of-the-stove rather than closed-oven cooking—all conditions that increase the general amount of evaporation of both water and alcohol—were found, not surprisingly, to increase the loss of alcohol.

Do you think you’re burning off all the alcohol as you march triumphantly into your darkened dining room bearing a tray of blazing cherries jubilee or
crêpes suzette
? Well, think again. According to the 1992 test results, you may be burning off only about 20 percent of the alcohol before the flame goes out. That’s because in order to sustain a flame, the percentage of alcohol in the vapor must be above a certain level. Remember that you had to use a high-proof brandy and warm it to make more alcohol vapor before it would even ignite. (You can’t light wine, for example.) When the alcohol burns down to a certain, still-substantial level in the dish, the fumes are no longer flammable and your fire goes out. That’s show biz.

How much weight should you give these test results when trying to accommodate your guests?

One thing you should consider is the dilution factor. If your recipe for six servings of
coq au vin
calls for 3 cups of wine, and if about half of the alcohol cooks off during a 30-minute simmer (as the researchers found), each serving will wind up with the amount of alcohol in two ounces of wine. On the other hand, those same 3 cups of wine in a six-serving
boeuf bourguignon
that simmers for three hours and loses 95 percent of its alcohol (according to the test results) will wind up giving each diner the alcohol equivalent of only two-tenths of an ounce of wine.

Still,
some
alcohol is still alcohol. Use your judgment.

HOT ENOUGH FOR YA?

Does it ever really get hot enough to fry an egg on the sidewalk?

I
t’s unlikely. But scientific opinion has never been known to discourage people from trying to prove an age-old urban legend.

When I was a kid in The Big City in the days before air conditioning, at least one newspaper would cook up an egg-on-the-sidewalk story sometime during the “silly season”—the dog days of summer, when even bank robbers were too lazy to make news and reporters had little to do. But to my recollection no one ever claimed to have actually pulled off the egg trick.

That hasn’t stopped the 150 citizens of the old Mojave Desert mining town of Oatman, Arizona, from holding an annual solar egg-frying contest every Fourth of July by the side of the fabled Route 66. According to Oatman’s exalted Egg Fry Coordinator, Fred Eck (get it?), the contestant who comes closest to cooking an egg in 15 minutes by sun power alone wins.

An occasional egg has indeed been cooked in Oatman, but the rules allow such gimmicks as magnifying glasses, mirrors, aluminum reflectors, and the like. No fair, I say. We’re talking here about breaking an egg directly onto the ground and leaving it alone.

A couple of years ago, finding myself in Austin, Texas, during a heat wave, I determined to find out whether it was possible to fry an egg on a sidewalk without any optical or mechanical aids. In order to draw meaningful conclusions, I had to measure the sidewalks’ temperatures. Fortunately, I had with me a wonderful little gadget called a non-contact thermometer. It’s a little gun that you point at a surface and when you pull its trigger, it instantly reads out the temperature of that surface, anywhere from 0°F to 500°F. The so-called MiniTemp, manufactured by Raytek in Santa Cruz, California, works by analyzing the amount of infrared radiation being emitted and/or reflected from the surface; hotter molecules emit more infrared radiation. My MiniTemp was an ideal tool for the sidewalk cooking experiment, because I already knew how hot it has to be to cook an egg, and if you keep on reading, so will you.

On a particularly scorching day I went around measuring the mid-afternoon temperatures of a wide variety of sidewalks, driveways, and parking lots, trying not to upset any Texans by looking as if I were pointing a real gun.

The ground temperatures varied quite a bit depending, not unexpectedly, on the darkness of the surface. Blacktop paving was much hotter than concrete, because dark objects absorb more light and therefore more energy. So there goes one cherished notion about outdoor egg-fries; you’d have a better chance in the middle of a blacktop street than on the sidewalk.

Although the air temperatures hovered around 100ºF, I never found a surface hotter than about 125ºF on concrete or 145º on blacktop (remember that number). In either case, the temperatures plunged almost immediately when the sun went behind a cloud (okay, a cloud went in front of the sun), because much of the infrared radiation coming from the surfaces is simply solar radiation that is being reflected back. Bright, shiny metal surfaces, in fact, reflect so much solar radiation that the MiniTemp won’t give accurate readings of their temperatures.

Now it was time for the crucial experiment. I had previously taken an egg from the refrigerator and warmed it to room temperature. I cracked it directly onto the 145ºF surface of an asphalt-paved parking lot at high noon. I didn’t use cooking oil, which might have cooled the surface too much. Then, I waited.

And waited.

If you don’t count the odd glances I received from passersby, nothing whatsoever happened. Well, maybe the egg white became slightly thicker at the edges, but there wasn’t anything remotely resembling cooking. The surface just wasn’t hot enough to cook an egg. But why not, I wondered?

First of all, only the white of the egg, or albumen, was in contact with the hot surface—the yolk floats on the white—so it’s a matter of what temperature might be required to cook the albumen. And what do we mean by “cook,” anyway? Egg white is a mixture of several kinds of protein, each of which is affected differently by heat and coagulates at a different temperature. (You expected a simple answer?)