What Einstein Told His Cook

Chapter Six

Fire and Ice

LOOK AROUND THE KITCHEN at all your modern conveniences: your toaster, your blender, your food processor, your coffee grinder, your mixer, your coffeemaker—all devices that you use only now and then for specialized purposes.

Now look at the only two appliances in your kitchen that you use daily and couldn’t do without: one that makes heat and one that makes cold. Compared with your food processor, you might not think of your stove and refrigerator as modern appliances, but they are surprisingly recent additions to the human arsenal of cooking and food-preserving equipment.

The first kitchen range, an enclosure containing a burning fuel (initially, coal) that heats a flat surface for cooking, was patented less than 375 years ago, heralding the end of more than a million years of cooking over open fires. And the electric refrigerator replaced ice for cooling only within the memories of some of the readers of this book.

When you bring fresh food home from the market, you may put it in the refrigerator, whose low temperatures will keep it from spoiling. Then you may use the stove’s high temperatures to convert some of that food into a form that is more palatable and digestible. After you’ve cooked and served the food, you may put some of the leftovers back into the refrigerator or freezer to keep. And some time later, you may take them out of the refrigerator and heat them up again. The manipulation of foods in our kitchens seems to involve a continual round of heating and cooling, of using figurative fire and ice. Only today, we do those jobs with gas and electricity.

What do heat and cold do to our foods? How can we control them to produce the best results? We can burn our food with too much heat, but on the other hand the freezer can “burn” it with…well, what is freezer burn, anyway? And just what is going on when we perform that most elementary of all cooking operations, the boiling of water? There’s more to it than you may think.

HOT STUFF

C IS FOR CALORIE

I know that a calorie is a unit of heat, but why does eating heat make me fat? What if I ate only cold foods?

A calorie is a much broader concept than just heat; it’s an amount of any kind of energy. We could measure the energy of a speeding Mack truck in calories, if we wanted to.

Energy is whatever makes things happen; call it “oomph” if you wish. It comes in many forms: physical motion (think Mack truck), chemical energy (think dynamite), nuclear energy (think reactor), electrical energy (think battery), gravitational energy (think waterfall), and yes, the most common form of all, heat.

It’s not heat that’s your enemy; it’s energy—the amount of energy-for-living that your body gets by metabolizing food. And if metabolizing that cheesecake produces more energy than you use up in walking from the refrigerator to the TV, your body will store the excess energy as fat. Fat is a concentrated storehouse of energy, because it has the potential of giving off lots of heat when burned. But don’t jump to any conclusions. When an advertisement promises to “burn off fat,” it’s only a metaphor; a blowtorch is not a feasible weight-loss device.

How much energy is a calorie, and why do different foods “contain” (that is, produce) various numbers of calories when metabolized?

Since heat is the most common and familiar form of energy, the calorie is defined in terms of heat—how much heat it takes to raise the temperature of water by a certain amount. Specifically, as the term is used in nutrition, a calorie is the amount of heat it takes to raise the temperature of one kilogram of water by 1 degree Celsius.

(Chemists, as opposed to nutritionists and dieticians, use a much smaller “calorie,” only one-thousandth as big. In their world, the nutritional calorie is called a kilocalorie. But in this book I use the word calorie to mean the common one that food books, food labels, and diets talk about.)

Here’s an idea of how much heat a calorie is: A nutritional calorie is the amount of heat it would take to raise the temperature of a pint of water by 3.8°F.

Different foods, as everyone knows, provide us with different amounts of food energy. Originally, the calorie contents of foods were measured by actually burning them in an oxygen-filled container immersed in water and measuring how much the water’s temperature went up. (The apparatus is called a calorimeter.) You could do the same thing with a serving of apple pie to find out how many calories it releases.

But is the amount of energy released when a slice of pie is burned in oxygen the same as the amount of energy released when it is metabolized in the body? Remarkably, it is, even though the mechanisms are quite different. Metabolism releases its energy much more slowly than combustion does, and mercifully without flames. (Heartburn doesn’t count.) The overall chemical reaction is exactly the same, however: Food plus oxygen produces energy plus various reaction products. And it’s a basic principle of chemistry that if the initial and final substances are the same, then the amount of energy given off is the same, regardless of how the reaction took place. The only practical difference is that foods aren’t digested or “burned” completely in the body, so we actually get out of them somewhat less than the total amount of energy they would release by being burned in oxygen.

On the average, we wind up getting about 9 calories of energy from each gram of fat and 4 calories from each gram of protein or carbohydrate. So instead of running into the lab and setting fire to every food in sight, nutritionists these days just add up the numbers of grams of fat, protein and carbohydrate in a serving and multiply by either 9 or 4.

Your normal basal metabolism rate—the minimum amount of energy you use up just by breathing, pumping your blood around, digesting your food, repairing your tissues, keeping your body temperature normal, and keeping your liver and kidneys, etc., doing their jobs—is about 1 calorie per hour for every kilogram (2.2 pounds) that you weigh. That’s about 1,600 calories per day for a 150-pound male. But that can vary quite a bit depending on sex (women require about 10 percent less), age, health, body size, shape, and so on.

Among other things, weight gain depends on how much your intake of food energy above and beyond your basal metabolism rate exceeds your expenditure of energy by exercise, not counting fork-lifting. For an average healthy adult the National Academy of Sciences recommends a daily intake of 2,700 calories for men and 2,000 for women—more for jocks and fewer for couch potatoes.

The hopeful theory about eating cold, calorie-deprived food has been bandied about in various forms for some time, but unfortunately, it won’t work. One variation that I’ve heard is that drinking ice water will help you lose weight because you must expend calories in warming the water up to your body temperature. That’s true in principle, but trivial. Warming an 8-ounce glass of ice water up to body temperature uses less then 9 calories, the equivalent of a single gram (one 454th of a pound) of fat. If dieting were that simple, “fat-farm” spas would have ice-water swimming pools. (Shivering also uses up energy.) And unfortunately, while most substances shrink when their temperatures are lowered, people don’t. Not for long, anyway.

THE EFFECTS OF FUDGE ON DIETING

If there are nine calories in a gram of fat, that means that there are more than 4,000 calories in a pound (454 grams) of fat. But I’ve read that in order to lose a pound of fat I must cut my intake by only 3,500 calories. Why the discrepancy?

Not being a nutritionist, I asked Marion Nestle, professor and chair of the Department of Nutrition and Food Studies at New York University.

“Fudge factors,” she said.

First of all, the actual energy content of a gram of fat is closer to 9.5 calories. But that would only make the discrepancy bigger. The fact is that the number of calories of energy we get from eating a gram of fat is quite a bit less than that because of incomplete digestion, absorption, and metabolism. That’s one fudge factor.

“Another fudge factor,” Nestle continued, “is applied to the number of calories in a pound of body fat. The idea is that body fat is only about 85 percent actual fat.” The rest consists of connective tissue, blood vessels, and other stuff that you’d probably rather not know about.

Thus, in order to lose a pound of real-life blubber, your bottom line, so to speak, is that you must deprive yourself of only about 3,500 calories.

And stay away from the fudge.

REALLY HAUTE CUISINE

My husband, daughter, and I will be returning to La Paz, Bolivia, to adopt another baby. Because of the high altitude, boiling water can take hours to cook things. Is there any rule of thumb about how long it takes to cook something at various altitudes? And will boiling bottles at this altitude kill germs?

The elevation at La Paz runs from 10,650 to 13,250 feet above sea level, depending on which part of town you’re in. And as you are aware, water boils at lower temperatures at higher elevations. That’s because in order to escape from the liquid and boil off into the air, water molecules have to fight against the downward pressure of the atmosphere. When the atmospheric pressure is lower, as it is at higher altitudes, the water molecules can boil off without having to get as hot.

The boiling temperature of water decreases about 1.9°F for every 1,000 feet above sea level. So at 13,000 feet, water will boil at 187ºF. Temperatures above 165ºF are generally thought to be high enough to kill most germs, so you should be okay on that score.

It’s hard to generalize about cooking times, because different foods behave differently. I’d suggest asking the locals how long they cook their rice, beans, and the like. Of course, you can always schlep a pressure cooker onto the airplane and manufacture your own high-pressure atmosphere at will.

Baking is a whole different ball game. For one thing, water evaporates more readily at high altitudes, so you will need to add more water to doughs and batters. And because there is less pressure to hold down the carbon dioxide gas released by baking powder, the gas can rise clear out the top of your cake, leaving it flat. So you must use less baking powder. All this can be very tricky. My advice is to leave the baking to the local pastelerías.

PROJECT HEAD-START

My husband claims that warm water takes longer to boil than cold water, because it is in the process of cooling as you place it on the stove. I think that’s ridiculous. But he took physics in college and I didn’t.

What grade did he get in physics? Apparently, your intuition is paying off better than his tuition, because you’re right and he’s wrong.

I can guess what he’s thinking, though. Something about momentum, I’ll bet, because if an object is already falling—in temperature, presumably—it should require extra time and effort to turn it around and make it rise. You first have to kill its downward momentum.

That’s all very well and true for physical objects, but temperature isn’t a physical object. When the weather report says that the temperature is falling, we hardly expect to hear a crash.

Temperature is just our artificial human way of expressing the average speed of the molecules in a substance, because their speed is what makes a substance hot; the faster its molecules are moving, the hotter it is. We can’t get in there and clock the speed of every single molecule, so we invented the concept of temperature. It’s really little more than a handy number.

In a pot of warm water, the zillions of molecules are flitting about at a higher average speed than in a pot of cold water. Our job in heating the pot is to give more energy to those molecules and make them move even faster—eventually fast enough to boil off. Obviously, then, warm molecules will require less added energy than cold ones, because they’re already partway to the finish line: the boiling point. So the warm water will boil first.

And you can tell him I said so.

Using hot tap water for cooking may be unwise for another reason. Older houses may have copper water pipes that are joined with lead-containing solder. Hot water can leach out tiny amounts of lead, which is a cumulative poison. So it’s a good idea always to use cold water to cook with. Yes, it’ll take longer to boil, but since you may live longer you can spare the time.

PUT A LID ON IT!

My wife and I disagree on whether a pot of water will boil sooner if you keep the lid on. She says it will, because without the lid a lot of heat would be lost. I say that it will take longer to boil, because the lid increases the pressure and raises the point at which water will boil, as in a pressure cooker. Who’s right?

Your wife wins, although you do have a point.

As a pot of water is heated and its temperature goes up, more and more water vapor is produced above the surface. That’s because more and more of the surface molecules gain enough energy to leap off into the air. The increasing amount of water vapor carries off an increasing amount of energy that could otherwise go into raising the water’s temperature. Moreover, the closer the water gets to its boiling temperature, the more energy each water vapor molecule carries off, so the more important it becomes not to lose them. A pot lid partially blocks the loss of all those molecules. The tighter the lid, the more hot molecules are retained in the pot and the sooner the water will boil.

Your point, that a lid increases the pressure inside the pot as in a pressure cooker, thereby raising the boiling point and delaying the actual boiling, is correct in theory but insignificant in reality. Even a tightly fitting, hefty one-pound lid on a ten-inch pot would raise the pressure inside by less than a tenth of a percent, which would in turn raise the boiling point by only four hundredths of a degree Fahrenheit. You could probably delay the boiling longer by watching the pot.

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?

Evaporating 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.

When shopping for a range, look for one that has at least one burner rated at 12,000 or preferably 15,000 Btu per hour. With that much heat output you’ll be able to boil water in no time, sear meats quickly, and stir-fry in your wok or stir-fryer like a Chinese chef.

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?

Does the vino lose its power in the Crock-Pot overnight?

In a flambé baked Alaska, does the brandy lose its bite?

Does the alcohol all burn off, as the cookbooks say it does?

Or can you eat a plate of coq au vin and get a little buzz?

Well, when you cook with wine or cook with brandy, here’s the scoop:

There will always be some alcohol remaining in the soup.

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?

It’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?)

But in an eggshell, it all boils down to this: Egg white begins to thicken at about 144ºF, it ceases to flow at 149ºF, and it becomes fairly firm at 158ºF. Meanwhile, a yolk will begin to thicken at 149ºF and lose its fluidity at 158ºF. So to cook an entire egg to a non-runny, sunny-side-up condition, you’d want both the white and the yolk to reach 158ºF and to stay there long enough for the rather slow coagulation reactions to take place.

Unfortunately, that’s quite a bit hotter than any reasonably attainable ground temperatures. But more important, when you break a 70ºF egg onto the 145ºF ground it cools the surface down considerably, and there is no continual replenishment of heat from below, as there would be in a frying pan over a fire. Also, pavement is a very poor conductor of heat, so none can flow in from the surroundings. Thus, even though a parking lot’s black surface might get close to the coagulation temperature of 158ºF on a really, really hot day, I’m afraid that actually cooking an egg on a sidewalk must forever remain but a midsummer night’s dream.

But wait! The roof of one sun-baked, dark blue, 1994 Ford Taurus station wagon measured 178ºF, more than hot enough to coagulate both white and yolk. And because steel is a good conductor of heat, that temperature could be maintained by heat feeding into the egg from other parts of the roof. Maybe cars, rather than streets and sidewalks, were the way to go.

Indeed, after I wrote about my experiments in my newspaper column, a reader wrote to tell me that in a World War II German newsreel he saw two Afrika Korps soldiers fry an egg on the fender of a tank. (Austin’s streets were mercifully free of tanks, although some SUV’s came close.) “They cleaned off a spot,” he wrote, “poured on a little oil, spread it around and then broke two eggs onto the surface. The whites turned opaque just as quickly as they do in my frying pan.”

I checked the Almanac and found that the highest weather temperature ever recorded was 136ºF on September 13, 1922, in El Azizia, Libya, not far from that German tank.

Another reader reported that she and some friends once cooked an egg on a sidewalk in Tempe, Arizona, when the air temperature was 122ºF, although she didn’t measure the temperature of the sidewalk.

“The egg came straight out of the refrigerator,” she wrote. “We cracked it directly on the sidewalk and immediately the white started cooking. In less than 10 minutes the yolk broke…and spread out and the whole egg cooked. We thought maybe it was a fluke that the yolk broke, so we tried another one and the yolk broke on that one, too, in about the same amount of time.”

Now, of course, I had to figure out why the yolks broke and spoiled the possibility of preparing sunny-side-up street food. I could only guess, but my reader gave me a clue.

“We went back inside the house,” she continued, “and a little while later my friend told us we’d better go clean up the eggs before her husband got home, so we went back outside. The eggs were completely dehydrated and broken into little pieces and there were a bunch of ants carrying off the pieces; we had nothing to clean up.”

Aha! That’s the answer: dehydration. In Arizona, the humidity can be so low as to be almost nonexistent, so liquids evaporate and dry up in a flash. What must have happened is that the surface of the egg yolk quickly dried out, became brittle, and cracked open, spilling its still-liquid contents. Eventually, the whole egg schmear dried out and cracked into small platelets, like mud does in a dry lake. The platelets were just the right size for the happy ants to cart off to wherever it is that ants take their afternoon tea.

The wonderful thing about science is that it can even explain things that nobody needs to know.

PLAYING WITH FIRE

What’s the best kind of fire for grilling: charcoal or gas?

The answer to that question is an unequivocal “It depends.” You can make burned-on-the-outside, raw-on-the-inside chicken equally well over charcoal or a gas flame.

As in all cooking, what matters is how much heat the food ultimately absorbs; that’s what determines its done-ness. Grilling infuses the necessary amount of heat by subjecting the food to a very high temperature for a short period of time, so a small difference in cooking time can make all the difference between succulence and cinders.

But the main reason that grilling is so tricky is that the temperature is hard to control. It’s easy to adjust a gas flame, but with charcoal you have to adjust the temperature continually by such antics as moving the food sideways to a hotter or cooler location, raising or lowering the grilling rack, and bunching up the charcoal to make it hotter or spreading it out to make it cooler. And the rules of the game differ, depending on whether you’re using a covered grill or cooking topless.

The ingredients of any fire are two: fuel and oxygen. If there isn’t enough oxygen available, the combustion process will be incomplete and some unburned fuel will show up as smoke and yellow flame. The yellow color comes from unburned carbon particles that are heated to incandescence. Because combustion is never 100 percent complete, there will also be some poisonous carbon monoxide, instead of carbon dioxide, produced. That’s why you should never barbecue or grill indoors, no matter how cute your hibachi.

For cooking, we want complete combustion, so it’s essential that the fuel receive enough air. (Smoked foods are made by deliberately starving the heated wood of oxygen.) In a well-adjusted gas grill, the gas is automatically mixed with the right amount of air on its way to the burner; in charcoal grills, you have to manipulate the vent openings.

When the cave persons discovered fire and grilled their first mastodonburgers, wood was undoubtedly the fuel. But wood contains resinous and sappy substances that don’t burn completely and therefore produce dirty flames. Hardwoods contain less of these substances, and hardwood is still preferred for grilling by purists who believe that there’s no fuel like an old fuel and who value the unique, smoky flavor that a wood fire imparts.

The fuelish question that most people ask is whether to burn charcoal or gas—and, of course, which equipment to burn it in. These days, the equipment can range anywhere from a fire-escape hibachi to a suburban behemoth equipped with everything but tail fins and radar.

Charcoal is wood that has been heated at a high temperature, but in the absence of air so it can’t actually burn. All the sap and resins are decomposed or driven off, leaving almost pure carbon that will burn slowly, quietly, and cleanly. Natural hardwood charcoal, still wearing the shapes of the chunks of wood it was made from, contains no additives and imparts no off-flavors to the food. Charcoal briquettes, on the other hand, are manufactured from sawdust, wood scraps, and coal dust, held together with a binder. Coal is far from pure carbon, however; it contains an assortment of petroleumlike chemicals whose smoke can affect the flavor of food.

The cleanest-burning fuel of all is gas, either the propane sold in tanks or so-called natural gas (methane) that’s piped into our houses. Gas grills are made for both kinds. The gases contain no impurities to speak of, and they burn to produce essentially nothing but carbon dioxide and water.

But what about that “charcoal flavor” that everyone values so highly? Can you really get it by cooking over a gas flame?

That wonderful grilled flavor comes not from the charcoal but from the intense browning that takes place on the seared surface of the food because of the very high temperature. It also comes from melted fat, which drips down onto a hot surface—a glowing briquette or a gas grill’s lava stones or porcelain bars—is vaporized, and sends its smoke back up to condense on the surface of the food.

But if too much fat drips you’ll have flare-ups, which are undesirable because fat, although a great fuel, doesn’t have the time or oxygen to burn completely, and it therefore produces a sooty, yellow flame that licks at your food, charring it and depositing horrible chemicals and unpleasant flavors. To avoid burning at the steak, trim off most of the fat beforehand and if a flare-up nevertheless occurs, move the meat off to the side until the flames subside.

Then there’s the problem of getting a charcoal fire started. No fuel will start burning until it gets hot enough for some of it to vaporize. Only then can its molecules mix with oxygen molecules in the air and react with them in the heat-producing reaction called combustion. Once the combustion reaction gets going, the heat it releases keeps vaporizing more fuel and the whole process becomes self-sustaining.

A chimney-type charcoal starter. Crumpled newspapers are ignited through the holes at the bottom.

Gas, of course, is already vaporized, so all you need is a spark or a match to get it going. But the bugaboo of charcoal grilling is getting the stuff hot enough to accomplish that all-important initial vaporization. Enter starter fluid, the fuel that kindles fuel. Starter fluid is a petroleum-derived liquid that lies somewhere between gasoline and fuel oil. If you wait about a minute for it to soak into the charcoal before lighting it, most of its fumes will be absorbed. But in my opinion charcoal is the world’s champion odor retainer (it’s used in water purifiers and gas masks), and the starter smell never really burns off completely. Electric loop starters work slowly but well, if you have electricity handy. But in my opinion the best way of starting a charcoal fire is the newspaper-fueled chimney, which is both fast and odorless. You just stuff some newspaper into it, load it with charcoal, light the paper, and in 15 or 20 minutes the charcoal will be well ignited and ready to be dumped into the grill.

The most burning question of all, however, is which fuel, gas or charcoal, is better? Well, which political party is better? There are staunch partisans of each. I personally prefer charcoal for two reasons. One, there are too many puny gas grills on the market that don’t produce much more heat than a Zippo lighter. And two, while burning charcoal produces only carbon dioxide, burning gas produces carbon dioxide and water vapor. Although I haven’t done any experiments, I believe that the water vapor might prevent the food from getting as hot as a charcoal fire would make it, and high, dry heat is the absolute essence of successful grilling.

Oven-“Grilled” Vegetables

Roasting the Garden

Outdoor grilling is great for meats and fish, but grilling most vegetables can be a problem. Put them on the grate and they tend to fall through into the fire; put them on skewers and some parts will burn while others steam.

Roasting vegetables in a hot oven is a lot easier. It results in nicely browned, tender vegetables with a flavor much like grilled, but sweeter. You can roast an assortment of brilliantly colored vegetables and serve them in the same dish in which they were roasted, a wide, shallow, ovenproof baking dish or casserole. Or you can roast them on a baking sheet and transfer them to a serving dish. The various vegetables will all cook in the same amount of time, because they are approximately the same size.

2 large Vidalia or sweet onions, peeled and scored across the top

1 whole red pepper, halved, cored, ribs and seeds removed

1 whole yellow pepper, halved, cored, ribs and seeds removed

1 whole medium green zucchini, stem removed

1 whole medium yellow squash, stem removed

4 ripe plum tomatoes, halved and seeded

3 large whole carrots, peeled

6 thick asparagus spears

1 head of garlic, top sliced off

Extra-virgin olive oil

Coarse salt

Thyme sprigs and basil leaves for garnish

1. Preheat the oven to 400ºF. Wash all the vegetables and arrange attractively in a shallow, wide ovenproof dish that is pretty enough for the table. Or arrange them in a single layer on a baking sheet with sides. Drizzle all over with olive oil.
2. Roast on a low shelf in the oven for about 50 minutes to an hour, until the edges of the vegetables are somewhat browned. Remove the baking dish or tray and allow the vegetables to cool.
3. If you used a baking sheet, transfer the vegetables to a serving platter. To arrange for serving, cut the onions into quarters. Rub off the skins of the peppers with your fingers, and cut the flesh into large segments. Slice the zucchini, squash, tomatoes, and carrots into chunks or strips. Leave the asparagus and garlic head whole. Be sure to save all the accumulated juices and spoon them back over the vegetables.
4. Drizzle the vegetables with extra-virgin olive oil and sprinkle with coarse salt. Garnish with the herbs. Serve at room temperature or warm, with toast made from a hearty bread. Butter the bread with the soft, roasted garlic cloves.

MAKES ABOUT 4 SERVINGS

REAL COOL STUFF

UNFREEZING YOUR ASSETS

What’s the best and fastest way to defrost frozen foods?

I know what you mean. You come home after a hard day’s work. You don’t feel like cooking, and you can’t face the hassle of going to a restaurant. Where do you turn?

To the freezer, of course. And like a crowd of football fans, a little voice in your head begins to chant, “DEE-frost! DEE-frost!”

Scanning your frozen assets, you’re wondering not so much about what is in there (“Why didn’t I label those packages?”), but about what would defrost in a minimum amount of time.

Your options are (a) leaving it out on the kitchen counter while you go through the mail, (b) soaking it in a sink full of water or (c) the best and fastest method of all, which I shall divulge in due time and which, I promise, will astound you.

For commercially packaged frozen foods, just follow the directions. You wouldn’t believe the armies of home economists and technicians who slaved away to determine the best methods of defrosting their company’s products in the home kitchen. Trust them.

While the defrosting directions on commercial packages often involve a microwave oven, that usually doesn’t work for thawing home-frozen foods, because it’s hard to keep the outer regions of the food from beginning to cook.

“Frozen food” is something of a misnomer. Technically speaking, freezing means converting a substance from its liquid form into its solid form by cooling it below its freezing point. But meats and vegetables are already solid when they are put into the freezer. It’s their water content that freezes into tiny ice crystals, and those ice crystals are what make the whole food hard. The job of defrosting, then, is to melt those tiny ice crystals back to their liquid form.

How do you melt ice? Why, you heat it, of course. Your first problem, then, is to find a source of low-temperature heat. If that phrase sounds paradoxical, please realize that heat and temperature are two very different things.

Heat is energy, the energy that moving molecules have. All molecules are moving to some extent, so heat is everywhere, in everything. Even an ice cube contains heat. Not as much as a hot potato, but some.

On the other hand, temperature, as I have pointed out earlier, is just a convenient number by which we humans express how fast the molecules are moving. If the molecules of one substance are moving faster, on the average, than those of another, we say that the first substance has a higher temperature, or is hotter, than the other.

Heat energy will travel automatically from a warmer substance into an adjecent, cooler one, because the faster molecules in the warmer one can bang against the molecules of the cooler one, making them move faster. Obviously, then, we could warm our frozen food most quickly by putting it in contact with a hot substance, such as the air in a hot oven. But that would cook the outer parts of the food before much heat could penetrate into the inner parts.

The air in your kitchen is at a very moderate temperature compared with the air in a hot oven, but it still contains a lot of heat that can be tapped to defrost frozen food. So should we just leave the food out in the air? No. It would take too long for the air to transfer its heat, because air is just about the worst conductor of heat that you can imagine. Its molecules are just too far apart to do much banging against other molecules. Besides, slow air-thawing is dangerous because bacteria can grow rapidly on the outside portions that are first to thaw.

How about soaking in water? Water is a much better heat conductor than air is, because its molecules are much closer together. If the food package is waterproof (and if you’re not sure, seal it in a zipper-top bag after pressing out most of the air), then by all means soak it in a bowl—or in the case of a whole chicken or turkey, a sink or bathtub—full of cold water. Since the frozen bird will make the water even colder, change the water every half hour or so and the whole process will go even faster.

The quickest method of all, I now reveal, is to place the unwrapped frozen food on an unheated, heavy skillet or frying pan. Yes, unheated. Metals are the champion heat conductors of all substances, because they have zillions of loose electrons that can transmit energy even better than clashing molecules can. The metal pan will conduct the room’s heat very efficiently into the frozen food, thawing it in record time. The heavier the pan the better, because thicker metal can conduct more heat per minute. Flat foods like steaks and chops will thaw fastest, because they make the best contact with the pan, so keep this in mind when making up your packages for the freezer. (Round, bulky roasts and whole chickens or turkeys won’t thaw much faster on the pan than on the counter; however, neither method is recommended because of the danger of bacterial growth. Thaw them either in cold water or in the refrigerator.) Nonstick pans won’t work, incidentally, because the coatings are poor heat conductors, nor will a cast-iron pan because it is porous.

I discovered the frying-pan gimmick while experimenting with one of those “miracle” defrosting trays sold in catalogs and cookware stores. They are reputedly made of an “advanced, space-age super-conductive alloy” that “takes heat right out of the air.” Well, the space-age alloy turns out to be ordinary aluminum (I analyzed it), and it “takes heat out of the air” exactly the way an aluminum frying pan does, and for exactly the same reasons.

So save the water method for the bulky stuff and just put that frozen steak or fillet on a heavy frying pan. It’ll be thawed before you can say, “Where did I put those frozen peas?” Well, not quite, but a lot sooner than you’d think.

HOW TO MAKE A COOL BUNCH OF DOUGH

Why do cookbooks recommend rolling out pastry dough on a marble surface?

Pastry dough must be kept cool during rolling so that the shortening—most often a solid fat such as butter, lard, or Crisco—doesn’t melt and soak into the flour. If it does, your piecrust will have the texture of a shipping carton. Flaky pastry is produced when many thin layers of dough are kept separated from one another by layers of fat. In the oven the separated dough layers begin to set, and by the time the fat melts, steam from the dough will have forced the layers permanently apart.

Marble is recommended for the rolling surface because, according to the books, it is “cool.” But that’s playing fast and loose with the concept of temperature, because the marble isn’t one bit cooler than anything else in the room.

But, you protest, the marble feels cold. Yes, it does. And so does the “cold steel” of your chef’s knife and every one of your pots, pans, and dishes. In fact, run into your kitchen right now (I’ll wait), pick up anything at all except the cat and hold it against your forehead. By George, everything feels cold! What’s going on here?

What’s going on is that the temperature of your skin is about 95ºF, while the temperature of your kitchen and everything in it is around 70ºF. Is it any surprise, then, that things should feel cold if they actually are 25 degrees colder than your skin? When you touch such an object, heat flows from your skin into the object, because heat always flows from a higher temperature to a lower one. Your heat-deprived skin then sends the message “I feel unusually cool” to your brain.

So it’s not that the object is cold; it’s that your skin is hot. As Einstein never said, “Everything is relative.”

But all things won’t feel equally cold, even though they are all at the same 70ºF room temperature. Go back to the kitchen, please. Notice that the steel blade of your chef’s knife feels colder than, say, the wooden cutting board. Is it actually colder? No, because the two objects have been in the same environment long enough to have come to the same temperature.

The steel knife blade feels colder on your forehead than the wooden cutting board does because steel, like all metals, is a much better conductor of heat than wood is. When in contact with your skin, it conducts heat away into the room much faster than the wood can, thereby cooling your skin faster.

Marble isn’t as good a heat conductor as metal, but it’s ten to twenty times better than a wood or plastic-laminate countertop. Just as marble feels cold to your skin because it steals away heat, the marble feels cold also to the pastry dough because it removes the rolling-generated heat rapidly. Thus, the dough doesn’t warm up enough to melt the shortening.

Okay, okay, so I’m splitting hairs. If something feels cold, acts cold, and does everything but quack like a cold duck, why the heck can’t we just say that it’s cold? So be my guest. Say that marble is cold. But take secret pleasure in the knowledge that it isn’t strictly correct.

Cold-Rolled Pastry

Easy Empanadas

In Spanish, empanada means “breaded,” derived from pan, meaning bread. But that’s a bit misleading, because in Latin America today an empanada is a filled pastry—almost any kind of pastry made from flour or cornmeal and filled with almost anything imaginable, but usually with meats or seafoods of some kind. We might call them turnovers or individual meat pies, and they can be either baked or deep-fried. Every Latin American country has its own versions. They go together quickly if you organize your work area like an assembly line.

In this variation, a traditional filling is wrapped in store-bought puff pastry instead of homemade pastry crust. This avoids the effort of making dough. But with puff pastry it’s particularly important to do the rolling on a “cool” surface such as marble. If marble is not available, roll it out as quickly as possible on a wooden board.

You will find frozen puff pastry sheets in the freezer section of your supermarket. Ground turkey or chicken can be substituted for the beef.

One 17-ounce package frozen puff-pastry sheets

1 tablespoon olive oil

½ cup finely chopped onion

½ cup finely chopped red bell pepper

1 clove garlic, finely minced

1 pound lean ground beef

2 teaspoons all-purpose flour

1 tablespoon chili powder

1 teaspoon salt

½ teaspoon hot pepper flakes

½ teaspoon dried oregano

½ teaspoon ground cumin

¼ teaspoon ground cloves

Freshly ground pepper to taste

3 tablespoons ketchup

1 large egg yolk mixed with 1 tablespoon water

1. Thaw the puff pastry for 8–12 hours in the refrigerator.
2. Heat the oil in a large skillet over medium-high heat and cook the onion and pepper until soft, 5 minutes. Add the garlic and cook 1 minute longer. Add the ground meat and cook until it is browned and crumbles, about 5 minutes. Pour off the accumulated fat. Remove from the heat.
3. In a small bowl, stir together the flour, spices, and seasonings. Add to the meat mixture and mix well. Add the ketchup and mix again. Check the seasonings. It should be spicy.
4. Transfer the mixture to a 10-by 15-inch cookie tray, and spread it out in a thin layer to cool. The empanadas are quickly made if you take an assembly line approach. Divide the filling into 18 small portions of 2 tablespoons each. Here is one way: Using a metal spatula, push the filling into 3 long rows, then divide each row into 6 sections so that the filling is now in 18 small portions. Set aside until needed.
5. Preheat the oven to 400ºF.
6. Remove one thawed puff pastry sheet from the refrigerator. Place it on a well-floured work surface. The sheet will be rather stiff. As soon as it is just warm enough to be unfolded without cracking, open it out flat. Dust both sides with a little flour.
7. With a sharp knife, cut the pastry sheet into three long strips along the fold lines. Cut each strip into three 3-inch squares. Using a rolling pin, roll each square into a 5-by 5-inch square. Flour the squares lightly and stack them to one side. Repeat with the second sheet of pastry. You will have 18 squares.
8. Make the empanadas: Place one square of pastry on the floured surface. Using a small, soft brush, paint a ½-inch strip of egg wash on the left and bottom edges of the square. Place one portion of the meat mixture onto the square slightly toward the brushed corner. Fold over the other half of the pastry to make a triangular turnover. Press the cut edges together. With the tines of a fork, pinch the edges together to seal. With a sharp knife, cut off the ragged edges, if necessary. Transfer the turnover to a baking sheet. Repeat until all of the pastry and filling are used.
9. Lightly brush the empanadas with the remaining egg wash. With the tip of a small knife, poke two holes in the top of each so that steam can escape. Bake for 18 to 20 minutes until puffed and browned. Wrap individually and freeze.

MAKES 18 EMPANADAS

HOT WATER FREEZES FASTER!

My guests were due to arrive for a party in three hours and I needed to make some ice in a hurry. I’ve heard that hot water freezes faster than cold water. Should I have put hot water in my ice-cube trays?

The hot-water-freezes-faster paradox has been debated since at least the 17th century when Sir Francis Bacon wrote about it. Even today, Canadians claim that a bucket of hot water left outdoors in cold weather will freeze faster than a bucket of cold water. Scientists, however, have been unable to explain why Canadians leave buckets of water outdoors in cold weather.

But believe it or not, hot water really may freeze faster than cold water. Sometimes. Under certain conditions. It depends on a lot of things.

Intuitively, it seems impossible because the hot water simply has further to go in its downhill race toward 32°F. In order to chill down by each four degrees, a pint of water has to lose about one calorie of heat. So the more degrees the water has to fall, the more heat must be taken out of it, and that means a longer cooling time, all other things being equal.

But according to Wolke’s Law of Pervasive Perversity, all other things are never equal. As we’ll see, hot and cold water are different in more ways than their temperatures.

When cornered and pressed for an explanation of how hot water could possibly freeze first, chemists are likely to mumble something about cold water containing more dissolved air, and dissolved substances lower the freezing temperature of water. True, but trivial. The amount of dissolved air in cold tap water would lower its freezing temperature by less than a thousandth of a degree Fahrenheit, and no hot-cold race can be controlled that precisely. The dissolved-air explanation just doesn’t hold water.

A real difference between hot and cold water is that the hotter a substance is, the faster it radiates its heat away into the surroundings. That is, warmer water cools off at a faster rate—more degrees per minute—than cooler water does. The difference is especially great if the containers are shallow, exposing large surfaces of water. But that still doesn’t mean that the hot water will reach the finish line first, because no matter how fast it cools off at first, the most it can do is catch up with the cold water. After that, they’re neck and neck.

A more significant difference between hot and cold water is that hot water evaporates faster than cold water. So if we start by trying to freeze equal amounts of hot and cold water, there will be less water remaining in the hot-water container when it gets down to rug-cuttin’ time at 32ºF. Less water, naturally, will freeze in less time.

Can that really make a significant difference? Well, water is a very unusual liquid in many ways. One of those ways is that an unusually large amount of heat must be removed from water before its temperature will go down very much. (Techspeak: Water has a high heat capacity.) So even if the hot container has lost only slightly more water by evaporation than the cold container has, it may require a lot less cooling time to freeze.

Now don’t go running into the kitchen to try it with ice-cube trays, because there are simply too many other factors operating. According to Wolke’s Law, the two trays can never be identical. They are not in exactly the same place at exactly the same temperature, and they are not necessarily being cooled at the same rate. (Is one closer to the cooling coils in the freezer?) Moreover, how are you going to tell exactly when the water freezes? At the first skin of ice on top? That doesn’t mean that the whole tray full has yet reached 32ºF. And you can’t peek too often, because opening the freezer door can cause unpredictable air currents that will affect the evaporation rates.

Most frustrating of all, undisturbed water has the perverse habit of getting colder than 32ºF before it freezes. (Techspeak: It super-cools.) It may refuse to freeze until some largely unpredictable outside influence perturbs it, such as a vibration, a speck of dust, or a scratch on the inside surface of its container. In short, you’re running a race with a very fuzzy finish line. Science isn’t easy.

But I know that won’t stop you. So go ahead and measure out equal amounts of hot and cold water, put them in identical (ha!) freezer trays, and don’t bet too much on the outcome.

HUMPTY DUMPTY NEVER HAD IT SO BAD

Can raw, whole eggs be frozen? I have almost two dozen eggs that I won’t be able to use up before I go on a trip and I’d hate to have them go to waste.

I hate to see food go to waste too, but in this case freezing the eggs might cause more trouble than they’re worth. For one thing, the shells will probably crack because, as you might expect, the whites expand when they freeze, just as water does when it turns to ice. There’s nothing you can do about that. There may also be some deterioration of flavor, depending on how long you keep them in the freezer.

More troublesome is the fact that the yolks will be thick and gummy when you thaw them out. That’s called gelation—the formation of a gel. It happens because as the eggs freeze, some of the proteins’ molecules bind themselves into a network that traps large amounts of water, and they can’t unbind themselves when thawed. The thickened egg yolks won’t be very good for making custards or sauces, where smoothness of texture is important. Using thick-yolked eggs in other recipes can be risky, and if a recipe bombs, you’ll be wasting a lot more than a few eggs.

Next time, leave them in the fridge if your trip isn’t going to last more than a couple of weeks, or hard-cook them all before you leave.

Manufacturers of prepared foods use tons of frozen eggs in making baked goods, mayonnaise, and other products. The gumminess is prevented by adding 10 parts of salt or sugar to every hundred parts of shelled, beaten eggs before they are frozen. I suppose you could do that too if you wanted to take the trouble, but the salt or sugar would sure limit your use of the eggs.

BURN, BABY, FREEZE!

What actually happened to food that is freezer-burned?

“Freezer burn” has to be one of the more ridiculous oxymorons going. But take a good look at that emergency pork chop that’s been in your freezer much longer than you ever intended. Doesn’t its parched and shriveled surface look as if it had been seared?

The dictionary tells us that seared doesn’t necessarily refer to heat; it means withered or dried out, no matter what did the drying. Notice that the patches of “burn” on your forlorn pork chop are indeed dry and rough, as if all the water had been sucked out.

Can the cold alone make frozen foods dry out, especially when the water is in the form of ice? Yes indeed. While your hapless chop was languishing in the freezer, something was stealing water molecules from its icy surface.

Here’s how water molecules, even when firmly anchored in solid ice, can be spirited off to another location.

A water molecule will spontaneously migrate to any place that offers it a more hospitable climate. And to water molecules, that means a place that’s as cold as possible, because that’s where they will have the least amount of heat energy, and “all other things being equal” (see Wolke’s Law of Pervasive Perversity on page 209), Nature always favors the lowest energy. So if the food’s wrapping isn’t absolutely molecule-tight, water will migrate through it, from the ice crystals in the food to any other location that happens to be the tiniest bit colder, such as the walls of the freezer. (That’s why nonfrost-free freezers have to be defrosted.) The net result is that water molecules have left the food, and the food’s surface is left parched, wrinkled, and discolored. Burned-looking.

This doesn’t happen overnight, of course; it’s a slow process that takes place molecule by molecule. But it can be slowed to practically zero by using a food-wrapping material that blocks wandering water molecules. Some plastic wraps are better at this than others.

Moral No. 1: For the long-range keeping of frozen foods, use a wrapping material specifically designed for freezing because of its impermeability to migrant water molecules. Best of all are vacuum-sealed, thick plastic packages like Cryovac, which are quite impermeable to water vapor. Freezer paper is obviously good; it has a moisture-proof plastic coating. But ordinary plastic food wraps are made of various materials, some better than others. Polyvinylidine chloride (Saran Wrap) is the best, and polyvinyl chloride(PVC) is also good. Read the fine print on the plastic wrap package to learn what it is made of. Thin polyethylene food wraps and ordinary polyethylene food-storage bags aren’t very good, but polyethylene “freezer bags” are okay because they’re unusually thick.

Moral No. 2: Wrap the food tightly, leaving no air pockets. Any air space inside a package will allow water molecules to float through it to the inner wall of the wrapping where it is colder, and settle there as ice.

Moral No. 3: When buying already-frozen foods, feel for ice crystals or “snow” in the space inside the package. Where do you think that water (to make the ice) came from? Right: the food. So either it’s become dehydrated from being kept too long in a loose package or it’s been thawed, which releases juices from the food, and then re-frozen. In either case, it’s been abused and, while still safe to eat, will have an off flavor and poor texture.

BLOWING HOT AND COLD

Why does blowing on hot food cool it?

As we have all learned from experience when the etiquette police were looking the other way, the cooling of hot food by blowing on it works best with liquids, or at least with wet foods. You won’t substantially diminish the heat of a hot dog by blowing on it, but hot tea, coffee, and soup are notorious for inspiring such gauche table manners. In fact, it works so well that there must be something more going on than the mere fact that the blown air is cooler than the food.

What’s going on is evaporation. When you blow, you’re speeding up the evaporation of the liquid, just as blowing on nail polish dries it faster. Now everyone knows that evaporation is a cooling process, but almost no one seems to know why.

Here’s why.

The molecules in water are moving around at various speeds. The average speed is reflected in what we call the temperature. But that’s only an average. In reality, there is a wide range of speeds, some molecules just poking along while others may be zipping around like a Taipei taxi. Now guess which ones are most likely to fly off into the air if they happen to find themselves at the surface. Right. The zippy, high-energy ones. The hotter ones. So as evaporation proceeds, more hot molecules are leaving than cool ones, and the remaining water becomes cooler than it was.

But why blow? Blowing on the surface speeds up evaporation by whisking away the newly evaporated molecules and making room for more. Faster evaporation makes faster cooling.

Miss Manners just doesn’t appreciate some of the applications of science to gastronomy.