the invisible ingredient

Physics + food = fun. This article's a good reminder (and taught me several things I didn't already know) about getting the best results when cooking.

The Curious Cook: The Invisible Ingredient in Every Kitchen
By HAROLD McGEE
January 2, 2008
OF all the ingredients in the kitchen, the most common is also the most mysterious.

It’s hard to measure and hard to control. It’s not a material like water or flour, to be added by the cup. In fact, it’s invisible.

It’s heat.

Every cook relies every day on the power of heat to transform food, but heat doesn’t always work in the way we might guess. And what we don’t know about it can end up burning us.

We waste huge amounts of gas or electricity, not to mention money and time, trying to get heat to do things it can’t do. Aiming to cook a roast or steak until it’s pink at the center, we routinely overcook the rest of it. Instead of a gentle simmer, we boil our stews and braises until they are tough and dry. Even if we do everything else right, we can undermine our best cooking if we let food cool on the way to the table — all because most of us don’t understand heat.

Heat is energy. It’s everywhere and it is always on the move, flowing out as it flows in. It roils the chemical innards of things, exciting their molecules to vibrate and crash into each other. When we add a lot of heat energy to foods, it agitates those innards enough to mix them up, destroy structures and create new ones. In doing so it transforms both texture and flavor.

There are, however, uncountable ways to misapply heat. In most cooking, we transfer energy from a heat source, something very hot and energetic, to relatively cold and inert foods. Our usual heat sources, gas flames and glowing coals and electrical elements, have temperatures well above 1,000 degrees Fahrenheit. Boiling water is around 212 degrees.

Cooks typically heat food to somewhere between 120 degrees (for fish and meats that we want to keep moist) and 400 degrees (for dry, crisp, flavorful brown crusts on breads, pastries, potatoes, or on fish and meats).

At the bottom of that range, a difference of just 5 or 10 degrees can mean the difference between juicy meat and dry, between a well-balanced cup of coffee or tea and a bitter, over-extracted one. And as every cook learns early on, it’s all too easy to burn the outside of a hamburger or a potato before the center is warm.

That’s the basic challenge: We’re often aiming a fire hose of heat at targets that can only absorb a slow trickle, and that will be ruined if they absorb a drop too much. Are you ever annoyed by pots that take forever to heat up, or frustrated by waiting for dry foods to soften? A kitchen that becomes hot enough to be a sauna? Big jumps in the utility bill when you do a lot of cooking? The problem, as you will notice if you pay more attention to your kitchen’s thermal landscape, even in terms of what you can feel, is how much heat escapes without ever getting into the food.

Among the major culprits here are inefficient appliances. According to the United States Department of Energy, a gas burner delivers only 35 to 40 percent of its heat energy to the pan; a standard electrical element conveys about 70 percent. Anyone thinking about kitchen renovation should know that induction cooktops, which generate heat directly within the pan itself, are around 90 percent efficient. They can out-cook big-B.T.U. gas burners, work faster, don’t heat up the whole kitchen, and are becoming more common in restaurant kitchens.

Maximizing the transfer of heat from burner to pot produces better food. In deep frying, the faster the burner can bring the oil temperature back up after the food is added, the quicker the food cooks and the less oil it absorbs. In boiling green vegetables, a fast recovery time means better retention of vibrant color and vitamins.

No matter how efficient an appliance is, the cook can help simply by covering pots and pans with their lids. Some of the heat that enters through the bottom of the pot exits through the top, but a lid prevents much of it from escaping into the air. This is especially true when you’re bringing a pot of water to the boil. With the lid on, it will start bubbling in as little as half the time. Turning water into steam takes a lot of energy, and every molecule that flies away from the water surface takes all that energy with it into the air. Prevent its escape, and the energy stays with the pot to heat the rest of the water.

Once a liquid starts to boil and is turning to steam throughout the pot — the bubbles of a boil are bubbles of water vapor — nearly all the energy from the burner is going into steam production. The temperature of the water itself remains steady at the boiling point, no matter how high the flame is underneath it. So turn the burner down. A gentle boil is just as hot as a furious one.

Cooking doesn’t get much more straightforward than boiling and steaming vegetables, grains, and the like. But sometimes it takes forever, which either delays dinner or results in crunchy beans and diners suffering for it later. And you can’t speed the process by raising the heat.

In fact it’s easy to save loads of time and energy and potential discomfort with grains, dry beans and lentils, and even pasta. But it requires a little thinking ahead. It turns out that the most time-consuming part of the process is not the movement of boiling heat to the center of each small bean or noodle, which takes only a few minutes, but the movement of moisture, which can take hours. Grains and dry legumes therefore cook much faster if they have been soaked. However heretical it may sound to soak dried pasta, doing so can cut its cooking time by two-thirds — and eliminates the problem of dry noodles getting stuck to each other as they slide into the pot.

The trickiest foods to heat just right are meats and fish. The problem is that we want to heat the center of the piece to 130 or 140 degrees, but we often want a browned, tasty crust on the surface, and that requires 400 degrees.

It takes time for heat to move inward from the surface to the center, so the default method is to fry or grill or broil and hope that the browning time equals the heat-through time. Even if that math works out, the area between the center and surface will then range in temperature between 130 and 400 degrees. The meat will be overcooked everywhere but right at the center.

The solution is to cook with more than one level of heat. Start with very cold meat and very high heat to get the surface browned as quickly as possible with minimal cooking inside; then switch to very low heat to cook the interior gently and evenly, leaving it moist and tender.

On the grill, this means having high- and low-heat zones and moving the food from one to the other. On the stove top or in the oven, start at 450 or 500 degrees, and then turn the heat down to around 250, ideally taking the food out until the pan or oven temperature has fallen significantly.

Another solution is to cook the food perfectly with low heat, let it cool some, and then flavor its surface with a brief blast of intense heat from a hot pan or even a gas torch. More and more restaurants are adopting this method, especially those that practice sous-vide cooking, in which food is sealed in a plastic bag, placed in a precisely controlled water bath and heated through at exactly the temperature that gives the desired doneness.

All these are two-step processes, but the same principle works for three steps or more. Rotisserie cooking alternates high and low heat many times: as the meat turns on the spit, each area of the surface is briefly exposed to high browning heat, then given time for that dose of energy to dissipate, part of it into the meat but part back out into the cool air. So the meat interior cooks through at a more moderate temperature. Similarly, steaks and chops cook more evenly on high grill heat — and faster as well — if you become a human rotisserie and turn them not once or twice but as often as you can stand to, even dozens of times, every 15 or 30 seconds.

Tough cuts of meat require longer cooking to dissolve their connective tissue, and stewing or slow braising in a low oven is a simple and popular method of doing so. But many recipes don’t give the best results, simply because they don’t take into account the vast difference between cooking with the lid on and off. Even in an oven set as low as 225 or 250 degrees, if the pot is covered, the contents will reach the boil, and the meat will overcook and dry out.

Leave the lid ajar or off, and evaporation of the cooking liquid cools the pot and moderates the meat temperature, keeping it closer to 160 to 180 degrees. This is hot enough to soften the connective tissue in a few hours without also driving out most of the meat’s moisture.

The challenge of heat management doesn’t end when you’ve cooked something to perfection. How often have you found that the dish that was perfect in the kitchen seems to have lost something by the time you sit down and take the first bite? That something certainly includes heat.

Heat knocks molecules at the surface of food into the air where we can sniff them, so it increases the aroma. Inside the food, agitated molecules make sauces more fluid and hot meat more tender. And the sensation of a food’s warmth is satisfying in itself. The moment hot food is put on a plate, its heat energy begins to flow out into the cooler surroundings. Aromas fade, sauces thicken, fats congeal.

So when you transfer heat’s handiwork from the kitchen to the table, take along some extra. Warm the plates to prolong the pleasure. And encourage everyone to sit down and eat it while it’s hot.