OUTDOOR COOKING CAN BE A pleasant part of camping or a key to survival in an emergency. It can also provide a study in thermal physics: how heat can be transferred from a heat source to food. This month I analyze several ways of cooking food with flames, coals or charcoal briquettes. The techniques require little or no equipment.

A few fundamental concepts of thermal physics underlie all cooking procedures. One concept involves what is meant by heat and temperature. The atoms and molecules of a substance move randomly at any temperature above absolute zero. In a solid the motion consists in rotation and vibration. In a gas or a liquid the phenomenon also includes the random motion of atoms and molecules that are traveling in straight lines, colliding and then again traveling in straight lines.

When a substance is heated, the heat represents the additional energy imparted to the random motion. Temperature is a measure of the amount of energy in the random motion. Thus when the substance is heated, its temperature increases and the substance is said to be hotter. The heat of cooking increases the energy of the random motion of the atoms and molecules in the food, and the food thereupon cooks by undergoing certain chemical and physical changes.

Conduction, convection and radiation are the three primary ways of transferring heat energy. In conduction the heat is conveyed through some intermediate material such as a metal pan or foil by means of atomic collisions. As the outside surface of the metal warms, the energy in the random motion of the atoms there increases. They collide with atoms somewhat deeper in the metal, giving those atoms some of the kinetic energy derived from the heat source. Eventually atoms on the inside surface receive the energy and collide with atoms on the surface of the food, heating the food. Conduction continues for as long as the temperature of the heat source is above the temperature of the food.

Convection involves the ascent of a heated fluid, either air or a liquid. Heat increases the energy in the random motion of the fluid and decreases the density of the fluid. The surrounding cooler and denser fluid then pushes the heated fluid upward. As the hot fluid passes the food, the atoms and molecules in the fluid collide with those on the surface of the food and transfer energy to them.

Radiation involves the emission and absorption of electromagnetic waves. In cooking the source is light. The surface of a heat source such as burning coal emits light in the infrared and visible parts of the electromagnetic spectrum. Since light has energy, this emission is a radiation of energy. When the light is absorbed by atoms and molecules on the surface of the food, the energy of their random motion increases, as does the temperature of the surface. Heating by radiation therefore requires that the food absorb some of the light (primarily the infrared) emitted by the heat source.

Many campfire-cooking techniques draw on more than one of these primary means of transferring heat. For example, a fire might heat a metal pan by both convection of hot air and radiation of light. As the metal warms, energy is conducted through it to the food. As the surface of the food then heats up, conduction brings the heat into the food.

One of the easiest ways to cook food such as meat is to spear it with a stick or wrap it around the stick and then suspend it over the fire or coals. The food is heated by the convection of rising hot air and by the radiation from the heated surfaces of the wood and from the hot regions in the flame. You can save work by propping the stick over the fire or suspending it across the fire by means of two forked sticks driven into the ground on opposite sides of the campfire.

A large piece of meat suspended over the fire must be turned frequently, because only the side toward the fire gets the effect of the rising hot air and the radiation. The rig known as a dingle fan, probably from the logging camp shed called a dingle, is helpful in this task. To make the apparatus attach a short chain to the upper end of a stick that is angled upward over the perimeter of the campfire. Suspend the meat from the chain by a string attached to a hook in one end of the meat. Tie a short stick to the string. One end of the stick holds a fan made of wire or branches wrapped in aluminum foil or leaves. To the other end attach a small rock to serve as a counterweight to the fan. Orient the plane of the fan somewhat off the vertical and arrange the entire assembly so that the fan is in the hot air rising from the campfire. The meat is not in that convection current but is exposed to the radiation from the fire.

The rising hot air pushes against the underside of the fan. The force moves the fan to one side, twisting the chain and rotating the meat. Once the fan is out of the convection current the chain untwists, rotating the meat in the opposite direction. It overshoots the original position, again twisting the chain. The cycle continues indefinitely, exposing about half of the meat to the radiation. After a while invert the roast and hang it from a hook on the other end to expose the other half of the meat to the radiation.

To fry food you can make a stove from an empty No. 10 can. Remove one end plate of the can and cut a flap at the open end. Bend the flap outward. Push the loose end plate into the can and against the other end plate. With a can opener (the kind that punches triangular holes) or a knife, cut flaps in the can in several places near the closed end. Push the flaps into the can and against the loose end plate to hold it near the fixed end one. Place the open end of the can over a heat source. The upper end plate serves as a surface on which eggs, bacon and other items can be fried.

The can functions as a chimney because cool air is sucked in through the open flap at the bottom to replace the hot air rising to the top and out through the holes there. The strong flow of air through the can fans the fire and keeps it burning briskly.

You could make the stove without the loose end plate held near the top. That plate, however, helps to produce a nearly uniform temperature over the entire cooking surface. Without this plate the part of that surface directly above the heat source would be hotter than the rest of the surface because it receives more radiation from the source. The loose plate is intended to heat the small layer of air above it, transferring heat to the cooking surface evenly by the conduction and convection through the air.

A popular heat source for the stove is a "buddy burner," a small can filled with corrugated cardboard over which hot paraffin has been poured. When the can is brought out for cooking, the paraffin is solid. A match melts and vaporizes some of it, and thereafter the vapor burns. The flame melts more paraffin, which is drawn to the top of the cardboard, where it vaporizes and burns. The cardboard also burns, but slowly, like the wick of a candle. A damper can be placed over part of the burner to slow things down if the stove gets too hot. Make the damper by fold ing the lid from the can over a doubled piece of wire.

Charcoal briquettes also serve well as a heat source, but it is hard to light them. One easy solution is to put them in a can that works like a chimney. Cut both ends off a large can and punch flaps into the side near one end. Fold the flaps inward. Push large wads of newspaper against them so that they hold the paper in place. Prop the can against a rock with the paper downward. Leave an air space between that end and the ground. Put the briquettes in the can and ignite the newspaper. The flame pulls air into the bottom of the can to replace the hot air rising through the briquettes and out of the can. The strong flow of air makes the flames ignite the briquettes. When the newspaper has burned up and the briquettes begin to fall to the ground, lift the can off them. You now have your cooking fire.

The can stove can be converted into a small oven by means of two more cans and several flat rocks. One of the cans should be larger than the other but no wider than the top of the stove. Remove one end of the smaller can and both ends of the other one. Wrap transparent plastic around an open end of the larger can, holding it in place with a piece of wire. Tuck the ends of another piece of wire under the first one to form a handle. The plastic, commonly called oven wrap, is designed for oven cooking; it will not melt at typical oven temperatures.

Lay the flat rocks on top of the stove. Position the smaller can on them with its open end upward. Put the larger can over that with the plastic upward. This assembly is an oven. Food can be baked in the smaller can. The plastic serves as the oven window.

Several other simple baking rigs are available to the campfire cook. In one of them you put a small can on several flat rocks on the ground and put a larger can over it. (Take one end plate out of the larger can but leave the other one in place.) Heap coals or briquettes on the top plate and around the sides of the larger can. As that can gets warm the air trapped in it heats up too, forcing hot air to flow around the smaller can, which holds the food.

You can also bake or roast food by wrapping it in heavy aluminum foil and putting it directly on the coals. It can be helpful to pile dirt on the foil. The dirt cuts down the supply of oxygen, causing the coals to burn incompletely, but it also traps the heat for as long as they do burn.

One disadvantage of this technique is that some parts of the food will cook too fast and may burn. To eliminate the problem wrap the food in two layers of foil with several layers of newspaper between them. The air trapped between the layers slows the transfer of heat; air is a poor conductor of heat and the pockets of air are too small to allow convection.

You can also protect the food from overrapid transfer of heat into the foil if at least part of the food is fairly moist. Then much of the heat goes into heating and vaporizing the water. The steam transfers heat to the rest of the food, cooking it.

The skin of an orange serves admirably as a baking dish. Slice off the top third of a large orange. Scoop out the rest of the orange and then partially fill it with dough. Replace the top of the orange and put the entire assemblage directly on the coals. Keep the orange upright so that the top stays on.

Heat conducted through the skin of the orange bakes the dough. Since the skin is thick and moist, it does not conduct the heat rapidly. Hence the dough is buffered against overheating at the places where the orange touches coals. Apples and bananas can be substituted for an orange. A large onion can be employed similarly to bake an egg or small pieces of meat.

You can also bake with a reflector oven that utilizes the heat radiated by a campfire. This type of oven can be made of aluminum foil or metal plates (such as cookie sheets) supported by sticks. In the simplest version the food is put on a flat piece of foil. Another piece of foil set at an angle over the flat one reflects heat onto the food.

One side of aluminum foil is ordinarily shinier than the other. Position the shiny side of the reflecting layer toward the food. The foil holding the food should be placed with the dull side upward so that it absorbs some of the radiated heat instead of reflecting it all away.

A more ambitious reflector oven includes a second tilted layer of foil below the level one; it reflects radiation onto the bottom surface of the cooking layer. Here the level sheet should be a rigid layer of metal instead of foil, which would buckle under the weight of the food.

A crude reflector oven can be made out of a large can from which both ends have been cut. Place it on its side so that an open end faces the coals. The food goes inside. The coals radiate heat into the can, where it reflects into the food.

You can make a skillet for frying by wrapping heavy aluminum foil around the forked end of a stick. Wrap one layer of

foil around the stick. Add several layers of newspaper and finally another layer of foil. The air trapped in the paper slows the transfer of heat, providing a more uniform temperature over the cooking surface.

In an emergency a flat rock put on the coals can serve as a skillet. Make sure that the rock is not wet or porous, water inside it may expand and blow the rock apart.

Some manuals for outdoor cooking describe how a whole chicken can be cooked with small rocks. Wrap the rocks in foil and put them on hot coals When they are quite hot, transfer them to the interior of the chicken. Wrap the chicken in foil and place it in a container insulated with layers of newspaper or leaves. It will cook in an hour or more, depending on the temperature of the rocks. To avoid the possibility of salmonella and other types of food poisoning make certain that the chicken cooks fully and is eaten soon afterward.

Another unusual cooking technique depends on the fact that liquid water can be no hotter than its boiling temperature of 212 degrees Fahrenheit. Relying on this fact, you can hard-boil an egg in a paper cup put directly on the coals. Paper will not burn until it reaches a temperature of at least 400 degrees F. The paper is so thin that the heat it receives from the coals is immediately conducted to the water.

Suppose you add coals below the cup or fan the coals already there so that heat is conducted faster into the water. Does the temperature of the water increase? No; the effect of the additional heat is to increase the frequency with which bubbles of water vapor form on the bottom. Because the water never gets hotter than 212 degrees F., the paper does not burn.

If the upper part of the cup is dry and receives enough heat by radiation from the coals or the rising hot air from them, it will ignite. To boil an egg in a paper cup you would do well to put enough water in the cup so that when the egg is in place, the cup is nearly full. The same principles apply when you boil water in containers made of canvas, bark, leaves or coconuts and placed directly on the coals.

This fact about water accounts for another curious technique in cooking. Crack an egg (or more than one) into a small paper bag. Close the bag and fold the top down several times. Pierce the folded section with a stick. Prop the stick so that the bag is suspended above hot coals. The eggs will cook. The water in them keeps the bag from reaching its ignition point.

One drawback of this technique is that the eggs tend to adhere to the paper. My son Christopher found that the problem can be surmounted by coating the bottom interior of the bag with margarine or butter. (Some people solve the problem by lining the bottom of the bag with bacon. I avoid this procedure; if I cook the eggs for the proper length of time, the bacon is not cooked enough. Moreover, in order to kill the parasitic worm Trichinella spiralis that might reside in the bacon, all of the meat should be heated to at least 149 degrees F. The bacon in the bag is not that fully heated.)

You could look into a variety of other outdoor-cooking schemes. The most intriguing ones require only components that can be found in nature. You might also study the relative reflectance of thermal radiation by the two sides of aluminum foil or the rate at which heat is conducted through material such as an orange skin. Let me know what you find.

Bibliography

ROUGHTNG IT EASY. Dian Thomas. Warner Books, 1975.

BOY SCOUT FIELDBOOK. Boy Scouts of America. Workman Publishing Company, 1978.

THE U.S. ARMED FORCES SURVIVAL MANUAL. Edited by John Boswell. Rawson, Wade Publishers, Inc., 1980.

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