THE SECRET OF A MICROWAVE oven's fast action is that water in the food rapidly absorbs the energy of the waves. At first thought this is surprising. In most cases where a material absorbs light or other types of electromagnetic radiation there must be a resonant match between the energy of the radiation and the energy changes possible in the atoms and molecules of the material. Water absorbs infrared radiation resonantly, but the frequency of microwaves is far too low for a resonant match. How, then, is energy transferred from the microwaves to the random thermal motion of the molecules of water? Several hypotheses I shall discuss represent a groping toward the answer. First one needs a bit of background on the physics of microwaves and the water molecule (H₂O), consisting of two atoms of hydrogen and one atom of oxygen.

Microwave frequencies range from 1 X 10⁹ hertz (cycles per second) to 5 X 10¹² hertz, well below the frequency of visible light (approximately 6 X 10¹⁴ hertz). My oven emits waves at the frequency of 2.45 X 10⁹ hertz, or 2.45 gigahertz. Except for the frequency, microwaves and visible light are similar. Each can be pictured as a wave of oscillating electric fields.

A water molecule can be rotated by the electric field of a microwave because of the arrangement of the electric charge within the molecule. The electrons (negatively charged) associated with the hydrogen atoms are shifted toward the oxygen atom because of their strong attraction to the eight positively charged protons in the oxygen. The shift leaves the oxygen end of the molecule negative and the hydrogen ends positive. Such a charge distribution is called an electric dipole. Although the molecule as a whole is electrically neutral, it contributes an electric field to its surroundings and can be rotated by an electric field imposed on it. The dipole moment is the product of the net charge at each end and the separation between the charges. The moment is represented by a vector that points from the negative oxygen end along the line of symmetry between the hydrogen ends.

Ordinarily the dipole moments in water are randomly oriented. If an electric field is imposed, however, it creates a torque on each molecule. The torque makes the molecule rotate to align its dipole moment with the field. Picture an electric field produced by two charged, parallel plates. The direction of the field is from the positive plate to the negative one. The water molecule rotates so that it presents its negative end to the positive plate and its positive ends to the negative plate, thereby aligning itself with the field.

Every molecule is constantly buffeted by the random thermal motion of the surrounding molecules. The random motion, which sometimes goes by the name of Brownian motion, is related to the temperature of the water. Heat gives the molecules more kinetic energy, so that in their random motion they strike one another more vigorously. The temperature rises.

The polarization of water is expressed as the net dipole moment per unit of volume. It is zero when the dipole moments are randomly oriented, because for every moment pointing in one direction another moment points in the opposite direction. When an electric field begins to align the dipole moments, polarization increases. It would be at a maximum if all the dipoles were in alignment. Random molecular motion, however, continuously knocks dipoles out of alignment, keeping the polarization below the maximum level.

Now for the first hypothesis. Early in this century the eminent Dutch physicist Peter J. W. Debye demonstrated mathematically why microwave energy is strongly absorbed by water. The key fact is that water molecules cannot rotate instantaneously into alignment with the electric field. Since they have a mass spread over a certain volume, it takes time for torque to make them rotate. The retarding forces from surrounding molecules also affect rotation.

The response time of water can be considered in terms of the decay of its polarization if an electric field is suddenly turned off. Random molecular motion then begins to destroy the alignment of the dipole moments. The effect is to reduce the polarization exponentially with time.

Water's response time determines whether the dipole moments can keep up with the oscillating electric field in an electromagnetic wave. At low frequencies the time taken by the electric field to change direction is longer than the response time of the dipoles, and polarization keeps in phase with the electric field. The field provides energy to make the molecules rotate into alignment. Some of the energy is transferred to the random motion each time a dipole is knocked out of alignment and then realigned. The transfer of energy is so small, however, that the temperature hardly rises. If the electric field oscillates rapidly, it changes direction faster than the response time of the dipoles. Since the dipoles do not rotate, no energy is absorbed and the water does not heat up.

In the microwave range of frequencies the time in which the field changes is about the same as the response time of the dipoles. They rotate because of the torques they experience, but the resulting polarization lags behind the changes in the direction of the electric field. When the field is at maximum strength, say in an upward direction, polarization may still be low. It keeps rising as the field weakens. The lag indicates that the water absorbs energy from the field.

Microwave ovens operate at a frequency that is lower than the frequency at which absorption is greatest. The practical reason is that the user wants to heat food throughout its interior. If the frequency is optimal for a maximum heating rate, the microwaves are absorbed in the outer regions of the food, penetrating only a short distance. If the frequency is lower, say 2.45 gigahertz, the penetration improves. Some ovens operate at a frequency of .915 gigahertz, and so the penetration is even greater.

As neat as Debye's mathematical solution is, it leaves unanswered the question of exactly how energy is transferred from the microwaves to the random motion of the molecules. Debye put forward a simple model, which serves as the second hypothesis. Consider a water molecule as being spherical. When the sphere is rotated by the electric field of a microwave, it experiences viscous drag from the water around it. The drag is important only at microwave frequencies. At lower frequencies the sphere rotates too slowly to encounter drag. At higher frequencies it does not rotate at all.

At microwave frequencies it rotates fast enough so that the drag fights the rotation, requiring the field to supply additional energy. This is the energy that goes into the random motion of the molecules surrounding the sphere, raising the temperature. Exactly why the sphere experiences a viscous drag and how the energy is transferred to the surrounding molecules was not understood in Debye's time and is still not understood in detail. A simple explanation can be made. Suppose the sphere is initially in some brief state of equilibrium with the electrical forces from the surrounding molecules. If the sphere is to turn, it must upset the equilibrium and move the molecules, thereby increasing the energy in their random motion.

As crude as Debye's simple model is, it is surprisingly accurate in predicting the frequency at which the absorption rate is greatest. Debye assumed that the molecule has a radius of 2 X 10^-10 meter and that the viscosity it encounters is the viscosity of bulk water. With his model he calculated that the response time should be about 2.5 X 10^-11 second. Since the frequency at which the absorption rate is greatest is approximately the inverse of the response time, it should be about 40 gigahertz, which is about right in terms of the model.

A more refined model explaining how microwaves heat water has since been proposed. It is the third hypothesis. In addition to individual molecules water is made up of many short-lived groups of molecules. The groups are held together by hydrogen bonds: the hydrogen ends of one molecule are attracted to the oxygen end of another molecule. Every time two or more molecules form a group they lower their electric-potential energy. The difference in energy is added to the kinetic energy of the group's random motion. The temperature does not rise because on the average the number of molecular groups that form equals the number of groups broken apart by collisions from the random motion, leaving no net gain in the energy of the random motion.

A microwave adds energy to the random motion if the torques it places on the molecules break some of the hydrogen bonds in the groups. (The electric field supplies the energy to break the bonds.) When a liberated molecule again forms a hydrogen bond with a group, the decrease in potential energy goes into the random motion of the molecular group.

Which of the various groups might be susceptible to such bond breaking? Individual molecules do not take part because the torque need not break a bond to rotate the molecule. Groups of two molecules are also not likely to participate because each molecule can be rotated about the hydrogen bond holding them together, with no bond breaking. Groups of four or more molecules probably do not participate either, because several bonds would have to be broken.

The best candidate for the process is a group of three water molecules in a particular arrangement [see Figure 8]. The middle molecule is attached to the second molecule by one of its own hydrogen ends. The other hydrogen end is free. Attached to the oxygen end of the middle molecule is the third water molecule. The attachment is by way of a hydrogen end on the third molecule.

The position of the attachment is important. Calculations indicate that there are two sites at the oxygen end of the middle molecule where the attachment would lower the potential energy the most. Attachments at other places are therefore less likely. For example an attachment is not likely to be made near the free hydrogen end of the middle molecule because that hydrogen end will repel the hydrogen end of the third molecule.

The torque on the middle molecule from the electric field of a microwave may be large enough to break the hydrogen bond with the third molecule. After the middle molecule rotates, the third one could reestablish its hydrogen bond at the other low-energy spot on the oxygen end of the middle molecule. Such a process puts energy into the random motion of the group. The field supplies energy to break the initial hydrogen bond. When the bond is reestablished, the energy goes into the kinetic energy of the group. In this way the microwave energy ends up as heat.

So much for hypotheses on how the water in food rapidly absorbs microwave energy. I shall now turn to some of the other things that go on in a microwave oven.

The presence of sodium chloride in the water increases the heating rate. The salt separates into positive sodium ions and negative chlorine ions. The positive ions are surrounded by up to four molecules of water, the negative ions by up to seven. In each case the positive or negative end of a water molecule is electrically attracted to the charged ion. The electric field of a microwave drives the hydrated ions through the water, pushing the sodium ions in the direction of the field and the chlorine ions in the opposite direction. Whenever the hydrated ions bump into water molecules, energy is transferred to the random motion of t hose molecules, heating the water.

The water molecules locked into the crystalline structure of ice cannot absorb energy from microwaves because they are immobile. How then does a microwave oven melt ice or cook frozen foods? The answer is that the objects are not completely frozen. Within seconds after you remove an ice cube from the freezer its surface begins to melt. If you put it in a microwave oven, the liquid layer on the outside absorbs microwaves, heats up and melts the ice.

Frozen foods exposed to air may also be coated with a layer of liquid. They melt there and at many internal points where

the water is liquid. If they are exposed to microwaves, the pockets heat up rapidly. As their surroundings thaw, the new water begins to absorb even more energy from the microwaves. This situation is called runaway heating. In order to avoid it food should be thawed at low power or exposed to high power only periodically so that the heat can get into the frozen sections by conduction, thereby thawing the food more uniformly.

In conventional cooking the food is kept in a hot environment so that heat can move from the surface into the interior by conduction or convection When a beef roast is prepared in this way, the environment may reach 170 degrees Celsius (338 degrees Fahrenheit), considerably hotter than the boiling temperature of water. The interior probably never reaches a temperature higher than 70 or 80 degrees C. At that temperature the myoglobin pigment in the meat changes to oxymyoglobin, which is bright red. Meanwhile the surface of the meat becomes so hot that its oxymyoglobin denatures, turning brown. The high temperature also changes the flavor and aroma of the surface of the meat.

When beef is cooked in a microwave oven, the water heats the solid material. The surface never reaches a, temperature higher than 100 degrees C. the boiling point of water. Since that temperature is not high enough to denature the oxymyoglobin fully, the surface never becomes dark brown. The meat also never develops the flavor and aroma of meat cooked in a conventional oven.

If the meat is thin, the entire interior cooks by the direct absorption of microwaves. If the meat is thick, as a roast usually is, the microwaves are absorbed before they reach the center Heat is conducted to the center from the region directly heated by the microwaves. Because conduction takes time, a large roast must be allowed to sit after it has been heated in a microwave oven.

Liquid water is heated so rapidly by microwaves that steam can present a problem. If an egg or even an exposed unbroken yolk is heated by microwaves, the rapid interior production of steam can make the object explode. If food is cooked in a closed container, an escape route must be provided for the steam. That is why plastic cooking bags must be slit at the top.

Metal containers should be avoided in microwave cooking for several reasons. Metal reflects microwaves, thereby shielding the food and possibly returning enough energy to the microwave emitter to overload it. Because metal conducts electricity, sparks jump between the container and the bottom or walls of the oven. As I learned by accident, even a wire twist closing a plastic bag can produce frightening sparks if it is near the bottom of the oven. Aluminum foil is sometimes used in microwave cooking to shield parts of a fowl that might overcook. The foil is not dangerous if there is not much of it and it is kept at least several centimeters from the bottom and walls of the oven. The containers usually recommended for microwave cooking are often plastic or glass. They contain materials that absorb microwaves poorly or not at all. The containers get hot only because energy is conducted into them from the food. Sometimes the containers do absorb a small amount of microwave energy, with the result that they help to heat the food.

In 1986 Robert E. Apfel of Yale University and Richard L. Day of the Yale University School of Medicine pointed out that when water is heated in a glass container in a microwave oven, it can superheat up to 110 degrees C. with no sign of boiling. The water in the center of the container heats so fast that convection to the top surface and the subsequent vaporization are insufficient to prevent superheating. Bubbles of water vapor do not form in the cooler water along the sides of the container, partly because the container does not absorb microwaves and conducts heat from the water. If an ice cube is dropped into the superheated water, vapor bubbles rapidly form in the abundant microscopic crevices on the surface of the ice. The cube continues to initiate boiling until the temperature of the surrounding water drops to about 102 degrees.

I investigated a similar example of superheating by partially filling a glass beaker with water and then pouring a layer of corn oil on the water. When the water superheated, vapor bubbles formed on the glass sides and then forced their way through the oil. In one trial the water apparently superheated so much that water and oil were blown throughout the oven by a sudden and extensive vaporization in the water. Similar minor explosions can result when grease from cooking meat lies on top of water in the roasting pan. I avoid a mess in the oven by enclosing the meat in a plastic bag that I have slit in a few places.

Apfel and Day also observed that a spoon can initiate boiling in superheated water. Anthony E. Siegman of Stanford University had previously written to me about the phenomenon. Anthony Parsons of the University of York recently noted that powder added to superheated water results in such rapid boiling that the contents may overflow the container.

Bibliography

AQUEOUS DIELECTRTCS. J. B. Hasted. Chapman and Hall, 1973.

INITIATING BOILING WITH ICE. Robert E. Apfel and Richard L. Day in Nature, Vol. 321, No. 6071, page 657; June 12, 1986.

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