THE SCIENCE OF MICROWAVE COOKING
Cooking – it’s about the heating of food
Cooking is about heating food to a desired temperature to change the chemical composition of the raw ingredients in order to enhance flavor, to change the texture so that the food is pleasant to eat, and to kill microbes which could cause disease or spoil the food. Heat is brought to the food by different mechanisms, according to the cooking method.
Heat transfer mechanisms
Heat can be brought from one place to another by three physical mechanisms: conduction, convection, and radiation.
All material including food, cookware, and gases and liquids contacting the food, are made of atoms, which may be joined together to form molecules and solids. These atoms and molecules are not stationary, but rather vibrate, rotate, and move back and forth, and collide with each other, to a degree which increases with their temperature. The “hotter” atoms have more energy and vibrate and move about faster. Each collision transfers some energy from one atom to another.
If one portion of a material is hotter than another portion, the atoms in the hotter portion will collide with their neighbors in the direction of the colder portion, and transfer energy to them, and thus heat them. These atoms in turn collide with additional atoms even closer to the colder region, and thus there is a flow of energy known as thermal conduction from the hotter part of the material to the cooler part.
Conduction can occur, for example, through the air in a conventional oven, i.e. from the hot air near the flame or heating element, to the food. There can also be conduction from a hot pot, to food in direct contact with the pot. And there will always be conduction within food, from the hot surface towards the interior.
Convection can be understood by considering the following example: heat water in a kettle on the stove, and then carry the kettle to a sink, and pour the hot water over some greens in order to blanch them. Heat was transferred by carrying the hot water in the kettle. This illustrates the basic idea of convection, but usually in cooking, heat is transferred by the flow of liquids and gases. This can occur naturally (i.e. natural convection), because hot gases and liquids rise, forming a flow. This can be seen, for example, when heating water in a pot. Water is heated at the bottom of the pot, which is in contact with the heat source, and then rises, initiating a circulation of water within the pot. Or the convection can initiated by some external means (forced convection). For example, in convection ovens, a fan forces a flow of air from the heating elements towards the food. In most cases, convection can transfer heat faster in gases and liquids than conduction.
Radiation in the form of electromagnetic waves can also heat food. Electromagnetic waves include radio waves, infrared radiation, visible light, ultraviolet radiation, and x-rays. The same physical principles govern all of these types of radiation – however they differ in frequency f and wavelength λ. An electric field In the electromagnetic wave can push or pull electrical charges. This field changes direction very rapidly, i.e. it goes from pushing a charge to pulling a charge and back to pushing gapproximately 100 million times a second (100 MHz) for FM radio waves. This number is the frequency f, and your FM radio allows you to tune in different stations by choosing f, i.e. specifying some particular f in the range of 88 to 108 MHz. As this wave travels through space, at any given instant there will be some places where the electric field will maximally push a charge, and this field will decrease in some direction to 0, then increase so that it will pull a charge, again, decrease to 0, and then increase so that again it pushes a charge, and so forth periodically. The distance between adjacent places where the pushing force is maximum is known as the wavelength. The wavelength of visible light is between 0.4 and 0.8 μm (μm = micrometer, or “micron”, is 1 millionth of a meter), while it is 0.8 to 10 μm for infrared radiation, and 3 m for a 100 MHz radio wave. The frequency and wavelength for a given electromagnetic wave are related through the formula λ=c/f, where c = 300 000 000 m/s is the speed of light, and all other electromagnetic waves.
Electromagnetic waves of any frequency can in principle heat food, but the nature of the heating will depend on how the food absorbs the energy carried by the electromagnetic wave. Radiation is the most important heat transfer mechanism in several forms of conventional cooking, such as grilling, broiling, and toasting, and also plays an important role in roasting. In these cases the electromagnetic waves are mostly in the infrared region.
Infrared radiation is very quickly absorbed by most foods, i.e. all of the energy is delivered to the surface. The inside of the food is heated purely by conduction in solid food, and by some combination of conduction and convection in liquids. In other words, when you grill meat on the BBQ, infrared radiation heats the surface, sometimes very rapidly, while the interior is heated from the surface by conduction, and this can take some time.
Microwave cooking also uses electromagnetic waves, but of much lower frequency and longer wavelength than the infrared radiation used in conventional cooking. In household microwave ovens, f = 2 450 MHz, and λ = 12.2 cm. Food does not absorb this wavelength as rapidly as infrared radiation, and hence the microwave penetrates into the food far deeper (e.g. typically several cm’s) than do infrared waves.
Food is comprised of molecules, the most common of which is the water molecule, H2O. This molecule contains two hydrogen atoms, and one oxygen atom. All atoms are built from an equal number of positively charged particles called protons, and negatively charged particles called electrons. When these atoms are put together to form a water molecule, the arrangement is not symmetric – the two hydrogen atoms are close together at one end of the molecule. While there is an equal amount of positive and negative charge in the molecule, so that the molecule is electrically neutral, these charges are not distributed within the molecule uniformly. The hydrogen end of the molecule has more positive charge, while the oxygen side has more negative charge. Such a molecule, in which the opposite ends have different local charges, is said to be polarized.
When a polar molecule, such as water, is placed in an electric field, the field will push the positive charge in one direction, and pull the negative charge in the other direction. The electrical field at any given time can be thought as having a direction, which can be represented as an arrow. Because of this pushing and pulling, polar molecules such as water will be aligned with this arrow, with the positive end in the direction of the arrowhead, and the negative charge in the direction of the tail. The electric field in an electromagnetic wave is reversing its direction with frequency f. This rotates the molecule at frequency f as well, and thus transfers energy to the water molecule. The water molecules collide with the other molecules in the food, and transfers part of this energy to them. This transfer of energy from the electromagnetic wave, directly to the water (and other polarized) molecules, and indirectly, through collisions, to the other molecules, heats the food.
The key difference in microwave cooking is that the microwaves penetrate some distance into the food. The penetration distance is determined by the chemical composition of the food, and its temperature. In particular, food containing a lot of water will have a shorter penetration distance than “drier” food, but in any event, the penetration depth of the microwaves will be much further than infrared waves used in grilling and toasting.
The penetration of the microwaves into the food has two important consequences in cooking. First, if the thickness of the food is less than the penetration depth, the cooking time will be much shorter than in conventional cooking. This is because in conventional cooking, all of the energy from the heating source is deposited at the surface of the food, and relatively long times are required for the heat to diffuse (i.e. be conducted) into the interior, while in microwave cooking the interior is heated directly by the microwaves which penetrate inside. It should be noted, however, that if the food is very thick, i.e. much thicker than the penetration depth, time must be allowed for the heat to diffuse into the center of the food, and in this case the cooking time might only be slightly less than in conventional cooking.
The second consequence is that the heating is more uniform with depth than in conventional cooking – it’s less likely that various “surface effects” such as scorching, browning, or charring will be encountered. Avoiding scorching, i.e. burning of the surface, is usually very desirable. However we often like the surface of foods to have a different degree of cooking than the interior, i.e. a slightly charred steak, or a browned casserole, contributes to taste and texture. This is difficult to obtain using only microwave cooking, but an additional cooking method may be employed to obtain these effects. High-end microwave ovens often deliver combined cooking techniques, i.e. combine infrared radiation and convection together with microwave radiation. And the food can be browned by finishing the cooking in a broiler, toaster oven or conventional oven, with an overall time savings.