Cooking 101: Heat Transfer

What is cooking? I have read many definitions, but the one I like best is simply the preparation of food for human consumption. Although an apple doesn’t require much more than washing before being eaten, much of the time cooking involves heating ingredients to change their structure and taste, and sometimes, to make them more digestible. When you heat your dinner, do you give much thought as to how the heat gets from its source into your food? Possibly not. A basic understanding of the physics involved with heating food can help all of us become better cooks.

When heating food, we may think that all we are doing is to make our food hot, but in the world of physics, we are attempting to bring the food we are heating and the source of the heat into thermal equilibrium. When we discuss the temperature of our food, we are referring to a measure of how hot it is. When we say something is 210 degrees Celsius or 410 degrees Fahrenheit, we are really saying how hot the item is relative to the boiling and freezing points of water.

When we place a pot of water on the stove to heat, we are causing heat to flow from the burner to the water. Thermal equilibrium is never reached because, while the burner is turned on, we are continually adding energy to the burner. Plus the burner is always going to be hotter than the water, which cannot exceed its boiling point. But that’s fine because our real goal is to boil the water in order to make tea, not to reach thermal equilibrium. (If we had enough patience and foolishness to boil all the water away, the pot would then come close to thermal equilibrium with the burner.)

The burner transfers heat to the pot, which in turn transfers heat to the water. The process whereby energy flows from the burner and eventually to the water involves a number of forms of heat transfer. But for now, let’s discuss just the first part of the process, heating the pot.

If our burner is an electric coil, most of the heat is transferred by means of conduction. The heat is conducted from the burner to our pot at the points where the two are in direct contact. Because the burner is sending energy in all directions, some of the energy directed downward is reflected by the metal surface below the burner, the reflector, up between the spaces of the burner coil to the bottom of the pot. This type of heat transfer is called radiation. Lastly, because the air between the burner and the reflector is hot, there is a small contribution to the heating of our pot by convection.

If our burner is gas, most of the heat transfer is by means of convection where the burning, high temperature gas flows along the bottom of our pot. There are also smaller contributions from the burning gas producing electromagnetic radiation in both the visible and infrared spectrums and conduction from the hot metal prongs that support the pot.

If we are using a ceramic cooking surface, heat transfer is mainly by means of conduction. But because pots tend not to be perfectly flat on the bottom, especially after some use, the pot is only in partial contact with the ceramic surface. In the narrow air spaces between the two where there is no contact, radiation and convection contribute to the heat transfer.

If our stove has an infrared cooking surface, there are electric coils sealed below a transparent ceramic surface which emit electromagnetic radiation in the form of infrared and visible light to heat our pot. Some of the energy is absorbed by the ceramic surface, which in turn contributes to heating our pot by conduction, where the two are in contact, and by both radiation and convection, where they are not.

If our stove is ultra-modern and has an induction cooktop, it uses none of the three aforementioned forms of heat transfer to heat our pot. With induction cooking, the “burner” uses electromagnetic induction to create eddy currents in the ferrous metal base of our pot. The metal’s resistance to the eddy currents causes it to rapidly heat. We can only use certain pots with an induction cooking system—our fancy copper pots or those lightweight aluminum ones we bought on sale just will not work.

Although not as efficient, we can also heat the pot in an oven. Then the form of heat transfer is primarily convection from the hot air in the oven and radiation from the heat being emitted from the hot walls of the oven. A convection oven will heat our pot faster than a conventional oven because the air in the convection oven is circulated forcefully past the pot, whereas in the conventional oven, the air is allowed to circulate by means of the natural currents established in the heated environment.

And there are other ways we could heat our pot. We could place it over a campfire, set it on an outdoor charcoal grill, cover it with hot rocks in a pit in the ground, or set it in front of a solar reflector. All of these would heat our pot through a combination of conduction, convection, and electromagnetic radiation.

One type of heating that we haven’t discussed yet, and I include it only for the sake of completeness, is heating by microwaves. In this method, water molecules are caused to vibrate rapidly by microwave radiation. The friction of the molecules bumping into each other produces the heat that will eventually cause the water to boil or the food to cook.

Note that in all of the examples I discuss above, except with microwave heating, we are not heating the water directly, but heating our pot, which in turn, heats the water by means of conduction. The inside surface of our pot heats the water that is in direct contact with it. The heat moves through the water by convection. By the time the water is brought to a simmer, the temperature of the water at the bottom will be warmer than at the top. Also, the sides of our pot will heat up, but not as hot as its base, because they are not directly affected by the burner and depend upon the energy conducted through the metal for heat. And while part of the heat in the sides is conducted into the water, the outside surface loses heat to the surrounding air by both convection and radiation. Nonetheless, water near the sides of our pot will be warmer than regions near the center. When the water reaches a rapid boil, the temperature of the water begins to approach equilibrium because there is now vigorous circulation within the water caused by the bubbles moving from the bottom to the surface. This is in addition to the natural convection currents.

As we continue to add heat to our pot, the water is gradually being turned from a liquid to a gas, which we call steam. The steam, being lighter than the water, rises to the top in the form of bubbles that escape the surface. The steam is eventually cooled by either the air or nearby surfaces in the kitchen, where it condenses back to a liquid. Most of the water is lost to our pot, and as a result of the steam escaping, the volume of liquid in our pot is reduced. If we forget that the pot is on the stove, we will eventually evaporate all the water (and cause quite a mess).

So far we’ve heated water, but it’s hard to call this cooking. (Although sometimes we describe someone who can’t cook as not being able to boil water.) Of course if we add some broccoli (and salt) to the boiling water, then we can cook the broccoli. To cook the broccoli, it is necessary to transfer heat from the water to the broccoli by means of convection. We must increase the broccoli’s temperature to cause the changes in texture and flavor we associate with cooked broccoli versus raw. (Depending upon how you were raised, the change in texture and color we associate with cooked broccoli may be either good or not so good.) The water can be thought of as the heat transfer agent used to cook the broccoli.

In almost all cooking, except for cooking in a microwave oven, either a liquid or gas is used as a heat transfer agent. The type of liquid or gas and the temperature at which you use it is determined by the method of cooking you choose. Note that in the strictest interpretation, the metal in our pot is also a heat transfer agent, but in our discussion, we are only concerned with the non-solid heat transfer agent around whatever we are cooking.

The cooking liquid is either primarily water-based or primarily fat-based. When cooking in a water-based liquid, such as braising, boiling, or poaching, the cooking is done at or below the boiling point. Because the temperature of liquid water cannot exceed 100 °C, no matter how much heat we apply to the outside of the cooking pot, the temperature of the cooking liquid and anything in it will never be hotter. (The ingredients will usually be cooler because of the difficulty of achieving thermal equilibrium.)

Poaching is usually done with a water-based liquid maintained at a constant temperature below the boiling point. Held at the right temperature, the liquid will prevent your dinner from overcooking. For example, if you place a raw chicken (along with some aromatics) into water at 70 °C, you can thoroughly cook the chicken without raising its temperature to a point where it becomes tough.

When braising tough meat, the goal is to slowly raise its internal temperature to a point where the connective tissue begins to dissolve. Whether done in the oven or on the stovetop, the meat is at least partially submerged in the water-based braising liquid so it never sees high temperatures. In this manner, it may take a couple of hours for the meat to slowly exceed the 90 °C required to actually dissolve the connective tissue. (If we raised the temperature of the meat rapidly, the outer areas would overcook and dry out long before the center would cook.)

If the ingredients and the water-based liquid are placed in a sealed pot, such as a pressure cooker, the cooking can proceed at a faster pace because the boiling point of the liquid will now be about 120 °C. But because the heat transfer agent is now capable of achieving a higher temperature, it also means that the solid ingredients can easily become overcooked.

In both of the above examples, the liquid transfers the heat from the pot to the solid ingredients. In addition, the liquid limits the temperature that the immersed ingredients can achieve. If the liquid is changed from being water-based to fat-based, the temperature that the immersed ingredients can be exposed to is much higher than the boiling temperature of water. Most fats begin to smoke at a temperature in excess of 150 °C and have much higher boiling points. When the braising liquid is fat-based, the temperature control comes from limiting how much heat is applied to the cooking vessel. If the solid ingredients are cooked to an internal temperature that is much above the boiling point of water, the water in the ingredients is cooked out and they become dry (and usually tough). When we use a fat-based braising liquid we control the input energy to limit the temperature of the liquid, which also acts as a heat transfer agent.

As noted above, cooking in fat can surround the food with a heat transfer agent at a much higher temperature than cooking in water. Deep frying is in reality just the process of simmering the food in hot fat. The fat appears to boil when the food is immersed in it because the water in the food boils and produces steam. As the water near the surface of the food being cooked is evaporated and the surface crusts over, sealing the remaining water in the interior of the ingredient, the bubbling ceases. While the water evaporates, the temperature of the fat will be lowered if the amount of steam is relatively high in comparison to the amount of fat. This is in addition to the basic temperature reduction that occurs when the cold ingredients are introduced to the hot fat.

Whether using a water-based or fat-based liquid to cook our food, the transfer of heat from the pot to the solid ingredients occurs by means of convection. The process occurs at both the interface of the interior of the pot and the liquid, where the liquid is heated and the pot surface is cooled in an attempt to reach equilibrium, and at the interface of the liquid and the solid ingredients, where the ingredients are heated and the liquid is cooled in another attempt to reach equilibrium.

As stated earlier, the base of the pot will be hotter than the sides because it is in contact with the heat source. Consequently, the shape of the pot can be a factor in how an ingredient cooks. In a wide pot with a shallow level of liquid, the temperature of the liquid will be closer to constant and the ingredients will cook more evenly. In a tall pot with a deep liquid level, the liquid at the bottom will be hotter than near the top so it will be necessary to stir the liquid and reposition the ingredients in order to achieve even cooking. In restaurants that make their own stocks, it is not uncommon to have a very large pot in the kitchen with built-in heating elements wrapped around the sides and the bottom so that all the interior surfaces can be directly and equally heated. With these pots, the temperature of the liquid in the pot will almost be uniform and the cooking requires less attention.

In the previous examples, we’ve discussed cooking food immersed in a liquid heat transfer agent. Oven cooking is very similar, especially in a convection oven with the ingredients open to the air in the oven. Although there is a radiant component to the energy that is absorbed by ingredients in the oven, much of the energy comes from the air flowing around the ingredients. The fan in a convection oven forces the hot air inside the oven to circulate at a higher speed than it would without the fan. By circulating the air, it is possible to maintain the interior of the oven at a more even temperature than in a conventional oven. With an oven, instead of being immersed in a hot liquid, the ingredients are immersed in hot gas.

Because the ingredients generally require some means of support in an oven, such as a baking sheet or roasting pan, the support mechanism is also heated during the cooking process. Over time, the support can heat to near the temperature of the oven. The parts of the food being cooked that are in contact with the support mechanism become more cooked than the parts which are not in contact. Conduction is a more effective way to transfer heat than convection. To prevent the parts in contact from burning, it is common to set the item supporting the ingredients being cooked into another container filled with water. This water bath, prevents the container holding the food being cooked from heating to a temperature greater than the boiling point.

Covered outdoor grills can function similar to an oven with the advantage of exposing the surface of the food being heated, usually meat, directly to the hot air inside the cavity of the grill. Like the oven, the heat transfer agent sending energy into the food is still the hot air that flows around it. Depending on the grill’s design, it may immerse the meat in high heat, greater than 260 °C or low heat, 95 to 120 °C. In most designs, the source of the heat is usually at the bottom of the grill. Air that is heated at the bottom flows upward in the grill’s cavity and escapes at the top. As the walls of the grill heat up, the speed of the air flowing through the cavity increases, providing less difference in temperature from bottom to top. Some designs include a means for a basin of water to sit just above the heat source. The steam produced by the evaporating water limits the temperature of the air flowing through the cavity of the grill to close to the temperature of boiling water.

In a similar method, we can use steam to cook food. Stovetop steamers usually consist of a two-part vessel. The bottom is a pot that holds the water we boil to produce the steam. The top part is another pot that sits tightly on top of the first. The bottom of the top pot is perforated so the steam can flow through it from below. Steam that does not condense on the food, condenses on the top and sides of the upper pot. All this liquid then flows back into the lower part of the steamer where it is converted back to steam. The food being cooked is usually supported on a plate or grill so it can be easily placed in the steamer and the later retrieved when cooking is finished. Once again, the heat transfer agent is a hot gas, in this case, water vapor.

Steaming is a very efficient and effective means of cooking. A chicken placed in an oven that is set to the temperature of boiling water will take many hours to cook whereas the same chicken placed in a steamer may take only about half an hour to cook. When the hot steam comes in contact with the cooler surface of the chicken, it condenses. The change of state from a gas to a liquid releases about five-hundred times more energy than what would be released by just cooling the same weight of water by one degree. Using steam as a heat transfer agent can be a very effective method of rapidly cooking food.

So far, all the examples we have examined involve immersing the food to be cooked in the heat transfer agent, either a gas or a liquid. Next let’s look at cooking with only a small amount of a heat transfer agent, a process we generally call frying. If you place a frying pan over a burner, add a little oil or some other fat to the pan, and place a piece of meat in the frying pan, there are a couple of heat transfer processes taking place. The meat that is in direct contact with the surface of the pan is being heated by means of conduction. The portion of meat that is in contact with the oil, is being heated by means of convection. The oil is acting as a heat transfer agent to move energy from the hot surface of the frying pan to the cooler surface of meat. This is important to remember when adding the oil to the pan. We have to add an amount appropriate to what and how we are cooking.

As the meat releases some of its juices, this liquid, which is primarily water, boils and is turned to steam. Some of this steam will contact the meat, condense, and further transfer energy into the meat. Even though the surface of the pan and the oil are much hotter than the steam, if too much liquid is present, the method of cooking may actually change from frying to steaming, or even boiling. This is why recipes instruct us to not crowd meat when browning it. If more juice is released by the meat than there is energy in the pan to quickly boil it away, it accumulates. Because the boiling liquid cannot be hotter than boiling water, the meat stews rather than browns. Browning requires the surface of the meat to reach a temperature much higher than that of boiling point.

Because most of the hot fat used in frying is near the surface of the pan, it is important to agitate the ingredients fairly often so that they cook evenly. Each time the ingredients are moved, their exposure to the hot fat changes. If the pan has a round bottom, such as with a Chinese wok, a small amount of fat can be used to cook a larger quantity of ingredients because the hot fat drains from the food and collects in the center of the wok to be reheated. This type of cooking requires a high source of heat to continually reheat the fat, which is usually of a lesser amount than we would use to cook the same amount of ingredients in a frying pan with a flat bottom.

Now suppose that you want to cook your food in a dry frying pan, one without any fat and thus no heat transfer agent. If the food to be cooked is a piece of meat that that consists of a lot of fat, this may be possible because it will then render its own heat transfer agent. But if the food to be cooked does not contain fat, such as most vegetables, then it will be difficult to evenly cook the food because the heat transfer will have to be mostly by conduction, where the food is in direct contact with the surface of the pan. There may also be a contribution to the cooking from heat radiating from the pan, but this will be minor. Some vegetables which easily release water, such as mushrooms, can be cooked in a dry pan, but they will not brown because the water released will boil and limit the cooking temperature. Food cooked in a dry frying pan tends to become burnt where it is in contact with the frying pan and remain uncooked elsewhere.

From the above examples, we can see how it is important to have some heat transfer agent to move energy from the heat source to the food being cooked. If the heat transfer agent is water-based, the cooking temperature will be limited by the water. If the heat transfer agent is oil-based, the cooking temperature will be limited by the initial source of energy.

We have only discussed heating the outside surface of the food. The heat must also travel to the center of the food in order to cook it throughout. The rate that the heat moves into the food is relative to the temperature difference between two adjacent molecules in the food. High-heat methods will cook food faster than low-heat methods, but they will also produce a greater difference between the temperature at the surface and the temperature at the center. This difference will also be a product of the size of the pieces being cooked. Smaller pieces will have less temperature difference than larger pieces. One of the reasons to let meat rest after roasting it is to allow the temperature to equalize throughout the whole piece.

How heat travels from its source into our food is important. The physics of cooking teaches us to choose our heat transfer agents and cooking methods wisely. If we do so, we can successfully place food on our table that is cooked exactly as we desire.

Note: for further reading about heat and heat transfer, a good starting point is Wikipedia.

©2006 Peter Hertzmann, Inc. All rights reserved.