In his book, What Einstein Told His Cook, the American chemistry professor asks: On a really hot day, would it be possible to fry an egg on the bonnet of a car? The answer would be “probably not.” The reason is the fact that as soon the egg lands on the bonnet, the surface will be cooled while the egg becomes hot. But since there is no other heat source than the sun radiating energy onto the bonnet, you end up with the egg partially fried. However, if you opened the bonnet and cracked the egg straight onto the engine block, chances would be considerably better for frying the egg all the way through, especially if the engine was running. Having the engine running would be similar to cooking on the stovetop, the combustion process constantly feeding energy to the metal. The amount of metal in the engine block is also much higher than in the bonnet, more energy would have been stored in the material, which would in turn be transferred to the egg to cook it. ×
Convection—boiling and baking. If you burn yourself holding your hand over a candle you experience energy transfer via convection. The phenomenon occurs when a liquid or gas of higher temperature moves to a place of initially lower temperature. In the case of the candle, warm gases flow upwards from the flame to reach your hand, after which your hand becomes heated by conduction when the hot gas comes in physical contact with your skin. Since convection is all about “hot molecules moving to new places,” it can only occur in media that are able to flow: liquids such as water or oil, or gases such as air or steam. Since these “hot molecules” have high energy, they move about or rotate in high speed, and when they collide with “colder” and slower molecules, some of their energy is transferred via conduction. Convection processes are common phenomena both in nature and in everyday life at home. While heating water for your pasta, you may see flows and currents in the water even though the pan stands perfectly still on the hotplate: hot water from the bottom rises and pushes colder water to the side, and so the circulation goes. Warm air rises while cold air sinks both as a meteorological phenomenon as well in your own living room.
The difference between conduction and convection can be illustrated using a dance floor as metaphor. Envisage a dance floor where some (colder) couple dance slowly cheek to cheek while other (hotter) couples dance a more intense swing, as shown in the illustration. The red steps illustrate fast swing, “hot molecules.” The blue pairs dance slowly and are “colder.”
During conduction, the cold couples surrounding the hot couple are affected, inspired, as they come in contact with the latter. After a while, the cold couples become hotter, while the originally hot couple has shared some of its energy and become somewhat cooler. This illustrates conduction in both solids (all maintain their original positions, but can still rotate on the spot and bump into each other) and in liquids and gases (the ones surrounding can move around when pushed by the “hot” couple).
^Conduction and convection illustrated by hot and cold couples on the dance floor. Red steps represent hot molecules, blue represents cold and purple represents temperatures between hot and cold.
Convection is illustrated by a room with one cold end, inhabited by couples dancing slowly, “cold molecules,” and the other end dominated by more energetic dance. Very similar to a hot oven standing in an otherwise cold room. Since molecules by nature move randomly and are reflected when they bounce against each other or a surface, they will in time distribute evenly across the room. We move toward an even temperature in the whole room for two reasons: 1) the various couples will over time distribute evenly throughout the room. 2) The “hot” couples will eventually bounce into the “cold” ones, sharing their energy (this is consequently conduction).
The most striking practical example of convection when cooking food is when you turn on the fan in your oven; indeed, ovens with such fans are called “convection ovens.” Many ready-made foods have instructions that you should heat at different temperatures depending on whether you use the fan or not, 200°C in ordinary ovens or 180°C with the fan running. This reflects the fact that the fan sets the air into motion to increase heating through convection. Even without the fan, convection also occurs via hot air moving out from the hot surfaces, pushing aside less hot air.
When barbecuing/grilling, at least over a campfire, you have the choice of holding your hot dog on the side of the coals or above them. Everyone having roasted hot dogs by a campfire has experienced that holding them above the coals is more efficient. Hot air moves upwards from the fire, and the hot dogs experience convection in addition to radiation. This is a more intense way of heating, which often results in a hot dog with a burnt surface while still cold in the middle. Apparently, sometimes convection can be a bit too effective way to transfer energy. However, if you hold the hot dog by the side of the fire or coals, which is also recommended as you avoid transfer of soot particles from a burning fire, you would be heating by radiation from the flames or embers, just like cooking Finnish loimulohi.
Condensation—steam in a closed pan. Rather than plunging your potatoes into hot water, you may place them on a grate with just a little water in the bottom of the pan. You place a glass lid on top, turn on the power. Your potatoes are ready almost just as quickly as when boiling them in water, without many visible signs of anything happening in or around the potatoes, possible except for some water dripping off the potatoes. How come?
Let’s return to the burnt hand because that experience is quite a powerful association for many of us. If you place your hand in front of the spout of a kettle with boiling water, you may be badly burnt. You will not only experience convection and conduction from 100°C air and steam reaching your hand transferring energy by contact (conduction), but in addition the steam, water in form of gas, will condense back to liquid water on your skin. It requires a large amount of energy to make water evaporate; indeed, it takes seven times more energy to evaporate water (already holding 100°C), as it takes heating the same amount of water from room temperature to 100°C. All the energy that went into evaporating the water is returned to your skin when the steam reaches your hand and condenses to become small drops of liquid water. This comes in addition to the hot water transferring its heat energy by ordinary conduction from temperature difference, like if you had stuck your finger into hot water. This is also the reason that you are more severely burnt if you come in contact with steam at 100°C compared with dry air of the same temperature. After all, you can stick your arm into an oven holding 200°C without getting burnt, but holding your hand just above a boiling pan of water will cause severe burns at “only” 100°C. However, what you actually see coming out of the kettle spout, or of a boiling pan, is not steam. Water vapor is a colorless and invisible gas. What you see is liquid water, steam that has already condensed into miniscule drops on its way out in the kitchen. If you look closely at the tip of the spout, or just above the boiling water, you would see (apparently) nothing. That area is where the water vapor is, and that is the place where the energy transfer to other bodies in the most intense.
When you steam food, you make use of this evaporation–condensation energy. Steam moves by convection from the boiling water below your food, hitting the surface of the food to release both condensation energy as well as some heat energy through direct contact, conduction. Consequently, steaming is quite efficient and competes well with boiling it in water even though steam is much less dense than liquid water. But don’t lift off the lid to have a look every second minute because this will release the efficient steam out into the room. A transparent glass lid is a good thing for the curious cook. If the water in your pot is boiling but you don’t see white clouds, you can be fairly certain that the pan is full of invisible steam condensing on your food to cook it.
Cooking with heat. It is very seldom that only one form of heat is in action when cooking. Two exceptions mentioned are the Finnish “flame-roasted” loimulohi, and microwaving, where radiation is the only source for heating. But even in these cases, conduction, convection, and condensation occur inside the food: the heat spr
eads and the temperature evens out throughout the food. Usually, several or all forms of heat are in action at the same time.
How often should you turn a steak in a skillet? This important question arises when pan frying, because there the heat transfer takes place both via conduction and convection: while the heat is conducted from the hot surface of the skillet to the food, energy is transferred by convection from the hot surface of the steak to cooler, surrounding air. The steak also cools via evaporation of water from the steak surface, the opposite of condensation. So at least three different heat transfer mechanisms are involved in the frying of a steak (in practice, there will also be radiation from the skillet surface onto the meat). Traditionally, many of us have learnt to turn the steak once during cooking—when you see juice seeping out on the top. But lately, chefs have started turning it more frequently, as often as every 30 seconds. An explanation for the superiority of this new technique lies in the optimization of heat transfer. If you turn the steak only once, the hot surface of the turned steak will cool down longer and more compared to the frequently turned steak, and the total cooking time is increased compared with the frequently turned steak. When turning frequently, the hot surface cools more evenly, avoiding significant temperature differences between the outer and inner parts. Furthermore, it has been found that the layer 1–3 mm beneath the surface is heated to about 10°C higher if turned only once compared with the frequent-turned steak, risking overcooking of the outer parts. In other words, flipping your steak frequently would both reduce cooking time and promote a more evenly cooked steak. ×
In a coal barbecue, the coals are very hot and the heat transfer occurs via radiation plus some convection by hot, rising air. But where the food is in direct contact with the grates, conduction occurs between the metal and the food. This heating is sufficiently intense to give the barbecued food its appealing grated pattern, also giving a more complex and appealing flavor. If you have a gas barbecue, the radiation is not as intense as in the glowing embers, and you get comparably more convection compared with radiation. The resulting process is somewhere between a coal barbecue and baking in an oven, particularly if you keep the lid on. In an oven, the surfaces are not as hot as the embers of a barbecue, so the radiation from the walls are less intense. However, the air in the oven is hot and kept locked inside the confined space, and, therefore, the most important factor when cooking in an oven, unless you have the grill function turned on, is convection in air.
Although it might seem self-evident, it is worth reflecting upon the fact that heat almost always moves from the outside of the food inwards when we cook, the seemingly only exception being microwaving. If we use high temperature when cooking a large piece of food, the outer parts will be overcooked before the heat reaches the inner parts. Keeping the temperature lower will make the cooking take longer, but the domino effect of heating the food throughout will have more time to occur without the outer parts becoming overheated. This is one of the main principles behind the sous vide technique, where the food is vacuumed and left in a water bath of desired temperature for a long time, allowing the heat to penetrate the food more gently. Chemical reactions are allowed to proceed, giving good flavor and tender meat. We will, however, not go into sous vide cooking here as great sources to information about this are only a few clicks away on the web (we recommend Douglas Baldwin’s book and web page or the grand cookbook, Modernist Cuisine—see the bibliography). In the illustrative table we have connected the various heating techniques to the various forms of heat and given each its own icon.
Important heat transfer mechanisms during cooking
^The table shows the different mechanisms for transfer of thermal energy, be it frying, deep frying/boiling, baking, grilling, microwaving, or using a cooking pit.
10
Cold Skillet—Juicy Fish
Tatu the chef came back from his summer holiday with great news: now he was positive that fish should be placed in a cold, and not a preheated, skillet. “I have tested it throughout the summer. When the fish is heated along with the skillet it turns out juicier and more succulent compared with if it is put in a preheated skillet.” His observation was thus quite the opposite as compared to the instruction generally given for frying fish. And what a perfect starting point for an experiment!
For the next food workshop, Tatu bought a large salmon fillet and put it into a 10% brine solution half an hour before the experiment was to start. Together with the participants we cut out two thicker (neck) and two thinner (tail) fillets. The pieces were weighed, and their thicknesses were measured in order to ensure the pieces were pairwise as identical as possible. A kitchen thermometer was inserted in the middle of each of the thick fillets. Both thermometers had been calibrated at room temperature (20°C) and in boiling water (100°C) to ensure that any measured differences were not due to the equipment. In addition, we had a nice laser thermometer to monitor the surface temperature of the skillet.
The stage was set for frying. A cold skillet with one thick and one thin fillet was placed on a preheated hotplate. After 120 seconds the temperature reached 160°C, whereby the two other fillets were placed in the now hot skillet. All four fillets were fried until the temperature inside the respective thicker piece reached 45°C, after which that pair was transferred to a separate plate, still monitoring the temperature for a few minutes. While Tatu was master of frying, two of the participants were in control of the measurements. Every 30th second, two participants monitored the temperatures in the thick fillets while two others documented the skillet surface temperature. The fifth participant planned how the measurements could best be visualized in a spreadsheet, and the rest watched eagerly. Not the most efficient use of manpower but fun it was. The temperature curves of the two versions of fish are shown in the diagram.
The pink line in the diagram shows the temperature curve of the “hot skillet” version and the blue line shows the temperature curve of the fish placed in the cold skillet. It is worth noting that the temperature continues to increase for a short while even after the pieces were removed from the skillet. We were not surprised that the curves ended up to be different, but they were opposite from what we had anticipated. To begin with, the temperature increase was highest in the fish placed in cold skillet, while evening out toward the end, and the two fillets ended up close to the same final temperature at around the same time. Weird and unexpected!
As usual in our workshops, we had to turn to discussions and review the literature to try to understand the results. We came up with three possible explanations of these counterintuitive graphs, presented below in the order of decreasing discomfort and fear:
1. Looking at the graphs, the skeptic would say that our rather relaxed experimenting group mixed the samples. Actually, we did not! We were under the close scrutiny of many critical eyes, and in this group much smaller mistakes than this are usually noticed.
2. Perhaps our thermometers were not working properly? Well, we did have quite ordinary kitchen thermometers, and the calibration procedure would not exactly stand up to scientific standards. It may also be that we did not succeed in placing the two thermometers exactly into the center of the fish samples. Definitely a weakness that asks for repeating the experiment. We are humble enough to accept this criticism as we did not bring another fillet for a second experiment. So, please go ahead and repeat our experiment!
3. Reading current literature of the field, however, indicates that there might be another explanation, and that the counterintuitive result might yet represent the actual state of affairs. In the great encyclopedias of modern cooking, Modernist Cuisine, we found a claim that placing meat onto a hot skillet might lead to formation of an insulating layer. The assumption for our subsequent reasoning would be that the same should apply to fish.
Fish frying experiment
^The diagram shows the development of internal temperatures during frying. The fish placed in hot skillet actually took longer to reach the desired internal temperature
(pink graph).
^The fish is fried from bottom up and at the same time from inside and out. Heat transfer occurs in different manners in the different zones of the fillet. See Chapter 9 for explanation of the icons.
The main author of Modernist Cuisine, Nathan Myhrvold, writes that a steak on the skillet, to a certain extent, is cooked from the inside out. When a steak is placed on a very hot surface, four different horizontal zones are rapidly formed from the surface touching the skillet surface and inwards: the zone closest to the skillet, the “drying zone,” is a very thin layer between the hot surface (approximately 200°C) and the food. Here, the temperature increases rapidly to pass the boiling point of water, resulting in the water evaporating quickly to give a dry surface or crust. Above (just inside) the drying zone the temperature increases to at least 140°C, enabling the Maillard browning reactions to occur (see info box). This “Maillard zone” receives heat by conduction from the drying zone. Since the temperature in this zone can reach up to ca. 150°C, water boils, rendering this zone dry as well. The next zone going inwards in the meat is a “boiling zone,” where hot water continuously evaporates and condenses, maintaining a fairly constant temperature around 100°C, and pushing steam both inwards and outwards. The steam moves mostly from below and upwards through existing and newly formed channels inside the food. Fats melt, connective tissues consisting of the protein collagen are transformed to gelatin, which can dissolve in the water. Meat proteins denature and coagulate. The meat is at the same time fried from the outside and boiled and steamed from the inside. As the steam forms new channels within the meat, the muscle fibers detach from each other breaking down the chewy structures.
A Pinch of Culinary Science Page 8