Cooking, difference between convection, conduction and radiation.
28 Oct 2019
Notes based on The Food Lab: Better Home Cooking Through Science
Cooking by rote--even when your mentors are some of the greatest chefs in the world--is paralyzing. Only by understanding the underlying principles involved in cookery can you free yourself from both recipes and blindly accepted conventional wisdom. Freedom.
Crossover with programming, underlying principles let you deviate from the mainstream because you know how and why things work, instead of following tutorials that could be outdated, or sound fancy and complicated but are actually making things more difficult. At the same time, just because someone tells you, here are the 'good' principles, don't take their word for it at face value, even if you consider them to be exceptional programmers. Test them, experiment, fail, succeed, learn.
According to the author, the kitchen is one of the easiest places to practice science every day. For example, the first time you ever bought a toaster, you may have burned your bread. So based on that feedback, you would have turned the dial down or down and back up a bit until you find the right 'lightness' for your bread. From then on, you might always use that setting, until the thickness of the bread you are using changes. The feedback loops based on observation are much quicker than other scientific disciplines.
What is cooking?
Cooking is about transferring energy. It's about applying heat to change the structure of molecules. It's about encouraging chemical reactions to alter flavours and textures... Heat and temperature are not the same thing.
Heat is energy. Everything that exists, is composed of tiny molecules that are either rapidly vibrating (in the case of solids), or rapidly bouncing around randomly (liquids and gases). The more energy (heat) is added to things (systems of molecules), the more rapidly their molecules vibrate or bounce around, and the more rapidly they transfer this energy (heat) the things they are touching. Cooking is where you transfer the energy from a hotter system (like a pan) to a cooler system (like a chicken). This energy raises the temperature of the chicken, but also gets used for other reactions like making moisture evaporate or the chemical reactions that cause browning etc.
Temperature is a system of measurement. It allows us to quantify how much energy is in a given system (pan or chicken). The temperature depends on the total energy (heat) of a pan or chicken, but also the density and heat capacity of the pan or chicken.
Density is how many molecules there are in a given amount of space. Solid objects have more molecules than gas or air, therefore they are denser than gas or air. The more dense something is, the more energy (heat) it will contain at a given temperature. If you put your hand in boiling water, you are likely to be burned. If you put your hand inside an oven that is the same temperature as the pan, you will not get burned or hurt in the slightest, because air is less dense than the pan, so the energy will also be less.
Heat capacity is the amout of energy (heat) it takes to raise a given amount of something to a certain temperature. For example, it takes one calory (calories are energy) to raise one gram of water to one degree celcius. As the heat capacity of water is higher than iron (an lower than air), the same amount of energy will raise the temperature of iron by at least ten times as much, and only half a calory will raise the temperature of air to the same number of degrees.
At a given temperature, denser materials usually contain more energy. Heavier pans will cook food faster, though it takes more energy to raise the temperature of heavy (denser) materials.
At any given temperature, materials that have a higher specific heat capacity will contain more energy than materials at the same temperature with a lower specific heat capacity. Similarly, the higher the heat capacity of a material, the more energy it takes to bring it to a certain temperature.
Most recipies call for cooking foods at specific temperatures. For most food, the temperature it's raised to is the main thing that determines it's final structure and texture. The following temperatures are from the book.
- 0°C - The freezing point of water or the melting point of ice.
- 52°C - Medium rare steak and the temperature at which most bacteria begin to die. Though it can take 2+ hours to safely sterilize food at this temperature.
- 64°C - Medium-well steak. Egg yolks begin to harden, egg whites are opaque but still jelly-like. Fish proteins begin to tighten to the point that white albumin will be forced out, which gives fish like salmon an unappealing layer of congealed proteins. After 3 mins at this temperature, bacteria reduces to the poin that only 1 will remain for every million that were there (7 log reduction).
- 71°C to 82°C - Well-done steak. Egg proteins fully coagulate (temp to which most custard or egg-based batters are cooked to set them fully) - bacteria experience 7 log reduction within 1 second.
- 100°C - The boiling point of water (or the condensation point of steam).
- 153°C + - The temp at which the reactions that produce deep brown crusts on steaks or loaves of bread begin to occur at a very rapid pace. The hotter the temperature the faster this happens. As these ranges are well above the boiling point of water, the crusts will be crisp and dehydrated.
Conduction is the direct transfer of energy from one solid body to another (like when you burn your hand by touching a hot pan).
Vibrating molecules from one surface will strike the relatively still molecules on another surface, to transfer energy). This is the most efficient method of heat transfer. Some examples: Searing a steak, Crisping the bottom of a pizza, cooking scrambled eggs, making grill marks on a burger, sauteing onions.
Convection is transferring energy through the intermediary of a liquid or gas. This is moderately efficient, but depends on the way that the fluid or gas flows around the food. The motion of this movement is referred to as convection patterns. The faster air or water travels over a surface, the more energy it can transfer. Convection ovens have fans that are designed to keep the air moving around to promote faster, more even cooking. Some examples: Steaming asparagus stalks, boiling dumplings in stock, deep-frying onion rings, barbecuing a pork shoulder, baking the top of a pizza in an oven.
Radiation is transferring enery via electromagnetic waves, which is the heat you feel when you sit close to a fire or hold your hand above a hot pan without touching it. This type of energy gets weaker according to the 'inverse square law'. If you hold your hand 1 foot away from a fire, then move it 2 feet away, the heat is only one-quarter as hot, even though you are double the distance away. Some examples: roasting a pig on a spit next to hot coals, toasting garlic bread under a broiler, getting a tan from the sun, broiling marinated salmon.
Most of the time, all three methods of heat transfer are used to some extent. For example, when you grill a burger, it touches the grill grate and the heat is conducted to it. The underside of the burger is cooked via radiation from the coals, and when you put the lid on it, heat is transferred to it via convection.
Most of these types of heat transfer heat only onto the surface of foods. To cook through to the center, the outer layer must transfer its heat to the next layer and so on until the center gets cooked. Foods are almost always more well done than their center.
Microwaves are the only standard cooking method that can penetrate through the exterior of food when heating it.