Read Cooking for Geeks: Real Science, Great Hacks, and Good Food Online
Authors: Jeff Potter
Tags: #COOKING / Methods / General
Cooking for Geeks: Real Science, Great Hacks, and Good Food (30 page)
“Great,” you’re probably thinking, “but how does knowing any of this actually help me cook?”
You can tell when something is done cooking by understanding what reactions you want to trigger and then detecting when those reactions have occurred. Cooking a steak? Check the internal temperature with a thermometer; once it’s reached 140°F / 60°C, the myosin proteins will have begun to denature. Baking crispy chocolate chip cookies at 375°F / 190°C? Open your eyes and keep your nose online; the cookies will be just about done when they begin to turn brown and you’re able to smell the caramelization occurring. Really, it’s that simple. Foods are “done” when they achieve a certain state, once they have undergone the desired chemical reactions. As soon as the reactions have occurred, pop the food out; it’s done cooking.
A small but critical detail: as we’ll discuss elsewhere, proteins don’t simultaneously denature at a given temperature. Denaturation is a function of duration of exposure at a given temperature. And there are many different types of proteins in different foods, each with its own temperature/time response rate.
Smell, touch, sight, sound, taste: learn to use all of your senses in cooking. Meat that has been cooked until it is medium rare — a point at which myosin has denatured and actin has yet to denature — will feel firmer and also visibly shrink. The bubbling sound of a sauce that’s being boiled and reduced will sound different once the water is mostly evaporated,
as bubbles pushing up through the thicker liquid will have a different sound. Bread crust that has reached the temperatures at which Maillard reactions and caramelization occur will smell wonderful, and you’ll see the color shift toward golden brown. By extension, this also means that the crust of the bread must reach a temperature of 310°F / 155°C before it begins to turn brown, which you can verify using an IR thermometer. (Bread flour has both proteins and sugars, so both caramelization and Maillard reactions occur during baking.)
This chapter shows you when and how these changes occur so that you can become comfortable saying, “It’s done!” We’ll start by looking at the differences between the common sources of heat in cooking and how differences in the type of heat and temperatures impact cooking. Since one of the main reasons for cooking is reducing the chances of foodborne illness, we’ll also discuss the key issues in food safety, including a look at how to manage bacterial contamination and parasites, along with some example recipes to demonstrate the principles behind food safety. The remainder of the chapter will then examine a number of key temperature points, starting with the coldest and ending with the hottest, discussing the importance of each temperature point and giving example recipes to illustrate the reactions that occur at each of these temperatures.
As with most recipes in this book, the recipes here are
components
, not necessarily entire dishes or meals unto themselves. Create your own combinations as you like! It’s usually easier to take each of the components in a dish and cook them separately: veggies in one pan, meats or proteins in another, and starches in a third. This allows you to isolate the variables for each component, then combine them at the end. Eggplant Parmesan might be your favorite dish, but if you’re new to cooking, it’s probably not the best place to start to learn about the reactions taking place.
Finally, cooking and baking share an axiom with coding and product development: it’s done when it’s done — not when the timer goes off. One of the best tips I can offer for improving your skills in the kitchen is to “calibrate” yourself: take a guess if something is done and then check, taking note of what your senses, especially smell and sight, notice in the process.
Timers are great guides for reminding you to check on a dish and a good safety net in case you’re like me and absentmindedly wander off occasionally. But timers are only a proxy for monitoring the underlying reactions. Given a fillet of fish that is done when its core temperature reaches 140°F / 60°C — which might take about 10 minutes — the primary variable is temperature, not time. If the fish is slow to heat up, regardless of the timer going off at the 10-minute mark, it won’t be done yet. Not to knock timers entirely: they’re a great tool, especially in baking, where the variables are much more controlled and thus the time needed to cook can be more accurately prescribed. But don’t be a slave to the timer.
*
Temperature
Since the primary chemical reactions in cooking are triggered by heat, let’s take a look at a chart of the temperatures at which the reactions we’ve just described begin to occur, along with the temperatures that we commonly use for applying heat to food:
Temperatures of common reactions in food (top portion) and heat sources (bottom portion).
There are a few “big picture” things to notice about these common temperatures in cooking. For one, notice that browning reactions (Maillard reactions and caramelization) occur well above the boiling point of water. If you’re cooking something by boiling it in a pot of water or stewing it in liquid, it’s impossible for high-heat reactions to occur, because the temperature can’t go much above 216°F / 102°C, the boiling point of moderately salted water. If you’re cooking a stew, such as the simple beef stew recipe in
Chapter 2
(
Simple Beef Stew
), sear the meats and caramelize the onions separately before adding them to the stew. This way, you’ll get the rich, complex flavors generated by these browning reactions into the dish. If you were to stew just the uncooked items, you’d never get these high-heat reactions.
Another neat thing to notice in the temperature graph is the fact that proteins denature in relatively narrow temperature ranges. When we cook, we’re adding heat to the food specifically to trigger these chemical and physical reactions. It’s not so much about the temperature of the oven, grill, or whatever environment you’re cooking in, but the temperature of the item of food itself.
Which brings us to our first major
aha!
moment: the most important variable in cooking is the temperature of the food itself, not the temperature of the environment in which it’s being cooked. When grilling a steak, the temperature of the grill will determine how long it takes the steak to come up to temperature, but at the end of the day, what you really want to control is the final temperature of the steak, to trigger the needed chemical reactions. For that steak to be cooked to at least medium rare, you need to heat the meat such that the meat itself is at a temperature of around 135°F / 57°C.
What’s all this talk about “denaturing” proteins? It’s all about structure.
Denaturing
refers to a change in the shape of a molecule (
molecular conformation
). Proteins are built of a large number of amino acids linked together and “pushed” into a certain shape upon creation. Since the function of a protein is related to its shape, changing the shape changes the protein’s ability to function, usually rendering it useless to the organism.
Think of a protein as a bit like the power cable between a laptop and an outlet: while it has a particular primary structure (the cord and wires inside it), the cord itself invariably gets all tangled up and twisted into some secondary structure. (If it’s anything like mine, it spontaneously “retangles” itself regardless of attempts to straighten it out, but proteins don’t actually do this.)
On the molecular level, the cable is the protein structure, and the tangles in the cable are secondary bonds between various atoms in the structure. Atoms can be relocated to different bonding spots, changing the overall shape of the molecule, but not actually changing the chemical composition. With its new shape, however, the molecule isn’t always able to perform its original function. It might no longer fit into places that it used to be able to go, or given the new conformation, other molecules might be able to form new bonds with the molecule and prevent it from functioning as it used to.
The idea that you can just cook a steak any old way until it reaches 135°F / 57°C sounds too easy, so surely there must be a catch. There are a few.
For one, how you get the heat into a piece of food matters. A lot. Clearly the center of the steak will hit 135°F / 57°C faster when placed on a 650°F / 343°C grill than in a 375°F / 190°C oven. The hotter the environment, the faster the mass will heat up, thus the rule of thumb: “cooking = time * temperature.” Consider the internal temperatures of steak cooked two ways, grilled and oven-roasted:
Schematic diagram of temperature curves for two imaginary steaks, one placed in an oven and a second placed on a grill.
Cooking a steak on a grill takes less time than in an oven, because energy is transferred faster in the hotter environment of the grill. Note that the error tolerance of when to pull the meat off the grill is smaller than pulling the meat from the oven, because the slope of the curve is steeper. That is, if
t
1
is the ideal time at which to pull the steak, leaving it for
t
1
+2 minutes will allow the temperature of the grilled steak to overshoot much more than one cooked in the oven.
This is an oversimplification, of course: the graph shows only the temperature at the center of the mass, leaving out the “slight” detail of the temperature of the rest of the meat. (It also doesn’t consider things like rate of heat transfer inside the food, water in the meat boiling off, or points where proteins in the meat undergo phase changes and absorb energy without a change in temperature.)
Another thing to realize about heat transfer is that it’s not linear. Cooking at a higher temperature is
not
like stepping on the pedal to get to the office faster, where going twice as fast will get you there in half the time. Sure, a hotter cooking environment like a grill will heat up the outer portions of the steak faster than a relatively cooler environment like an oven. But the hotter environment will continue to heat the outer portions of the steak before the center is done, resulting in an overcooked outer portion compared to the same size steak cooked in an oven to the same level of internal doneness.
What’s the appeal of cooking on a hot grill, then? For the right cut of meat, you can keep a larger portion of the center below the point at which proteins become tough and dry (around 170°F / 77°C) while getting the outer portion up above 310°F / 154°C, allowing for large amounts of Maillard reactions to occur. That is, the grill helps give the outside of the steak a nice brown color and all the wonderful smells that are the hallmark of grilling — aromas that are the result of Maillard reactions. The outside portion of grilled meat will also have more byproducts from the Maillard reactions, resulting in a richer flavor.
Juggling time and temperature is a balancing act between achieving some reactions in some portions of the meat and other reactions in other parts of the meat. If you’re like me, your ideal piece of red meat is cooked so that the outer crust is over 310°F / 155°C and the rest of the meat is just over 135°F / 57°C, with as little of the meat between the crust and the center as possible being above 135°F / 57°C. The modern technique of sous vide cooking can be used to achieve this effect; we’ll cover this in
Chapter 7
.
This has to be one of the hand-waviest formulas ever. I hereby apologize. To make up for it, here’s an actual mathematical model for temperature change as a function of heat being applied. Remember to cook until medium-rare...
SOURCE: M. A. BELYAEVA (2003), “CHANGE OF MEAT PROTEINS DURING THERMAL TREATMENT,”
CHEMISTRY OF NATURAL COMPOUNDS
39 (4)
This balancing act — getting the center cooked while not overcooking the outside — has to do with the rate at which heat energy is transferred to the core of a food. Since cooking applies heat to foods from the outside in, the outer portions will warm up faster, and because we want to make sure the entire food is at least above a minimum temperature, the outside will technically be overcooked by the time the center gets there. This difference in temperature from the center to outer edges of the food is referred to as a
temperature gradient
.
Choose the method of cooking to match the properties of the food you are cooking. Smaller items — skirt steak, fish fillets, hamburgers — work well at high heats. Larger items — roasts, whole birds, meatloaf — do better at moderate temperatures.
All parts of our example steak are not going to to reach temperature simultaneously. Because grill environments are hotter than ovens, the temperature delta between the environment and the food is larger, so foods cooked on the grill will heat up more quickly and have a steeper temperature gradient.
Carryover
in cooking refers to the phenomenon of continued cooking once the food is removed from the source of heat. While this seems to violate a whole bunch of laws of thermodynamics, it’s actually straightforward: the outer portion of the just-cooked food is hotter than the center portion, so the outer portion will transfer some of its heat into the center. You can think of it like pouring hot fudge sauce on top of ice cream: even though there’s no external heat being added to the system, the ice cream melts because the hot fudge raises its temperature.
Lower heat sources bring up the temperature of the meat more uniformly than hotter heat sources.
The amount of carryover depends upon the mass of the food and the heat gradient, but as a general rule, I find carryover for small grilled items is often about 5°F / 3°C. When grilling a steak or other “whole muscle” meat, pull it when it registers a few degrees lower at its core than your target temperature and let it rest for a few minutes for the heat to equalize.
To see how this works, try using a kitchen probe thermometer to record the temperature of a steak after removing it from the grill once it reaches 140°F / 60°C, recording data at 30-second intervals. You should see the core temperature peak at around 145°F / 63°C three minutes into the rest period for a small steak.
Get a cast iron pan good and hot over medium-high heat. Take a steak that’s about 1” / 2.5 cm thick, rub lightly with olive oil, and sprinkle with salt and pepper. Drop the steak onto the cast iron pan and let it cook for two minutes. (Don’t poke it! Just let it sit and sear.) After two minutes, flip and let cook for another two minutes. Flip again, reduce heat to medium and cook for five to seven minutes, until the center is about 135°F / 57°C. Let rest on cutting board for five minutes before serving.