by Jeff Potter
What does the home cook need to understand about what they’re doing in the kitchen?
A scale and a good thermometer are absolutely essential if you’re going to try to understand things and do experiments carefully enough to draw real conclusions. You need to be able to measure, and temperature and weight are the main variables.
Is there something that really surprised you in the kitchen?
I suppose the one moment in my life that really confounded my expectation was the copper bowl versus glass bowl for beating egg whites. I was reading Julia Child while I was writing the book [On Food and Cooking] the first time in the late 1970s. She said that you should whip egg whites in a copper bowl because it acidifies the whites and gives you a better foam for meringue and soufflés, but the chemistry was wrong. Copper doesn’t change the pH of solutions, so I thought that since the explanation was wrong, there probably was nothing to the claim either.
Then a couple of years later, when it came time to get ready for publication, I was looking at old graphic sources for illustrations for the book. I looked at a French encyclopedia from the 17th century that had a lot of professions illustrated. One of them was a pastry kitchen. In the engraving, there was a boy beating egg whites, and it said that the boy was beating egg whites in a copper bowl to make biscuits. It specified a copper bowl, and it looked exactly like today’s copper bowls: it was hemispherical and had a ring for hanging. I thought if a French book from 200 years ago is saying the same thing that Julia Child said, then maybe I should give it a try.
I tried a glass bowl and a copper bowl side by side, so I could look at them and taste them, and the difference was huge. It took twice as long to make a foam in the copper bowl; the color was different, the texture was different, the stability was different. That was a very important moment for me. You may know that somebody else doesn’t know the chemistry, but they probably know a lot more about cooking than you do. That certainly got me to realize that I really did have to check everything I could.
A French chef told me a story. He’d made a million meringues in his life, and one day he was in the middle of whipping the egg whites in a machine. The phone rang—there was some kind of emergency and he had to go away for 15 or 20 minutes—so he just left the machine running. He came back to the best whipped egg whites he’d ever seen in his life. His conclusion from that was, in French, "Je sais, je sais que je sais jamais." It sounds a lot better in French than it does in English, but the English is, "I know, I know that I never know."
Thanks to that experience with the copper bowl, that’s been my motto as well. No matter how crazy an idea sounds or how much I distrust my own senses when I do something, and it somehow seems inexplicably different from what it should be, I know that I’m never going to understand everything completely, and there’s probably a lot more to learn about whatever it is that’s going on.
Chapter 4. Time and Temperature: Cooking’s Primary Variables
EVER SINCE CAVEMEN FIRST SET UP CAMPFIRES AND STARTED ROASTING THEIR KILL, MANKIND HAS ENJOYED A WHOLE NEW SET OF FLAVORS IN FOOD. Cooking is the application of heat to ingredients to transform them via chemical and physical reactions that improve flavor, reduce chances of foodborne illness, and increase nutritional value.
From a culinary perspective, the more interesting and enjoyable changes are brought about when compounds in food undergo the following chemical reactions:
Protein denaturation
The native form of a protein is the three-dimensional shape (conformation) assumed by the protein that is required for normal functioning. If this structure is disrupted (typically by heat or acid), the protein is said to be denatured. Changes in the shapes of proteins also alter their taste and texture.
Different proteins denature at different temperatures; most proteins in food denature in the range of 120–160°F / 49–71°C. Egg whites, for example, begin to denature at 141°F / 61°C and turn white because the shape of the denatured protein is no longer transparent to visible light. In meat, the protein myosin begins to denature around 122°F / 50°C; another protein, actin, begins to denature around 150°F / 65.5°C. Most people prefer meat cooked such that myosin is denatured while keeping the actin native.
Maillard reaction
A Maillard reaction is a browning reaction that gives foods an aromatic and mouth-watering aroma. Usually triggered by heat, this occurs when an amino acid and certain types of sugars break down and then recombine into hundreds of different types of compounds. The exact byproducts and resulting smells depend upon the amino acids present in the food being cooked, but as an example, imagine the rich smell of the crispy skin on a roasted chicken.
For culinary purposes, the reaction generally becomes noticeable around 310°F / 154°C, although the reaction rate depends on pH, chemical reagents in the food, and amount of time at any given temperature. Many meats are roasted at or above 325°F / 160°C—at temperatures lower than this, the Maillard reaction hardly occurs.
Caramelization
Caramelization is the result of the breakdown of sugars, which, like the Maillard reaction, generates hundreds of compounds that smell delicious. Pure sucrose (the type of sugar in granulated sugar) caramelizes at between 320–400°F / 160–204°C, with only the middle range of 356–370°F / 180–188°C generating rich flavors.
In baking, those goods that are baked at 375°F / 190° C generally have a noticeably browned exterior, while those baked at or below 350°F / 175°C remain lighter-colored.
"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.
Note
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 temperat
ure 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.
Note
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.
Cooked = Time * 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.
Denaturing Proteins
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.
Heat Transfer and Doneness
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 t1 is the ideal time at which to pull the steak, leaving it for t1+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.
"Cooking = time * temperature"
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)
Temperature gradients
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.
Note
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.
