Cooking for Geeks: Real Science, Great Hacks, and Good Food
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Lower heat sources bring up the temperature of the meat more uniformly than hotter heat sources.
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
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.
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.
Note
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.
Simple Seared 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.
Methods of Heat Transfer
There are three methods of transferring heat into foods: conduction, convection, and radiation. While the heating method doesn’t change the temperature at which chemical reactions occur, the rate of heat transfer is different among them, meaning that the length of time needed to cook identical steaks via each method will be different. The table below shows the common cooking techniques broken out by their primary means of heat transfer.
Conduction
Conduction is the simplest type of heat transfer to understand because it’s the most common: it’s what you experience whenever you touch a cold countertop or grasp a warm cup of coffee. In cooking, those methods that transfer heat by direct contact between food and a hot material, such as the hot metal of a skillet, are conduction methods. Dropping a steak onto a hot cast iron pan, for example, causes thermal energy from the skillet to be transferred to the colder steak as the neighboring molecules distribute kinetic energy in an effort to equalize the difference in temperature. For more on thermal conductivity, see the Metals, Pans, and Hot Spots sidebar in Chapter 2.
Conduction
Convection
Radiation
Description
Heat passes by direct contact between two materials
Heat passes via movement of a heated material against a colder material
Heat is transferred via electromagnetic radiation
Example
Steak touching pan; pan touching electric burner
Hot water, hot air, or oil moving along outside of food
Infrared radiation from charcoal
Uses
Sautéing
Searing
Dry heat methods:
- Baking/roasting
- Deep-fat frying
Wet heat methods:
- Boiling
- Braising/water bath
- Pressure cooking
- Simmering/poaching
- Steaming
Microwaving
Broiling
Grilling
Cooking methods listed by type of heat transfer. (Frying is a dry-heat method because it does not involve moisture.)
Convection
Convection methods of heat transfer—baking, roasting, boiling, steaming—all work by circulating a hot material against a cold one, causing the two materials to undergo conduction to transfer heat. In baking and roasting, the hot air of the oven imparts the heat; in boiling and steaming, it’s the water that does this.
Those heat methods that involve water are called wet heat methods; all others fall into the dry heat category. One major difference between these two categories is that wet methods don’t reach the temperatures necessary for Maillard reactions or caramelization (with the exception of pressure cooking, which does get up to temperature while remaining moist). The flavorful compounds produced by Maillard reactions in grilled or oven-roasted items won’t be present in braised or stewed foods: steamed carrots, for example, won’t undergo any caramelization, leaving the food with a subtler flavor. Brussels sprouts are commonly boiled and widely hated. Next time you cook them, quarter them, coat with olive oil and sprinkle with salt, and cook them under a broiler set to medium.
Note
Water is an essential material in cooking, and not just for its heat transfer properties. Rice cookers work by noticing when the temperature rises above 212°F / 100°C. At that point, there’s no water left, so they shut off.
Another key difference between most of the dry versus wet methods is the higher speed of heat transfer typical in wet methods. Water conducts heat roughly 23 times faster than air (air’s coefficient of thermal conductivity is 0.026, olive oil’s is 0.17, and water’s is 0.61), which is why hard-cooked eggs finish faster in a wet environment even at a lower temperature.
Note
Try it! Cook one egg for 30 minutes in a 325°F / 165°C oven and another for 10 minutes in a 212°F / 100°C water bath. You need to leave the egg in the oven for 20 minutes longer to get the same results.
One exception to this wet-is-faster-than-dry rule is deep-fat frying. Oil is technically dry (there’s no moisture present), but for culinary purposes it acts a lot like water: it has a high rate of heat transfer with the added benefit of being hot enough to trigger a large number of caramelization and Maillard reactions. (Mmm, donuts!)
Wet methods have their drawbacks (including, depending on the desired result, the lack of the aforementioned chemical reactions). While the subtler flavors achieved without browning reactions can be desirable, as in a gently cooked piece of fish, it’s also much easier to overcook foods with wet methods. When cooking meat, the hot liquid interacting with it can quickly raise its temperature above 160°F / 71°C, the point at which a significant percentage of the actin proteins in meats are denatured, giving the meat a tough, dry texture. For pieces of meat with large amounts of fat and collagen (such as ribs, shanks, or poultry legs), this isn’t as much of an issue, because the fats and collagen (which converts to gelatin) will mask the toughness brought about by the denatured actin. But for leaner cuts of meat, especially fish and poultry, take care that the meat doesn’t get too hot! The trick for these low-collagen types of meats is to keep your liquids at a gentle simmer, around 160°F / 71°C, and minimize the time that the meat spends in the liquid.
Note
Even water in its gaseous form—steam—can pack a real thermal punch. While it doesn’t conduct heat anywhere nearly as quickly as water in its liquid form, steam gives off a large amount of heat due to the phase transition from gas to liquid, something that air at the same temperature doesn’t do. As the steam comes into contact with colder food, it condenses, giving off 540 calories (not to be confused with "food calories," which are technically kilocalories) of energy per gram of water, causing the food to heat up that much more quickly (1 calorie raises the temperature of
1 gram of water by 1°C).
Steamed vegetables, for example, cook quickly not just because they’re in a 212°F / 100°C environment, but also because the water vapor condensing on the food’s surface imparts a lot of energy. Cheetos, like most "extruded brittle foams" we eat, gain their puff by being expelled under pressure and heat, which causes them to "steam puff." (Think of it as the industrial version of popping popcorn.) There’s a lot of energy in steam. For this reason, when pouring boiling water through a colander over a sink, you should be sure to pour away from yourself so that the steam cloud (and any splashed liquid) doesn’t condense on your face!
Radiation
Radiant methods of heat transfer impart energy in the form of electromagnetic energy, typically microwaves or infrared radiation. The warmth you feel when sunlight hits your skin is radiant heat.
You can create a "heat shield" out of aluminum foil if part of a dish begins to burn while broiling. The aluminum foil will reflect the thermal radiation.
In cooking, radiant heat methods are the only ones in which the energy being applied to the food can be either reflected or absorbed by the food. You can use this reflective property to redirect energy away from parts of something you’re cooking. One technique for baking pie shells, for example, includes putting foil around the edge, to prevent overcooking the outer ring of crust. Likewise, if you’re broiling something, such as a chicken, and part of the meat is starting to burn, you can put a small piece of aluminum foil directly on top of that part of meat. It might be a hack, but in a pinch it’s a decent way to avoid burning part of a dish, and nobody but you, me, and everyone else who reads this book will ever know.
Combinations of heat
The various techniques for applying heat to food differ in other ways than just the mechanisms of heat transfer. In roasting and baking we apply heat from all directions, while in searing and sautéing heat is applied from only one side. This is why we flip pancakes (stovetop, heat from below) but not cakes (oven, heat from all directions). The same food can turn out vastly different under different heat conditions. Batter for pancakes (conduction via stovetop) is similar to that for muffins (convection via baking) and waffles (conduction), but the end result differs widely.
To further complicate things, most cooking methods are actually combinations of different types of heat transfer. Broiling, for example, primarily heats the food via thermal radiation, but the surrounding air in the oven also heats up as it comes in contact with the oven walls, then comes in contact with the food and supplies additional heat via convection. Likewise, baking is primarily convection (via hot air) but also some amount of radiation (from the hot oven walls). "Convection ovens" are nothing more than normal ovens with a blower inside to help move the air around more quickly. All ovens are, by definition, convection ovens, in the sense that heat is transferred by the movement of hot air. Adding a fan just moves the air more quickly, leading to a higher temperature difference at the surface of the (cold) food you’re cooking.
To a kitchen newbie, working with combinations of heat might be frustrating, but as you get experience with different heat sources and come to understand how they differ, you’ll be able to switch methods in the middle of cooking to adjust how a food item is heating up. For example, if you like your lasagna like I do—toasty warm in the middle and with a delicious browned top—the middle needs to get hot enough to melt the cheese and allow the flavors to meld, while the top needs to be hot enough to brown. Baking alone won’t generate much of a toasted top, and broiling won’t produce a warm center. However, baking until it’s almost done and then switching to the broiler achieves both results.
Note
The convenience food industry cooks with combinations of heat, too, cooking some foods in a hot oven while simultaneously hitting them with microwaves and infrared radiation to cook them quickly.
When cooking, if something isn’t coming out as you expect—too hot in one part, too cold in another—check to see whether switching to a different cooking technique can get you the results you want.
If you’re an experienced cook, try changing heat sources as a way of creating a challenge for yourself: adapt a recipe to use a different source of heat. In some cases, the adaptation is already common—pancake batter, when deep-fat fried, is a lot like funnel cakes. But try pushing things further. Eggs cooked on top of rice in a rice cooker? Chocolate cookies cooked in a waffle iron? Fish cooked in a dishwasher? (See Nathan Myhrvold on Modernist Cuisine.) Why not?
It might be unconventional, but heat is heat. Sure, different sources of heat transfer energy at different rates, and some are better suited to transitioning the starting thermal gradient (edge to center) of the food to the target thermal gradient. But there are invariably similar enough heat sources worth trying. And you can push it pretty far: fry an egg on your CPU, or cook your beans and sausage on an engine block like some long-haul truck drivers do! As a way of getting unstuck—or just playing around—it’s fun to try.
Cooking methods plotted by rate of heat transfer. This plot shows the amount of time it took to heat the center of uniformly sized pieces of tofu from 40°F / 4°C to 140°F / 60°C for each cooking method. Pan material (cast iron, stainless steel, aluminum) and baking pan material (glass, ceramic) had only minor impact on total time for this experiment and are not individually listed.
Foodborne Illness and Staying Safe[2]
The American food supply is one of the most interconnected and interdependent ones in the world. As I write this, I’m eating my morning bowl of cereal, yogurt, bananas, and almonds. The muesli cereal is from Switzerland, the yogurt local to New England, the bananas from Costa Rica, and the almonds from California. The only direction from which food hasn’t come 3,000 miles is north, and that’s probably only because not much grows at the North Pole!
As our food system has become more interconnected, the number of people that can be affected by a mistake in handling food has also increased. Today, a single bad batch of water sprayed onto a field of spinach can sicken hundreds of American consumers because that crop can be transported thousands of miles and make its way into so many dishes before the contamination is noticed.
Handling food carefully—taking note of what has been washed in the case of produce and cooked in the case of meats, and being careful to avoid cross-contamination—is among the easier ways of keeping yourself healthy.
Bacteria related to common foodborne illnesses begin to multiply above 40°F / 4.4°C. The standard food safety rule provided by the FDA for mitigating foodborne illnesses from bacteria states that food should not be held between the temperatures of 40°F / 4.4°C and 140°F / 60°C for more than two hours. Below 40°F / 4.4°C, the bacteria remain viable but won’t have a chance to multiply to a sufficient quantity to bother us. Above 140°F / 60°C, the bacteria won’t be able to survive long. (Bacterial spores, however, can.)
This is called the "danger zone rule," and as you’d probably imagine, a vast simplification of what’s really going on in the bacterial world. Still, as an easy safety rule there’s really no reason to violate it, because there are few dishes that I can think of that actually need to violate it to be made.
Note
For those recipes that say to marinate meat at room temperature: don’t! Let it marinate in the fridge.
Keep in mind that it’s cumulative time here that matters. Say you buy a chicken at the store, and that it was kept chilled the entire time before you picked it up. Between the time you put it in the cart and when you stick it in your fridge, it’ll be in a warmer environment, and any time it spends above the temperature at which bacteria begin to multiply will increase the bacterial count in the meat.
While cooking food kills off most of the bacteria, a minor (yet safe) number can survive even post-cooking. Given the right temperature range, they can reproduce back up into unsafe quantities. When cooking, stick any leftovers in the fridge right away, as opposed to letting them sit around until post-meal cleanup. The bacteri
al level is all about exposure—the amount of time and rate of multiplication at a given temperature.
Note
This is why you should defrost large pieces of meat in the fridge overnight. Letting it thaw, even under cold running water, can take too long to be safe—unless your cold water happens to be below 40°F / 4.4°C!
One detail this rule glosses over is that some bacteria can reproduce at lower temperatures. Luckily, most bacteria related to foodborne illness don’t multiply very quickly at near-freezing temperatures, but other types of bacteria do. Spoilage-related bacteria, for example, are happy breeding down to freezing temperatures. These are the ones that cause milk to go bad even below 40°F / 4.4°C and break down the flesh in things like raw chicken, causing uncooked meats to go bad after a few days. The danger zone rule addresses only the common pathological bacteria, which don’t reproduce very quickly at the temperature of your fridge.
Another area that the danger zone rule glosses over is the different rates of reproduction at different temperatures. Salmonella, for example, is happiest breeding around 100°F / 37.8°C. It’s not like the bacteria go from zero multiplication at 40°F / 4.4°C to full-on party mode at 41°F / 5°C; it’s a gradual ramp up to an ideal breeding temperature. The two-hour window is given for the worst-case scenario: that the food is being held at the ideal breeding temperature for the most aggressive of the common bacteria, Bacillus cereus.
Since food safety codes are currently adopted on a state-by-state level, some states still use a danger zone rule of "40 to 140 for four hours," on the basis that B. cereus accounts for only a minor amount of foodborne illness and that a four-hour exposure isn’t likely to produce much risk. If you’re getting the impression that food safety is a probability game, you’re right. The rules reduce the odds to an acceptable level. Still, that lunch you took to work and forgot to toss in the fridge is probably safe, given that the total amount of bacterial multiplication is likely to be well below the level necessary to trigger any sort of foodborne illness.