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Cooking food: how it works

It turns out that understanding what actually happens when we cook (i.e. heat) food is very helpful in making choices about how to cook food. This gets a bit science-y, but hang in there — it’s not that hard, and you’ve got this.

Why do we cook food anyway?

We cook food for three reasons:

  1. To kill bacteria, parasites, and other bad stuff on and in the food that could make us sick.
  2. To tenderize food to make it easier to digest.
  3. To add flavor (see “Browning“).

All three of these are extremely dependent on heating food to specific temperatures, and usually holding it at that temperature for a specific period of time.

How does cooking work?

To cook something, we add heat energy to it, either directly by putting it in contact with an original source of heat (like a flame) or indirectly by heating the air, water, oil, pot or pan that the food is touching.

Having more heat energy means that sub-atomic particles will vibrate more, and at some point the increased vibration becomes stronger than the bonds holding atoms and molecules together. Several things can then start happening:

  • Matter transitions to “looser” states in which the atoms and molecules can more around more freely: solids become liquids, liquids become gases.
  • Proteins and enzymes, which are folded up into specific shapes that are critical to their function, become “denatured”: the bonds that hold them in that shape start to fail and they begin to unravel. That disrupts biological processes — and kills pathogens.
  • Cell walls start to come apart, spilling out the inner contents of the cells.
  • Muscle tissue fibers can soften.

The threshold temperature where all this starts happening is not a constant; it varies depending on the molecules, proteins, enzymes and cells that are involved. That’s why we have to cook different foods at different temperatures, and why not all pathogens die at the same temperature. Fortunately, scientists have documented the important ones, and there are handy and easily accessible guides that tell us what temperature we need to heat food to in order to properly cook it and make it safe for consumption.

How does heat move around?

If you’ve ever taken a physics class, then you may recall learning about the Second Law of Thermodynamics. It tells us two things that are important to cooking:

  1. Over time, heat energy will spread out evenly.
  2. Heat energy moves between things that are touching each other, from the one with more energy to the one with less energy, until they are equalized.

So if a hot thing comes in contact with a cold thing, heat energy moves from the hotter one to the colder one; the colder one gets warmer, and the hotter one gets cooler. And this will keep happening until they are the same temperature. This is called “conducting” heat.

Exactly how much heat energy transfers between them, and how quickly, depends on three factors:

  1. The temperature difference: the greater the difference, the faster heat will transfer.
  2. How much surface area they share. You can think of this like water pressure or air pressure equalizing: it does it faster and at greater intensity with a wide pipe than a narrow one.
  3. The heat capacity of each. Heat capacity is how much energy in the form of heat an object can store, and this varies depending upon the material you are heating or cooling. If you’ve ever removed aluminum foil from an object you just took out of the oven, you know that it pretty much instantly cools; that’s because it has an extremely low heat capacity (and it’s very thin and has huge amounts of surface area). But water and oil take longer to cool, because they have higher heat capacities. Likewise, metal pans cool quickly, but glass and ceramic ones take longer both to heat up and to cool because the materials they are made of have larger heat capacity.

Consider what happens when you drop an ice cube into a large pot of hot water. The ice and the hot water are now in contact with each other, so heat energy will move from the hot water to the ice cube until they are the same temperature. But there was a lot more hot water than ice, so while the hot water temperature drops a little bit, it’s almost too little to measure — the hot water doesn’t have to give up much heat in order to heat a small amount of ice up. But if you dumped a large amount of ice into a pot of hot water, you’d see a much more significant drop in the temperature of the hot water, and when it all equalized it would be at a temperature somewhere in between the two extremes.

Now in reality it’s a little more complicated than that, because the hot water that is directly in contact with the ice transfers some of its heat energy over; but then it’s at a lower temperature than the hot water next to it, so some heat moves from the hotter water to the slightly cooler water next to the ice. And so on and so on, as the temperature drop works its way around the the pot. While this sounds like small details, understanding this idea is very important when we start looking at cooking solid things like meat.

Most of the cooking we do takes the form of heat conduction, and for solid objects like meat and bread we apply heat to the outside of the object. Then, when the outside is hotter than the part just a little farther inside, heat is conducted a farther in, step by step.

An object shortly after it starts being heated: the heat is starting to work its way inside.

After a while, we have a “gradient” of heat: the outside is hottest, the center is coldest, and the temperature gradually changes in the area between the two. How steep that gradient is, and how quickly heat is moved to the center, depends on the three factors we discussed before: the difference between the outside and the center, the amount of surface area, and the heat capacity.

An object after being heated for a while: there is a “gradient” of temperatures, with the outer part hottest and the center coolest

Where this gets tricky is when we need to start making choices about how much heat to apply to the outside in order to cook the inside. If we want to get the center to a nice, safe 165 degrees, we can apply 165-degree heat to the outside — and wait a long time. Or we can crank up the oven to 350 degrees, in which case the heat will move faster to the center and it will reach 165 degrees in less time. But in the process we’re going to make the outer parts even hotter; they will keep trying to get up to 350 degrees, since that’s how much heat they are in contact with. So long as the center is cooler than the outside, there will always be a temperature gradient across the thickness as heat tries to move farther in to equalize the temperature.

We care about this because food becomes less edible if it gets too hot. If it reaches 212 degrees, then the water will start boiling off and it will dry out. Between 300 and 350 degrees, the sugars and proteins will start browning — and eventually will burn. The browning and burning happens even faster at higher temperatures. This is why most recipes for roast chicken and turkey suggest starting at 425 degrees for about 15 minutes to give the outside a little bit of browning, and then turning the temperature down to 325 and letting it cook more slowly until the center reaches 165.

This also means that when you remove that roast from the oven, there is still a temperature gradient: the outer layers are hotter than the center. That means that even though you are no longer applying heat to the outside, heat is still moving from the middle layers to the center as it tries to equalize the temperature.

An object immediately after being removed from the oven; there is still a temperature gradient.

Five minutes later, the center will be warmer than when you removed it from the oven. This is called “carryover cooking.”

An object five minutes after being removed from the oven: carryover cooking has continued to raise the center temperature.

Of course, the outside has been exposed to room-temperature air, so it’s cooling. That means the middle layers are hotter than the center AND hotter than the outside — and so they are sending heat energy in both directions.

An object ten minutes after being removed from the oven: more carryover cooking, which has almost equalized. The hottest part is the middle layer, which is sending heat in both directions as the outside cools.

Carryover cooking is weird. But understanding it is essential to making sure that your cooking comes out right. In a perfect world, you know how much carryover cooking will happen and you can remove that roast from the oven at just the right time, when it’s still a little below your target temperature, so that after the carryover cooking is done it’s exactly where you want it. But reality just doesn’t work that way; there are no magic formulas to calculate carryover cooking. Making a recipe a few times will give you a good sense for how much carryover cooking will happen, if you make the effort to measure it.

And unfortunately there is little you can do to control carryover cooking. Cooks are often taught to rinse their pasta in cold water to stop the carryover cooking, which is complete nonsense; the whole notion of carryover cooking is that changing the outside temperature doesn’t immediately change what’s happening inside. If moving pasta from a 212-degree pot of water to 70-degree air doesn’t stop carryover cooking, then rinsing it in 50-degree water won’t make a meaningful difference either except to the outside temperature. The only way to stop (or reduce) carryover cooking is to slice up the thing you’re cooking to physically separate the hotter part from the cooler part — but by doing that you’re also increasing the surface area, so every piece will cool down to room temperature faster.

Let’s talk turkey

Before we conclude, there is one more practical example worth discussing: should you stuff that Thanksgiving turkey with hot or cold stuffing (or not at all)? If you don’t stuff it, then the middle is hollow (full of hot oven air) and you’re effectively cooking your turkey both from the outside in and the inside out — though really you’ve just redefined the “center” to be the middle of the thickest part of the turkey. Regardless, it will cook the fastest this way.

If you stuff it with cold stuffing, then you’ve made the whole thing thicker. It will take longer for the oven heat to penetrate all the way to the center, and you will need to take care to measure the temperature of the stuffing as well as the meat. A hazard of using cold stuffing is that it can sit in the “danger zone” for hours while the turkey meat cooks, a perfect environment for bacteria to grow, and if you don’t check to make sure that you’ve heated the stuffing enough (and thoroughly) that bacteria won’t die and you and your guests can end up very sick.

If you stuff it with hot stuffing, then in many ways it’s similar to not stuffing it at all: you’re cooking it outside-in and middle-out simultaneously. But while the stuffing has its own heat capacity, it’s not a heat source: as it transfers heat to the turkey, its own temperature will drop, potentially into the “danger zone.”

The moral of this story: any of these three options CAN be safe, but it requires checking the temperature of both the turkey meat at its thickest point AND also the stuffing, to make sure both are cooked enough to be safe to eat.

And that advice generalizes: don’t guess. Get a good, accurate insertion thermometer, and check the temperature of the things you’re cooking both to ensure that they are safe to eat, and also to learn about how much carryover cooking they will have. Your thermometer is your best friend.

Convection vs. standard ovens

Many ovens today come with a “convection” setting (in older days, ovens were either standard or convection, but not both, so this is a nice advance). It’s worth taking a moment to explain the difference between the two, which is pretty easy stuff now that we’ve discussed how cooking works.

Ovens are indirect heating: the heating elements heat the air inside the oven, which then comes into contact with the food and transfers heat energy. But in a traditional oven there isn’t a great deal of air circulation, so the air itself has a heat gradient: the air nearest the heating elements is the hottest, and the air directly in contact with the food is the coolest since it’s actively transferring its heat. Even worse, ovens tend to have “hot spots” and “cold spots” depending on how well they are insulated, and every time you open the door you replace a lot of the hot air with cooler air.

Convection ovens have a fan that keeps the air circulating, ensuring that much more of the air is close to the desired temperature. Air isn’t trapped next to the food; it’s constantly moving along, making room for other hot air to come in and transfer more heat energy.

What this means is that convection ovens are more efficient at transferring heat energy to food, and as a result a convection oven will cook food faster than a traditional oven set to the same temperature. Or, to be more specific, it will raise the temperature of the outside layer of the food faster, creating a steeper temperature gradient and thus moving heat to the center a bit faster. Though it also means that the outer layers will get hotter, and unless you manage that, you may not like the effect that has.

Air fryers work on the same principle as a convection oven: moving hot air around. In fact, it wouldn’t be wrong to say that an air fryer is essentially a very small convection oven with a very powerful fan.

Microwave ovens

Microwave ovens heat food using a very different principle. Rather than transferring heat energy through conduction or convection, they irradiate food with electromagnetic waves. The microwave frequencies used will cause water molecules to vibrate, essentially turning electromagnetic energy into heat energy.

It is often said that microwave ovens cook food “from the inside out.” This is not true — at least not often true. Microwaves can only penetrate about a centimeter into a piece of solid food, so anything thicker than two centimeters will not be cooked through by a microwave oven. Instead, the outer layers will generate heat, and that heat will then conduct itself further inside as it does in a traditional oven. So the microwave generates less ambient heat in the air and focuses the heat into the food, but it’s still outside-in; it just gets a bit of a head start. And food cooked in a microwave can have carryover cooking too (which is why that bag of microwave popcorn still has some residual kernels popping after you remove it from the microwave oven).

Microwave ovens are much more efficient when used for heating liquids and for cooking thin foods, where the microwaves can penetrate more fully.

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