I’ve written about thickeners and emulsifiers, but I’ve been ignoring one of the most basic components of cooking. Warming a soup; boiling a potato; searing a steak; baking a souffle; microwaving a cup of tea; they all have one thing in common: the application of heat.
Understanding the different ways that heat moves is the key to understanding the different effects heat can produce. Most of what I’m going to describe will be effects you already know — melting, freezing, boiling, toasting, browning, convection — but it’s worthwhile to have a really clear mental image of the how and why of these processes.
Let’s start with a grilled cheese. The bread is room temperature; the butter on the outside and the cheese on the inside are both still cold from the refrigerator. The pan is hot.
What does that mean, “the pan is hot”? The pan is made of molecules. Even though the pan is solid, the molecules can jiggle around. The faster they jiggle, the hotter the pan. Heat is just the molecules being speedy.
So, all the little pan molecules are jiggling pretty fast, but the butter molecules are moving slowly — the butter’s still cold. Once the butter comes in contact with the pan, the hyperactive, hot-and-jiggly molecules in the pan start hitting the slow molecules of butter. Just like when a fast-moving pool ball hits a stationary one, the butter molecules that are closest to the pan molecules end up with a fraction of that extra speed. They jostle around a little faster; and they in turn run into butter molecules that are a little higher up — ones that aren’t directly touching the pan. And so a fraction of a fraction of the pan-speed gets up a little further into the butter, and gets those molecules bumping around. And a fraction of a fraction of a fraction of the speed is transferred even higher…
And slowly, slowly, some of the heat of the pan moves up into the butter.
This process of moving heat through molecules hitting other molecules hitting other molecules, is called diffusion. It’s important to remember that diffusion always starts at the hot part — the pan — and moves (slowly!) towards the cold part — the center of the butter.
Once the butter molecules get jiggling fast enough, they can no longer hold on to one another. It’s a little like trying to ice-skate with a group of people, all holding hands; it works if you’re slow and careful, but if everyone starts heading in a different direction, they all break apart. When the butter molecules get jiggling fast enough to fly apart, the butter melts. That’s all a liquid is — a material whose molecules are moving fast enough that they can’t hold on to each other. Slow them down — cool the liquid down — and it will turn into a solid again. (The particular speed at which the molecules can hold on to each other depends on what kind of molecules they are. That’s why water is ice at 0 degrees celsius but alchohol is still a liquid all the way down to -114. It’s much easier for water molecules to hold hands than alcohol molecules.)
But let’s keep our attention on the grilled cheese. The butter has melted, now — it’s good and hot — so lets turn our attention to the bread.
Bread is spongy; it’s full of air.
Both the metal of the pan and the butter have LOTS of molecules packed into a small space. Air has relatively few. Because there are so few air molecules, it takes a long time for a hot pan molecule to hit a cool air molecule. Air heats up much slower than other materials — and it heats other things up slowly, too.
Remember: bread is full of air.
Bread doesn’t make much contact with the pan by itself — but once the butter is melted, the bread makes plenty of contact with the butter. The butter helps transfer heat (speed) from the pan to the bread. But because the bread is full of air, most of that heat stays right at the surface of the bread.
We’ve been pretending that molecules are like pool-balls — they’re identical, solid, and unbreakable. That’s not really true. The molecules of bread are varied and complex. There are sugar molecules, starch molecules, fat molecules, protein molecules — all kinds of big complicated molecules. Once those molecules get really hot, they start to fall apart, and change shape, and recombine, and do all sorts of complicated things. It’s as if our ice skaters got going so fast that their arms started flying off, heads and legs and torsos all bending around and running into each other and making a really interesting mess. That interesting mess? In the case of the bread, it’s tasty:
The bread is toasting.
It’s only toasting on the surface because most of the heat is staying on the surface. Only some of it is migrating slowly through the bread, and reaching the cheese; just enough to warm it.
A gentler heat means the outside of your food will take more time to reach the point of toasting and browning; and so the center of the food will have more time to heat up. A higher heat will heat the outside rapidly, before the heat has had time to bounce and jiggle its way into the center.
High heat to sear; low heat to cook. True of grilled cheese; true of steak.
The process of moving heat around by waiting for molecules to bump other molecules that bump still other molecules — it’s called diffusion. Baking, frying, and grilling all make use of diffusion.
Lets talk about boiling water, heating soup, and other liquids. You’ve put a pot of cold water on the stove, and the pot’s gotten hot, and the lowest layer of water has warmed up but the heat hasn’t yet jostled it’s way to the top.
If you took a ladle and stirred the water up, mixing the hot water (and all its fast-moving molecules) into the cold water, all those speedy moecules would have many more slow-moving targets to hit, and warm the rest of the water faster.
Luckily, hot water is less dense than cold water — precisely because its molecules are moving faster, and so are (on average) further apart. The denser, colder water will tend to sink around the hotter, faster water — the water will stir itself!
Moving heat around by stirring — by literally moving the fast-jiggling molecules around — is called convection. Convection will evenly heat a liquid faster than diffusion. Trying to defrost a loaf of sliced bread? Rearrange the slices now and again, to help the heat make its way into the center of the loaf.
What about when you put food under a broiler? Diffusion is present — the air gets hot, and the hot air heats the food — but there’s a more important process going on.
The broiler is radiating. When atoms get excited and xtra-jiggly., they can get rid of some of that excess energy by spitting out a photon — a tiny bit of light. The mean red glow of an electric broiler? That’s light made by extra-jiggly atoms.
The same process can happen in reverse — a photon can be swallowed by an atom, and give that atom some extra jiggle.
Much of the heat that moves from the broiler into a steak moves this way — carried in the light (only some of which is visible). Shiny, reflective things — things like aluminum foil — will reflect the light, and prevent it from being absorbed. If you want to brown part of a dish, but not the whole thing, put some foil over the part you don’t want to brown — and put it shiny side up!
So far, everything we’ve talked about involves heat starting at the outside of the food and heading inward. What about a microwave?
Microwaves are a little more complicated. Do you remember, from when I talked about emulsifiers, that water is polar? A microwave is a machine for spinning polar molecules. Microwaves are a form of radiation — just like the light from the broiler — but microwaves aren’t absorbed by atoms very easily. That means that they can penetrate further into the food, exerting their polar-molecule-spinning powers.
And what’s a spin except a special kind of jiggle?
And once the water is jiggling good and fast, the heat in the water will move (by diffusion) into the rest of the food.
This is why it’s hard to brown food in the microwave — it’s not the food that’s getting hot at first, it’s the water. It also explains why if you put something like dry noodles into a microwave, they won’t heat up very well at all — there’s no water there to jiggle!
Moving heat this way — by spinning polar molecules — is called dialectric heating.
We’ve talked about diffusion, convection, radiation, and dialectric heating.
See if you can now explain:
- Why, if you want to cool a bowl of pasta, should you stir it?
- Why should you cut up potatoes first, to get them to boil faster?
- If you’re re-heating leftover rice in the microwave, why should you get it wet first?
- Why does a thin sauce heat all the way through all by itself, but a thick sauce needs to be stirred?