What heat
does to food.
cook and chemist at once.
Cooking is chemistry you can eat.
Crack a raw egg into a cold pan and you have a clear, runny liquid. Switch on the heat and within a minute it has turned white, firm and opaque — and it will never go back. You added no flour, no setting agent, nothing. All you did was raise the temperature. That single change rearranged the egg at a level too small to see, and the result is a different food. Cooking, at heart, is the art of using heat to change food on purpose.
Heat does three big jobs in the kitchen. It makes food safe — high temperatures kill the bacteria you met in the hygiene lesson. It makes food easier to digest — softening tough fibres and breaking down starch the body can then use. And it builds flavour, colour and texture — the crust on bread, the brown of a roast, the snap of a crisp chip. Three quite different chemical changes are doing most of that work, and they all start the moment the temperature climbs.
Temperature is really a measure of how fast the tiny particles in food are jostling. Heat them and they move faster and harder — fast enough to shake apart the bonds holding some molecules in their original shape, and fast enough to push others into brand-new molecules. Cooking is exactly that: giving particles enough energy to rearrange into something new.
Humans have been cooking for an astonishingly long time — there is good evidence of controlled fire used for cooking going back roughly 800,000 years. One leading idea, the "cooking hypothesis", argues that softened, cooked food gave our ancestors so much more usable energy that it helped fuel the growth of the large human brain.
Proteins set; starches swell.
Two of the most important changes in cooking happen to the two big building blocks of food: protein and starch. Once you can picture what heat does to each, half the kitchen makes sense.
A protein in its raw state is a long chain folded up into a neat, specific shape — think of a length of wool wound into a tidy ball. Heat shakes that ball loose: the chain unravels. We call that denaturing. The loosened chains then bump into each other and tangle together into a firm mesh — that step is coagulation. That is exactly why egg white goes from clear liquid to firm white solid: the proteins denature, then coagulate, trapping water in a solid net. The same setting happens in a cooking sausage, a grilling fish fillet, a firming custard.
Starch behaves quite differently. A raw potato or a piece of dried pasta is packed with tightly bundled starch granules — hard, and difficult for the body to digest. Add water and heat, and those granules drink up the water, swell, and burst open into a soft, thickened mass. That change is called gelatinisation. It is why pasta turns from brittle to tender, why a sauce thickens when you stir in flour, and why a raw potato is inedible but a boiled one is soft and digestible.
Protein setting needs mainly heat. Starch gelatinising needs heat and water together — which is why you boil pasta and rice rather than dry-roast them, and why a flour-and-water paste thickens only once it gets hot. Notice the pattern as you cook: is this food mostly protein, or mostly starch? That tells you what it needs.
Why a steak browns and bread gets a crust.
The setting of an egg is dramatic, but it is colourless. The reactions that make cooked food look and smell so good are two browning reactions — and they need higher temperatures, and a dry surface, to happen at all. That is the real reason you can't get a brown crust on something you've boiled: water keeps the surface stuck at 100°C, and browning needs hotter than that.
The first and most important is the Maillard reaction. When the dry surface of a food climbs above roughly 140°C, its proteins and its sugars react together to create hundreds of brand-new molecules — the brown colour and the deep, savoury, roasted smell of a seared steak, toasted bread, roasted coffee, fried onions, the crust of a cake. It is one of the most important reactions in all of cooking, and it has a name because a French chemist worked it out.
The second browning reaction is caramelisation. This one is simpler: it is sugar browning on its own, with no protein needed, once it climbs above about 160°C. It gives the golden edge of a roast onion, the toffee note in a caramel, the dark sweetness of well-roasted vegetables. Maillard browning is savoury; caramelisation is sweet; and in a real roast dinner the two often happen side by side.
Water boils at 100°C and won't get hotter while it stays liquid — so anything boiling sits at 100°C, well below the 140°C the Maillard reaction needs. That is why boiled chicken stays pale and a roasted one goes golden, and why you pat a steak dry before searing: a wet surface wastes the heat on boiling the water off before browning can even start.
The Maillard reaction isn't only about taste — it's also what makes roasted coffee, baked bread and toasted nuts smell so powerful. A single roasting coffee bean releases hundreds of different aroma molecules, almost none of which exist in the green, unroasted bean. The smell of a bakery is the Maillard reaction drifting out of the door.
Three ways heat reaches your food.
None of those reactions can happen until the heat actually gets into the food. And heat travels in three different ways — conduction, convection and radiation. Every cooking method is really one or more of these three at work. Here they are, all three at once, in a single pan on a hob.
Conduction
- Heat passing through a solid by direct contact, particle to particle.
- The pan base heats; the food touching it heats; the handle slowly warms.
- Frying is mostly conduction — straight from hot metal into the food.
Convection
- Heat carried by a moving liquid or gas — hot fluid rises, cool sinks, a current forms.
- Boiling water and a fan oven's circulating air both cook by convection.
Radiation
- ·Heat travelling as invisible rays, needing no contact and no air between — the way the sun warms you across empty space.
- ·Grilling and toasting cook by radiation — heat beams down (or up) onto the food's surface, browning it fast.
Real cooking rarely uses only one. An oven roast uses convection (hot circulating air), radiation (from the hot oven walls) and conduction (from the tray into the food) all at once. Knowing which mode dominates tells you what to expect: grilled food browns on top fast (radiation); boiled food cooks evenly but never browns (convection in water, capped at 100°C).
Every method, by its science.
Now the whole picture joins up. Each everyday cooking method is a particular way of getting heat into food, working at a particular temperature, and so producing — or not producing — that prized brown crust. Read the table as the science behind the cooker.
| Method | Heat mostly travels by… | Roughly | Browns? |
|---|---|---|---|
| Boiling | Convection (in water) | 100°C | No — too cool, too wet |
| Frying | Conduction (hot metal) | 160–190°C | Yes — Maillard crust |
| Roasting | Convection + radiation | 180–220°C | Yes — golden all over |
| Grilling | Radiation | very hot, from one side | Yes — browns the top fast |
| Steaming | Convection (in steam) | 100°C | No — gentle and moist |
Notice the rule hiding in the last column: the methods that brown (frying, roasting, grilling) are the dry, hot ones; the methods that don't (boiling, steaming) are the wet ones, capped at 100°C. If you want a crust, you need dry heat above 140°C. If you want gentle, even cooking with no browning, you reach for water or steam.
This is why a cook chooses the method to suit the food and the result they want. Steaming keeps a delicate fish soft and pale. Roasting gives a chicken a crisp golden skin. Boiling softens a hard potato through to the middle. None is "better" — each is the right tool for a different job, and now you know why.
Name the three ways heat travels.
Here is the pan again, with three numbered arrows showing how heat is reaching the food. Pick a label, then place it on the matching pin. Two of the chips name something that isn't a way heat travels — read each one before you place it.
Match each label to its arrow
The egg that sets.
Fresh one. Before a protein can set, heat first unfolds it from its neat folded shape into a loose tangle. What is the name for that unfolding step?
Why pasta softens.
The savoury brown.
Who gave the reaction its name?
Heat through the metal.
Heat that beams down.
Why boiled food won't brown.
Sugar on its own.
Explain the science of one cooked breakfast.
Picture a plate of scrambled eggs on toast. Using today's words, write a few sentences explaining the science of what happened in the kitchen. Try to weave in at least three ideas: what heat did to the egg (think proteins), what happened to the bread to give it a brown, crisp surface (think the browning reaction), and which way the heat travelled in at least one of the steps. There's no single right wording — explain it as if to a curious friend.
The egg (what heat did to its proteins) · the toast (why it browns) · and how the heat travelled. A few sentences is plenty, Florence. No rush.
On your explanation, Florence
strong You've got the egg right — the proteins unfolding (denaturing) and then linking into a firm mesh (coagulating) is exactly why a runny egg turns solid. Naming the change, not only describing it, is the move that turns a description into real science.
try this One strand is quieter than the others — see if you can add a sentence on it. If the toast is covered but the heat-transfer mode is missing, name how the heat reached the bread; if it's the other way round, say which reaction browns the crust.
to add A lovely finishing touch would be one because: "the toast browns because its dry surface gets above 140°C, where the Maillard reaction starts." One reason like that lifts the whole answer.
Films and series about the science of food.
Sit down with Dad for any of these — they turn the kitchen into a laboratory and make the everyday feel wonderful.
The words from today.
- Denaturing
- When heat unfolds a protein from its neat folded shape into a loose tangle — the step before it sets.
- Coagulation
- When unfolded protein chains link into a firm mesh, turning a runny liquid solid — what makes an egg set.
- Gelatinisation
- When starch granules absorb water and heat, swell and burst — softening pasta and thickening sauces.
- Maillard reaction
- The browning of sugars and proteins above about 140°C that creates the savoury colour and aroma of cooked food.
- Caramelisation
- The browning of sugar on its own, above about 160°C, giving sweet, toffee-like flavour.
- Conduction · convection · radiation
- The three ways heat travels: through a solid by contact; carried by a moving liquid or gas; and as invisible rays across a gap.
Worth watching.
Two short films to watch alongside today's lesson — each shows you something the words and pictures can't.
You can read a kitchen now.
You know what heat does to food — proteins denature and coagulate so an egg sets; starch gelatinises so pasta softens; the Maillard reaction and caramelisation brown and flavour the surface. And you know the three ways heat travels to get there. Next time you cook, watch for it: the white firming, the crust browning, the steam rising. That's chemistry on a plate, Florence — and you understand it.
A pressure cooker is a clever trick around the 100°C ceiling. By trapping steam it raises the pressure inside, which lets the water get hotter than 100°C without boiling away — often around 120°C. That extra heat cooks tough food far faster, which is why a stew that takes hours on the hob can be done in under an hour in a pressure cooker.
Film recommendations are factual reference only — see each title's own copyright owner.