eli5: What is the difference between cooking food and burning it, at a chemistry level?

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In both cases you’re heating up food to change it in some form, but cooking changes it in a good way that maintains (or enhances) its flavor and nutritional value, while burning destroys the food.

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“Burning” a food is generally a chemical reaction called pyrolysis, where a complex organic molecule gets broken down into carbon and small volatile molecules like water that evaporate away. It also produces some chemicals that are harmful to you (like polycyclic aromatics) that you’d find in something like oil (in the sense of petroleum, not cooking oil). Since burnt food loses most of its nutritional value because most vitamins and useful organic molecules break down and gains toxic chemicals, it makes sense that it would taste bad to you. (Humans have been cooking food for a very long time, long enough for it to be reflected in our evolution.)

Cooking at a lower temperature, on the other hand, tends to preserve most of the nutritional value while killing bacteria and parasites and often making food softer and more digestible. So it’s reasonable that the chemicals produced in the cooking process tend to taste good.

In short: “burnt” food has gone past the point where your body can usefully process it.

Heating food isn’t instantaneous. Even when the food does get up to temperature the resulting chemical reactions aren’t instantaneous.

When you heat food at the right temperature for the right amount of time the right chemical reactions go on for the right amount of time to make them taste good. If you use a higher temperature or a longer time those reactions will go on for too long.

Too long may just mean that too much water has evaporated and it’s dried out. It may mean that compounds have started to form or break down in undesirable ways.

In particular if it’s really burnt you’ve oxidized it. At the extreme end of that you have a pile of ash.

An analogy in my mind is the difference between partially disassembling a an intricate rubber band ball and thoroughly taking scissors to it.

It’s the difference between drinking water and drowning. When you’re applying heat to food, you’re drinking water. Try to drink too much water (too hot), or try to drink for too long, and you’ll drown (the food will burn).

If you drink slowly and consistently (low/medium temp for an extended time), you’re less likely to drown, although it’s still possible (technically, water intoxication). If you’re chugging water (high heat) and not careful about the timing, you more likely to drown (burn). But you can chug water for a short period of time and be okay (searing or toasting). You can also sip water very consistently for a very long period of time (slow cooking).

You should be aware that water boils at 100 C, 212 F, and while it boils the heat from the flame under the pot goes into making it boil, not into raising the temperature of the water. So water has the effect of keeping the food from heating up so much that it burns; as long as there’s (a decent amount of) water present in whatever you’re cooking, the temperature will go to 100 C and stay there.

So that’s the difference between cooking and burning food; in general there’s water in the food and it keeps the food at 100 C, 212 F and basically prevents it from being burned.

Even deep-frying things in oil (which boils at much higher temperatures), for example chicken or fries, there’s enough water in the pieces of chicken and in the fries that boils first, to keep the temperatures from getting too high.

Same thing happens with ice; water freezes (and ice melts) at 0 C (32 F), and as long as you have *some* ice left in your drink, the temperature of the whole drink will stay at roughly 0 C despite all the heat from the room and from your hands on the glass. All that heat energy goes into melting the ice, not into raising the temperature of the drink.