eli5: why isn’t it possible to cook e.g. cookies at twice the temperature for half the time? obviously i know it isn’t possible, but *why*? what’s the physics behind it?
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The hot air only causes the outside of the food to get hot. The hot air doesn’t touch the inside of the food. The inside of the food only gets hot because heat “leaks” its way through the food from the outside to the inside. This needs to take time to happen. The food is a good insulator, taking time to let heat “leak” its way through.
So if you set the temperature high and try to cook it too fast, the heat hasn’t had time to “leak” all the way to the inside by the time the outside starts to get overdone. Try to do that and you end up with food that’s burned on the outside when it’s still raw in the middle.
If you notice, whether the instructions say to go high temperature for a short time versus lower temperature for a long time often depends on how thick the food is. If you have two different store-bought frozen pizzas from the same company that are mostly the same ingredients except one is the thin-crust version and the other is the thick-crust version, the baking instructions will differ. The thicker pizza will have the slower instructions (lower temperature, longer time). If you try to cook it with the same faster instructions the thin crust version has, you’ll end up with a crust that’s all floppy and made of goo. The heat didn’t have time to “leak” all the way through and cook the the inside of the dough.
(EDIT) Note, this is for conventional ovens and for toaster ovens. They work by making the air hot, and then the hot air touching the food cooks the food. A microwave oven doesn’t do it that way. In fact it barely heats the air at all. A microwave raises the food temperature by using radio waves that make molecules vibrate, making the food cook throughout, inside at the same time as outside. In fact, the inside often cooks a little faster than the outside in a microwave. Even though both the inside and outside get an equal amount of heat input from the waves, the inside can’t “bleed” that heat away to the outside air so its temperature rises faster. The problem on a microwave can end up being the opposite of on a conventional oven. Instead of “outside overdone and inside underdone” it can be “outside underdone and inside overdone”. This is why things like “Hot Pockets” and little microwave pizzas are cooked using a little cardboard table or sheath with a kind of grey coating on it. That coating is just a little bit metallic. The metal “curves” the microwaves toward it so stuff next to the grey coating heats up faster than other stuff in the microwave does. The crust needs to heat a bit more than the inside does, which would naturally happen in a conventional oven but doesn’t in a microwave oven.
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