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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When you put something in the oven, the outside heats first, and then the inside slowly absorbs heat from the outside.
If you raise the temperature, the outside heats way faster but the inside still lags behind.
In the most extreme scenario, the outside is burnt while the inside isn’t even warm.
The amount of time it takes heat to penetrate the cookie is always the same. So when you cook higher for less time, the inside doesn’t get the heat before the outside burns
Food cooks by transferring heat from a source (e.g. oven, skillet, boiling water, etc.) to the interior of the item being cooked. The speed of cooking is limited by the capacity of the food to conduct heat. The heat conducting capacity changes very little regardless of the heat source. Raise the temperature of the heat source, the food simply cooks faster at the surface, but not at its interior. The result is that the surface of the food is done much faster than the interior. This is good if you want a very rare steak, i.e. crispy brown surface with cool, red center. On the other hand, if you’re going for medium rare, it’s best to start with very high heat to brown the surface, then lower the heat to allow the interior to warm slowly while remaining moist and juicy. The same is true in baking. If you set the oven too high, the surface of your cake will char while the interior is still raw dough.
The energy from the heat can’t make it into the center of the cookie in half the time, and at the same time you’re causing unwanted reactions in the outer part of the cookie by heating it too high (burning it).
You need to get the heat to conduct through your food to the center, which takes time because a solid material can’t be stirred. If you expose the outside to a greater temperature, it may get scorched before the center is cooked because it dries up quickly, and without water there is nothing to limit the temperature.
You’ll want to look up something called the Biot number. This number is the ratio of heat transfer (how much heat into the surface of something, in this case, from the oven to the cookie), relative to how quickly that heat can go from the outside of the cookie to the inside (thermal conductivity of the cookie). Imagine a case where the heat into the cookie is 10000X higher than how quickly that same amount of heat could travel to the center of the cookie. You’d end up with a cookie that is very burnt on the outside, and raw cookie dough in the middle.
Bc the entire cookie doesn’t immediately become the same temperature. The outer most layers heat up first and continue to heat and thus cook and then burn, while the inner layers are coming up to temperature more slowly. In scientific terms it’s about heat transfer from the air in the oven to the dough. The dough is a solid and cannot be penetrated 100% immediately, it happens in layers and the heat is transferred from the outer to the inner layers. But at a higher temperature, the outer layers are still getting more heated all the time which burns it before the inner layer can be cooked.
Different chemical reactions happen at different temperatures. Not just the same ones but faster. You have to cook at a temperature that’s above the
>eggs + flour = cookie
threshold, but below the
eggs + flour + oxygen from air = charcoal
threshold. If you double the temp and reach the burning/charring reaction threshold temp, then that reaction starts happening along side the baking reactions you want. Also you could have the burning outside before the inside even reaches the eggs+flour = cookie temp, so at every point along the timeline some part of it is either raw or burnt. You need to use a temp low enough that there’s at least some window where it’s all cooked but not burnt.
The material properties of heat propagation is covered by the other answers, but there’s another thing you touched on that makes this question kind of impossible: What does “twice the temperature” mean?
Doubling the degrees changes depending on where your 0 is. 10°C ‘doubled’ would be 20°C, right? But in Fahrenheit those temperatures are 50°F and 68°F. The only absolute way to double temperature would be basing 0 at 0°K, but that would have the baking temperature of 350°F/180°C/453°K to 1160°F/630°C/900°K, which is quite ridiculous to expect in any conventional kitchen given that aluminum melts only 30°C higher at 660°C. Most of the kitchen’s materials will be degrading pretty quickly.
Heat is just molecules bumping into each other faster. Heat that is too high is like using a battering ram to hammer in a nail. There’s too much unnecessary force for what you’re trying to do so you “burn” it and destroy it instead of joining things together just right.
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