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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It’s similar to how you can’t eat all your veggies for the month in one sitting. You need time for it to clear your stomach and absorb into your body.
Similarly, the goal is not to dump all the heat energy into a cookie as quickly as possible, it’s to distribute heat in a certain way that leaves the right consistency in different parts – a little crisp on the outside, chewy or crispy on the inside (and the flour cooked throughout!) It takes a certain pace of heat delivery for all the parts to reach this at the right time.
Think of it in an extreme.
If you want to make Mac and Cheese, you’ll boil water for a few minutes, put in the pasta, let it boil for several minutes and you’re done, at like 350-400 degrees.
It heats the noodles and it slowly is absorbed into pasta…too short and it’ll be too stiff, too long and they’ll get soggy.
You could instead heat the water at 4000 degrees for like 30 seconds… but not it’s furiously boiling and turned to steam, and the macaroni will not heat up and absorb water, it’ll burn to a dry crisp.
The interaction between the ingedients and the reactions to different temperatures is what makes it happen.
Can do same with chocolate chip cookies, if you just heat it to 1000 degrees, the chocolate doesn’t nicely melt, it would caramalize and harden.
To answer this, let’s talk about toilets. Imagine that in a cold bathroom you have two toilets: one with a plastic seat, and one with a metal seat. When you sit on the plastic seat, it may feel a little cool, but otherwise not too bad. If you sit on the metal seat, it will feel horrifically cold, and you’ll probably be concerned about the safety of your genitalia. They’re in the same room and are the same temperature, but the metal one feels much, much colder. Why is that? It’s because the heat in your butt is stolen much more quickly by the metal toilet seat. The plastic one steals the heat much more slowly.
Cookies are like plastic toilet seats (especially ones made by my mother! She never really got the hang of cooking…). The heat moves slowly into them. If the oven is too hot, by the time that the heat reaches the middle of the cookie and finishes cooking it, the outside will be burnt.
If you want a great illustration of the answer, cook a steak in a sous vide. You can set the water bath to 140F and the steak will cook to a perfect medium rare – and then stay there. You can leave it for an hour or more and it won’t overcook. That’s because it’s not about the time, really – it’s about getting the internal temperature of the meat right. At a certain temperature, the proteins start to denature and change, giving it a “cooked” texture – and the proteins belonging to bacteria that may be living in the meat also denature, killing the bacteria and making the meat safe to eat.
When it comes out, it has no crust – it’s medium rare all the way through. Slap it in the cast iron or the grill at high heat for a minute on each side, and only the outside of the meat heats up really hot, cooking it past well done to a nice hot crispy burnt texture but not penetrating to the soft inside, which stays at the same 140 temperature and remains pink and tender. Pop it on a plate and there you have it, a perfect steak!
Edit: I made myself hungry for steak but that ain’t happening tonight 😩
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.
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.
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.
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.
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.
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