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?


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?

In: 9

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

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 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).

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.

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.

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.

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.

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.

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 😩

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.

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.

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.

Chemical compounds have a non-linear response to temperature. Things that are stable for very long periods of time at one temperature will decompose very quickly at higher temperatures. For example, the flash point or smoke point of cooking oils. You can heat the hell out of them, well under that point, for quite a while and not much will happen, but if you go over it they will decompose and give off smoke and maybe even catch fire. If you tried to blast cookies at a really high temperature the dough would just burn to carbon.

It’s because of the speed at which heat moves through the whole cookie.

If you double the heat outside, you do not double the speed at which the heat moves through the cookie.

Also at certain temperatures you get chemical reactions on the outside that actually slow the speed at which heat moves through the cookie, so the recommended temperature to bake a cookie is often the optimum in regards to many factors.

If the cookies you made were very thin, it is possible to speed up cook time with increased heat.
Although they would cook quite quickly anyway, so the benefits of the increased heat would be miniscule.

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.

The incredibly simple explanation is the outside is exposed to a higher heat so Cooks quicker but it takes time for heat to reach the centre and due to the extra heat the outside burnt in that time.

Why not? It is called a microwave. I get the jist of the question but it is possible. It is sad that few people actually understand how a microwave works. It is a magical beast that gets around so many evaporation corners in cooking. FYI, that power button on your microwave is a useful thing.