Ever notice how a potato in boiling water cooks a LOT faster than a potato in the oven? A whole potato might take 20 minutes in water at around 200 degrees, where a potato may take an hour or longer at twice that temperature.
Potato’s are one of the worst conductors of heat in the food kingdom. All sorts of studies have been executed trying to figure out the perfect way to cook this darn thing. And they all point to the same result, spuds take forever to cook. It’s this slow transfer of heat that caused the Belgians to come up with a double frying strategy for spuds, once at 325 F, and another at 375 F, to counteract the slow transfer of heat. The lower initial cooking method cooks the spud through, and the higher temperature browns the exterior.
How does all this relate to an oven?
Let’s talk a little bit about the factors that help an oven cook food.
The first factor is a bunch of hot air. Air is a TERRIBLE conductor of heat (ever stuck your hand in a 212 degree oven, and then in a boiling pot of water (also 212). Where the oven seems uncomfortably warm, the water will give you serious burns. Even though they’re both the same temperature, the results are dramatically different. Therein lies the challenge with air as a method of cooking. It doesn’t get the job done. It’s no wonder that the phrase “a bunch of hot air” means a lot of hype, but no action.
The second influence is the motion of the air in your oven. Think of this as a reverse windchill. A 32 degree day may seem nice with no wind, but when it’s combined with a 30 mile per hour gale, it seems a LOT colder than it is. As your oven elements cycle off and on, air warms and cools. This creates a natural convection in your oven that forces hot air molecules into your food. Although this cycle in a normal oven occurs at a very low speed, it has a significant impact on transferring heat from the air of your oven to your food.
The last, and most important part of cooking your food in an oven is the infrared radiation emitted by the heating elements and oven enclosure itself (walls/ceiling). Radiation does more to cook your food than both of the previous factors combined. To prove this, try holding your hand in the middle of a 300 degree oven with the element off, then try it again with the element on. Although the temperature in the oven is close to the same in both scenarios, you’re MUCH more likely to get burned when the element is on. This same radiation makes wearing a black shirt on a sunny day a bad idea. This same radiation that causes dark pans to burn the bottom of your cake. It’s blocking this radiation that prevents sunburns, and burned pie crust edges.
The way that infrared radiation is emitted is not linear. So if you increase the temperature of your oven by 10%, you won’t see a 10% increase in radiation. The total radiation will increase by the absolute temperature to the fourth power. In other words… a LOT. For more discussion on this, please read this article on blackbody radiation and this article on the Stefan-Boltsmann Law. It’s important to note that the sides and back of your oven are radiating heat near to the temperature of the air in the oven, but the heating element is roughly 2000 degrees. Something to think about next time you stick your hand into the oven while the element is on.
So what does this mean to the average person?
1 – Just because the air in your oven is to temperature, doesn’t mean that the walls and ceilings of the oven are hot enough to properly cook your food. If you’ve ever made cookies and noticed that the more batches you make, the shorter your cooking time is? The walls and ceilings of the oven are finally emitting enough infrared heat to help in the cooking process. To get more consistent results, let your oven go at least 15 minutes after it’s been preheated before you begin cooking. (I know some chefs that swear you should go at least an hour).
2 – Consider the type of dish you will be cooking your food in and how it will absorb radiation. Dark metals will absorb much more heat, meaning a pizza with a killer crust. On the flip side, it can also mean the bottom of your cookies will be burned before the cookie is cooked through. By understanding a lighter pan will cook slower, and a darker pan will cook faster, you’ll be able to tailor your pan selection to get the best results.
3 – The side of the food exposed to the heating element will always cook faster. If you’re making two pans of cookies at a time, the top pan will always be browned on top before the bottom pan (assuming you’re using similar pans). However, the bottom pan will have a browner cookie bottom. To account for these differences, switch your product halfway through. You’ll get better results.
4 – Aluminum Foil prevents burning. After all, it’s reflective nature will prevent the infrared radiation from coming into contact with either your pan, or your food, depending on where you place the foil. Once the power of the radiation has been diminished, the miniscule effects of air temperature and convection will be left to do the work. Many a Thanksgiving turkey has been saved from burnt skin by a well placed piece of foil, at just the right time. (Just make sure your foil isn’t touching your food, or it will still transfer heat).
In closing, I know of some people that put unglazed quarry tiles in the bottom of their oven, and let it preheat for 30 minutes or more. The tiles do a pretty good job holding heat, preventing the oven from cycling on and off when the door is opened. In my experience, this practice does a great job at creating consistent results in applications where a well done bottom crust (such as pizza) aren’t important.
Then again, you can always put the pizza directly on the quarry tiles.