Before we begin, I'd like everyone to get any "that's what she/he said" jokes out of their system regarding the title of this post. All done? Ready to go on? Good.
Don't you just hate when this happens to the bottom of your pizza? I mean, sure, it's better than the dreaded soggy bottom,* but this guy is so charred that even Dom DeMarco might have a problem serving it.
*I've had a fear of soggy bottoms ever since that incident in grade school.
So what gives?
The problem all comes down to the relative difference between how the top of the pie cooks and how the bottom does. Unless you want to futz around with skillets and broilers (a noble futz, to be sure, but not one you want to deal with all the time), the goal when baking a pizza is to get the top and the bottom to both cook A) fast, and B) at the same rate. Your undercarriage should come out burnished and lightly charred in spots just as your pizza top achieves a spotty golden brown crispness.
In his recipe for thin crust pizza from Cook's Illustrated, Andrew Janjigian takes the novel approach of placing the stone on the top rack of the oven. This is totally contradictory to what most pizza authorities recommend: putting the stone on the bottom rack (or even the floor of the oven) in order to maximize the amount of heat it absorbs.
For NY-style pizzas, I've recommended using the middle rack, but I may well switch over to the top now that I've tried Andrew's method with success. This week I decided to explore a bit of the thermodynamics of pizza stone placement. No recipe, but I hope you find it interesting nonetheless.
An excerpt from my lab journal:
- 6 batches pizza dough. Use NY style (Neap. won't work - don't want spotting, more even browning desired.
- One oven. Hot Point brand, 500°F maximum temp on temp gauge. Previous tests have showed 560°F is possible on broiler setting.
- One pizza stone. Wililams-Sonoma brand, well-used.
- Wooden pizza peel.
- 1 exceedingly sexy instant-read infrared thermometer.
- 1 belt-mounted holster for exceedingly sexy instant-read infrared thermometer.
- Adjustable oven rack.
- At least three mouths for oral assay of samples. (mine, Irving the doorman, Dumpling)
Method: 6 pizzas will be cooked successively on a pizza stone preheated for at least 45 minutes. The oven will be set to the "broiler" setting to maximize temperature (air temperature of ~560°F is the target). Each pizza will be cooked with the stone on a different rack: top, upper, middle, lower, bottom, and one with the stone directly on the oven floor. Each pizza will be cooked for exactly 5 minutes.
In order to make sure that any temperature differences in the way the stone preheats in various rack settings are accounted for, the stone will be allowed to rest at room temperature in between tests for 30 minutes before being placed back in the oven for preheating. Total time for each test: 1 hour 20 minutes. Total time for all testing 7 hours 30 minutes (1'20" x 6 - 30"(no resting for the first pie)).
Since 9pm is the usual time for kitchen testing to commence, it's gonna be a late one. Note to self: Look into slipping sleeping pill's into wife and dogs food to avoid late night post-baking confrontations.
During each test, the temperature of the stone will be taken by whipping out the exceedingly sexy instant-read infrared thermometer, pointing the laser sight at the center of the stone, and making a "piu piu piu" noise. The temperature of the top of each pizza will be taken after it has finished baking using the same method.
Data: There were some very interesting results indeed. First off, a look at the bottoms of the pies:
It's clear that the pizza baked on the very bottom of the oven severely overcooked within its 5 minute baking time. What's interesting is that the gradation between the pizza on the very top and the pizza in the middle is not so severe—indeed, they've achieved a pretty close to identical amount of browning. Now here are the tops of the same pies:
It's no surprise that the pizza cooked on the top shelf browned better on top, but again, what's interesting to me is that pizza cooked in the middle and on the bottom browned nearly as well—this time it's the top rack pizza that's the outlier. Why is this?
On Convection and Radiation
Before we get to the pretty little diagrams, let's talk briefly about heat, what it really means, and how it gets transferred from one place to another.
The biggest misconception that a lot of people have is the correlation between heat and temperature.
Heat is energy that is transferred from one body to another. So when you place a steak in a pan in order to cook it, what you are really doing is transferring energy from the pan-burner system to the steak system. This added energy partially goes to raising the temperature of the steak, but much of it also gets used up for other reactions: it takes energy to make moisture evaporate. The chemical reactions that take place that cause browning require enrgy.
"different bodies take different amounts of energy to change their temperatures by the same number of degrees."
Temperature is a system of measurement which allows us to quantify how much energy is in a given body based on its thermal characteristics, most importantly, its heat capacity. Depending on its density and the materials it's made out of, different bodies take different amounts of energy to change their temperatures by the same number of degrees. For instance, to raise the temperature of 1 gram of water by 1°C, it takes 1 calorie of energy,* while the same amount of energy will raise the temperature of a gram of iron by almost 10 times as much and a gram of air by only half as much.
*for point of reference, the calories that you count in your food are the equivalent of 1,000 real calories, which means that for every 2 calorie Tic Tac you knock back, you've just consumed enough energy to bring 20 grams of water from 0° to a boil!
Conversely, this means that given the same mass and temperature, water will contain about 10 times as much energy as iron and air about half as much. Not only that, but remember that air is far less dense than water, which means that the amount of heat energy contained in a given volume of air at a given temperature will be only a small fraction of the amount of energy contained in the same volume of water at the same temperature. That's the reason why you'll get a bad burn by sticking your hand into a pot of 212°F boiling water but can stick your whole arm into a 212°F oven without a second thought.
Three Types of Heat Transfer
So let's say that your rack is in a fairly average position: right in the middle. How does food actually cook on there?
This diagram represents a head-on view of your oven with a pizza stone sitting on a rack. The blue arrows represent air currents in the oven, while the red arrows represent radiation.
Heat energy gets transferred to food via three different methods:
Conduction is the direct transfer of energy from one solid body to another. In pizzas, this occurs on the spots where the dough is in direct contact with the stone (or oven floor). Although it may seem like the entire bottom surface of a pizza is in direct contact with the stone, in fact, as soon as you place it down, bubbles of steam and air rapidly form underneath the surface, elevating it. The dough actually only touches the stone in a few spots. Since conduction is an extremely efficient form of heat transfer, those spots of contact cook rapidly and eventually develop into the nice charred spots you get on a well-cooked pie.
Convection is the transfer of one solid body to another through the intermediary of a fluid (that is, either a liquid or a gas). In this diagram, convection currents are represented by the blue lines. Air near the heating element in the base of the oven is heated up. As it heat, colder air from the top of the oven forces the hot air up creating a current. The bulk of this current is blocked by the pizza stone set in the middle, so the air is forced around the edges where it flows up, hits the roof of the oven, meets again in the center, then flows down and over the surface of the pizza.
"the faster the air is traveling over a given surface, the more energy it can transfer"
As a general rule, the faster the air is traveling over a given surface, the more energy it can transfer. Still air will rapidly give up its energy, but with moving air, the energy supply is constantly being replenished by new air being cycled over the food. Convection ovens have fans in them that are designed to keep the air moving around the space at a good clip in order to promote faster, more even cooking.
Radiation is transfer of energy directly through space via electromagnetic waves. It doesn't require any medium to transfer it—the sun's energy reaches the earth via electromagnetic waves directly through the vacuum of space.
In an oven, the primary source of radiant heat is the metal floor, which absorbs energy from the heating element (or burner), then re-emits it as electromagnetic energy. In a properly pre-heated oven, the sides and roof will have absorbed enough heat that they too will be secondary sources of radiant heat. An important fact to remember about radiant energy is that it decays by the inverse square law—the energy that reaches an object from a radiant energy source is proportional to the inverse of the square of its distance.
So, for example, hold your hand 1 foot away from a fire then move it away to 2 feet. Even though you've only doubled the distance, the fire will feel only 1/4 as hot. Though the reflective and absorptive sides of an oven will mitigate this effect to some degree, it sill holds true—he further away your food is from the base of the oven, the less radiant energy reach its underside.
Check out what happens when you move the pizza stone to the top shelf:
"hot air transfers more energy than still hot air, even at the same temperature"
As you can see, with so much distance from the base of the oven, relatively little radiant energy reaches the stone. At the top of the oven, the stone only reaches around 560°F after a full 45 minutes of preheating. By contrast, it gets closer to 580°F when it's at the center of the oven. On the other hand, there is a great deal of radiant energy coming from the top surface of the oven, and even more importantly, because of the small space between the pie and the roof of the oven, convection currents are moving quite fast. This means that even though the temperature of the top surface of the stone when placed on the top rack is lower than the temperature of the top surface of the stone when placed on the middle rack, you actually end up transferring far more energy to the food being cooked via convection (remember—fast moving hot air transfers more energy than still hot air, even at the same temperature).
That explains why the top surface of the pizza cooked at the very top of the oven browned so much better than the ones cooked below it.
And what about the bottom shelf?
When heated near the floor of the oven, the stone gets an extraordinarily intense blast of radiant heat from the bottom (remember the inverse-square law!). After preheating, it comes to nearly 680°F, even though the ambient temperature of the air in the oven is only at around 560°F. That's the power of radiation! In addition, with so much room for air to circulate above it, there is far less transfer of energy via convection on the top surface of the pie. What this means is that with a pie cooked on the bottom of the oven, your base is going to burn long before the top even gets a chance to brown.
The most interesting part to me is that when baking in an oven with a stone, it's only the extreme-most positions that really make a difference in how things cook. This graph represents the temperature of the stone and the final temperature of the top of each pizza after 5 minutes of baking (a pretty good indicator of how fast things will cook).
With the stone temperature, placing it directly on the floor of the oven causes a sudden 80°F jump in its temperature, while anything above the lower rack is within a 20°F. Conversely, when you examine the effects of convection on the top surfaces, it's only when you place the stone on the very top shelf of the oven that you see a sudden jump in how fast the top surface cooks.
So I've got to admit: I have an ulterior motive with this post. I'm doing some demos and talking to a bunch of (very smart) kids at my old high school next week about...heat! I apologize if this post seemed overtly lecture-like. I'm honestly not a lecturer, I promise! And if you disagree, well then I'm just gonna have to sit you down for a little talk.
Hopefully there's some useful information buried in all this that can extend well-beyond the world of pizza. The most important being that the position of your oven rack can have a great bearing on the your final results. Have you always wondered why the bottoms of your pies seem to burn before the top turns golden brown? All you've got to do is bake them on a higher rack. Is the crisp crust of your mac & cheese turning black while the bottom of the casserole is still ice cold? You're baking it too high!
It also, of course, explains why Andrew's all-the-way-on-the-top method works better than my in-the-middle method. At the very top of the oven, the base of the pizza cooks almost as quickly as it does in the middle, but the top cooks much faster, resulting in better oven-spring, hole structure, and more browning.
There's no recipe here, but I promise I'll make up for it: Spinach, Provolone, and Pepperoni Calzone recipe coming soon!