We once declared this to be the best way to reheat leftover pizza. But we've learned some new tricks, thanks to Slice contributor Andrew Janjigian. Here's Andrew with the actual best way, and the science behind slice reheats.
While I'm a big fan of cold pizza, I oftentimes find myself wistfully wanting to relive the glory of the fresh pie. Knowing that it's impossible to roll the clock backward on a pizza—which begins its decline almost immediately after coming out of the oven—I've never given much thought to sorting out whether there was an ideal way to go about reheating cold pizza. I've just assumed that the best you could do was throw it into a hot oven until it's heated through, and resign yourself to the fact that the crust on a reheated slice never approaches the texture of a freshly-baked one. While the oven can restore to a slice some of the crispness and pliability it had when fresh, it does so at the expense of much of the moisture it contained. A reheated slice, no matter how tasty, is inevitably crisper, drier, and firmer than it had been originally, since some percentage of the water it contained has been driven off in the process.
Griddle Me This
But I've recently found a better method, one that minimizes this moisture loss as much as possible. I stumbled upon entirely by accident, when one day I'd decided I didn't want to waste the time to fire up my entire oven to reheat a few measly slices. Lacking a toaster oven—the standard weapon of choice for reheating pizza slices—I decided to try heating up my slices on the (little-used) griddle I have on my stove, placing them underneath the stainless steel cover that is meant to hide it away when not in use.
Doing so, I thought, would heat the slices far more quickly and efficiently than the oven, and the cover would create a shallow oven-like space, allowing them to be heated through completely and not merely from below. But I was so hungry at the time, I decided I couldn't wait even long enough for the griddle to heat up. Instead, I just threw the slices onto the cold griddle, covered it, and turned it on. Since the slices would be sitting so close to the gas burner itself, I did not want to scorch or overcook them, so I set the griddle to its lowest temperature (200°F).
About thirty minutes later, what emerged from beneath the cover was a puff of steam, along with my slices, now completely transformed. Instead of the stiff, slightly warped slices they had been when cold, they were flat once again, and the cheese on them was gooey and glossy. And with one bite, I knew I was onto something. While these slices were still not quite as good as they had been when fresh—they remained a touch more crisp and slightly drier—they had a far fresher texture than I had experienced ever before in a reheated slice. The crust was soft in the interior and crisp on the bottom, the cheese fully melted, and the toppings supple and hot.
The next few times I had leftover pizza with which to experiment, I played around with the method to make sure I understood the technique. I knew that the moisture- and heat-trapping cover was essential, but wasn't sure about the heating method itself. I tried placing the slices onto a preheated griddle, but that only served to over-crisp and dry out the slices, leaving them stiff as planks. As did starting out with a cold griddle set to a higher temperature. In the end, I determined that the three essential elements of the method were starting from cold, covering the griddle, and heating the slices at a temperature below the boiling point of water (212°F).
To make sense of why this method worked so well, it's important to understand what happens to the crust once a slice of pizza begins its inevitable post-bake decline. It doesn't merely turn stiff because it's cold, it actually stales, just like a loaf of bread does over time.
Staling, or retrogradation, is the process by which the starches in the bread release the water they contain into the surrounding spaces and firm up*. So long as the water released by the starches remains in the surrounding gluten, the staling process can be partially reversed by reheating the bread above 140°F, the temperature at which wheat starches absorb water and gelatinize (e.g., become soft and pliable). This is why toasting a partially stale slice of bread can restore softness to its interior.
But if the water released by the starches escapes from the bread, either through gradual evaporation, or rapidly, when it is heated above the boiling point (212°F), the starches will no longer have water to reabsorb, and all hope is lost.
So what this means is that to get the best possible reheated pizza slice, you want to heat it to above the gelatinization point, but below the point that the moisture will be driven off. A slice of pizza placed into a hot oven acts like a slice of bread in a toaster: the exterior (including the cheese and toppings) rapidly heats up, loses water, and hardens. Only the very interior of the slice retains enough water to re-gelatinize. When instead heated gently on a covered griddle, the slice is slowly and uniformly brought up to the gelatinization point, and only a small amount of water is lost to evaporation, mostly at the one place you actually want it to happen: the bottom crust. Reheating the slice in a small, enclosed space further helps to minimize the amount of water that is lost during the process, and the trapped moisture—which escaped in the puff of steam that I saw when I removed the griddle cover—actually helps to soften the cheese and toppings.
So does this mean you are S.O.L. if you don't have a griddle on which to reheat your pizza? Thankfully, no. The method works just as well in a covered skillet, preferably one that has a tight-fitting lid and is as shallow as possible, to minimize the amount of water that is lost as steam. The only limitation to the skillet method is that you can only reheat at most two or three slices at one time.
*Retrogradation happens most rapidly at temperatures just above freezing, which is why you are better off leaving leftover pizza on your countertop and finishing it off before it has a chance to spoil, rather than refrigerating it.