When you microwave pizza to reheat it, you run the risk of turning the best food in the world into a chew toy fit only for Fido. Microwaving melts the cheese and reheats the toppings, but toughens the crust. But, it doesn’t have to be that way! You can nuke your leftover pie and enjoy the result if you put a mug of water in the microwave with the pizza.
The Microwave Hack
Ready to reheat that pizza? Here’s what you do:
- Place a slice or two of pizza on a plate.
- Place a (microwave-safe) mug of tap water in the microwave right next to the plate. Corelle or Pyrex works best. Don’t use stoneware because it gets crazy hot. You can use a glass of water, but let it cool before retrieving it.
- Heat the pizza on high power about 45 seconds until it’s hot.
Why Microwaving Pizza With Water Works
It seems counter-intuitive to heat pizza with water to prevent the crust from getting soggy. So how does it work? The answer has to do with the interaction between microwaves and matter.
First, heating a glass of water for 30-45 seconds in the microwave isn’t long enough to bring the water to a boil. So, you won’t fill the microwave with water vapor, which could ruin the crust.
The water works by acting as a “microwave sink” or a “dummy load.” While the technique may be new to you, chemists and physicists have known about it for a while. Heating chemicals in a microwave is different from heating them on a hot plate!
If you heat pizza sans water, the pizza absorbs all the microwaves. Water in the sauce and toppings quickly vaporizes and soggifies the crust. So, you can get a nice crust with unmelted cheese and cold toppings or a tough crust with hot toppings. Gross. Using a dummy load absorbs some of the microwaves and moderates the reaction, allowing the water and oils to heat at a more uniform rate. Basically, the cheese re-melts and the toppings and crust get hot all about the same time.
The Importance of the Dielectric Constant
The more technical explanation has to do with the dielectric constants of water (in toppings and sauce) and oil (in cheese and some crusts). The magnetron of the microwave oven takes electricity and converts it into microwave radiation. Microwaves pass through a waveguide into the “oven” part of the appliance, but at a preset frequency (typically 2.45 GHz). The energy output varies depending on the appliance, so the calories of heat delivered depend on your oven. Most microwaves have rotating tables to increase the interaction between the beam of microwaves and your food. Chemists add a dummy load when heating samples to protect the appliance from reflected radiation (basically so the microwave isn’t running “empty,” which can cause damage). Usually, this is a glass flask of water or a beaker of water topped with a watch glass. The neck of the flask or watch glass condenses water so the oven doesn’t fill with vapor. Glass is chosen because it doesn’t absorb microwaves.
This is where the dielectric constant comes into play. Microwaves heat by changing electromagnetic energy into heat. Microwaves are electromagnetic waves that cause dipoles to rotate. The dielectric constant of a material reflects how efficiently microwaves produce heat. Water has a high dielectric constant; oil has a low constant. The set frequency of a commercial microwave oven is designed to maximize heating of water-rich foods. In other words, a microwave oven is meant to heat water. Hydrocarbons (like the oils in cheese and crust) don’t absorb microwaves, but they are heated indirectly by water. To heat your pizza, you want to slowly heat the water. Slow heating keeps the crust dry while giving the cheese and meaty toppings a chance to heat.
The Best Way to Reheat Pizza
If you’re fortunate enough to have leftovers, the best way to reheat pizza is in the oven. Preheat the oven to 350 °F, place the pizza on a cookie sheet (or pizza pan, if you have one), and heat it until it’s hot. Note, this is a lower temperature than you’d use to cook a frozen pizza. This is because the crust is already cooked. You want to revive the pizza, not char it.
Of course, none of this matters if you’re the type who relishes cold leftovers. Enjoy!
- Adamo Fini and Alberto Breccia. Chemistry by microwaves. Pure Appl. Chem., Vol. 71, No. 4, pp. 573–579, 1999.
- A. Fini. Microwave and High Frequency Heating Principles and Chemical Application (A. Breecia, A. C. Metaxas, eds), pp. 69–83. UCISCRM, Bologna, Italy (1997).