You ask, we answer. That's the heart and soul of WGBH's Curiosity Desk. We recently heard from Evie Frost, a loyal listener in Brookline, who reached out with a question that's been nagging her for years.
"I'd like to inquire why food that is microwaved does not hold the heat the same way things conventionally heated hold heat."
For answers to Evie’s question, I turned to Vayu Maini Rekdal, a PhD candidate in biochemistry at Harvard University.
"I’m also a scientist chef and educator that thinks a lot about and works around cooking and science," he explained.
The first question is whether Evie's premise is sound. After all, isn’t all heat just, well, heat?
"Yes," said Rekdal. "Heat is heat. It doesn’t matter where the source is from."
But while all heat may be the same, all cooking is not. And Rekdal says there are some fundamental differences about cooking in a microwave versus other methods that could be factors here.
"There’s different ways that heat is transmitted," he said. "On a stove, it’s conduction — direct contact between the material and the piece of food."
This is when, for example, a gas flame or an electric coil heats a griddle, and then a griddle heats a grilled cheese sandwich.
"In an oven, it’s called convection," said Rekdal. "The energy is coming, more evenly distributed, from the environment around it as ... heat waves are slowly diffusing into the material."
But in a microwave, tiny waves of electromagnetic radiation are fired at the food. And these microwaves penetrate the food on the hunt for one very specific thing: water.
"The microwaves are essentially exciting the water molecules to make them move around and become warm," explained Rekdal. "So, you are targeting water specifically."
This means that the water content will influence what gets heated to what extent. So let’s imagine a lasagna. In an oven, the heat slowly works its way from the outside in, through all those layers, regardless of amount of water present.
Not so in a microwave.
"If you have a really dry piece of lasagna with not a lot of water in it, it’s not going to really get heated that well in the microwave," said Rekdal.
So, it could be that some microwaved food, while perhaps warm enough, simply never gets as warm as it would in an oven or on the stove. And if it starts out cooler, it will also seem to cool faster.
Crucially, the amount of water will also often vary throughout any given food. In our lasagna example, there could be more water in the sauce than in the noodles. Or maybe the edges are a little dry, but the center is moist.
"And so, when you take that out of the microwave, let’s say it’s really hot in one place and cool in one place," said Rekdal. "All that heat is going to move to the cooler place in the lasagna, and so overall it's going to be cooler.
Rekdal says that another factor could be the vessel you cook in. Metal is great at conducting and holding heat, and it’s a common material in stove-top and oven cooking. But it’s a no-no in the microwave. So, once the food comes out of the oven, if it stays in a metal pot or pan, it will cool more slowly than it would just out of the microwave on a plate or in a plastic container.
Of course, Rekdal is a serious scientist and that means without data, his caveat is that all of these theories are simply his best guesses.
"I don’t think anyone has really done the solid experiments side-by-side," he said. "And I think it really needs to be investigated in more detail for us to really know the nuances."
Our thanks to Brookline's Evie Frost for her question that led to this story. What's yours? Let us know what you are curious about at CuriosityDesk@wgbh.org.