Probing Question: How do microwaves cook food?

Photo by Emily Rowlands

By Tom Fitzgerald
Research/Penn State

Think about it -- most people don't go through an entire day without using a microwave oven. But how does it work? What actually happens when a person "nukes" yesterday's pizza or pops a bag of popcorn in the microwave? And does the term "nuking" mean there's really radiation inside that box?

"Although technically microwaves are a kind of electromagnetic radiation, we shouldn't use the word 'radiation' to describe them -- it scares a lot of people unnecessarily," said Swamy Anantheswaran, Penn State professor of food science. "Actually, microwave ovens operate in the electromagnetic spectrum at about the same frequency as a lot of telephones -- 2450 megahertz. But they have more power output as compared to cell phones. That's why the FCC tightly regulates their manufacture. They want to make sure they're built well enough to contain all of the radio emissions."

So, microwaves are just high-frequency electromagnetic waves, and microwave ovens create a high-intensity electromagnetic field to cook food. But what are those waves really doing in there?

Microwaves do most of their work on the water in food, Anantheswaran said. "Water molecules constitute what are known as 'dipoles,'" he explained. "A dipole is sort of like a bar magnet, with a positive pole and a negative pole. The oven's electromagnetic field oscillates as it passes through the water molecules in the food, changing the polarity of the field and causing the dipole/water molecules to flip themselves in order to be aligned with the new polarity." Heat is created by the resulting friction of the water molecules reversing direction millions of times a second.

Conventional ovens rely on conduction to slowly spread the heat from the outside of the food to the inside; by the time the inside is cooked, the outside may be over-cooked. In microwave cooking, the energy reaches everywhere almost at once, resulting in more-or-less even heating. "It's like each water molecule is a little heater heating the food," Anantheswaran said.

While protein, fat and starch molecules also absorb microwave energy to a lesser degree, the presence of salt in foods can have a large influence on microwave heating. "Salt molecules tend to break apart in the presence of water," noted Anantheswaran. The sodium and chlorine ions create heat by colliding in the rapidly oscillating electromagnetic field, leaving less microwave energy available to heat the center of the food.

"When you place a refrigerated bowl of soup in the microwave," he explained, "the soup near the outside encounters the microwave energy first and very rapidly heats. It even appears to be boiling. So you take the soup out -- but then discover the central portion is still cold. This is because the salt ions absorb so much energy around the outside of the bowl that they produce localized boiling before the rest of the soup absorbs enough energy."

Microwave ovens can be great time-savers, Anantheswaran said. "Total cooking time can be much less, allowing for greater retention of nutrients. However, most consumers manage to overcook their food by leaving it in the microwave too long or by using too high of a power setting."

The solution is simple, he said, and as important for safety as for good eating.

"Use the appropriate power setting for the food you're microwaving," said Anantheswaran. "And make sure you read the manual."

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Ramaswamy C. Anantheswaran is professor of food science and graduate program coordinator. He can be reached at rca3@psu.edu

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