Can You Toast Bread in the Microwave? Ask Paul

No, you can’t toast bread in the microwave for a couple of major reasons.

First, it helps to understand how a conventional toaster cooks. Those glowing bars emit radiant heat in the form of infrared (as well as a bit of visible light). The infrared radiation falls on the surface of the bread where it gets absorbed, turning the surface golden and crisp. Heat slowly makes its way through the slice to the interior, which just warms through and develops a nice chew.

A microwave oven can’t do that kind of outside-in cooking. Instead of infrared radiation, which has a short wavelength, it emits microwave radiation, which consists of much longer waves. The longer waves aren’t stopped by the surface of the bread, so they pass right through into the middle of the slice and cook it.

Instead of a toasted surface and tender middle, microwaved bread starts browning on the interior first, and it happens fast, so you wind up with scorched spots on the inside, while the outside of the slice is still white and raw.

And that’s not the worst part. Microwaving bread or other starch-based foods is notorious for what food science refers to as “objectionable textural changes”: It gets chewy for a brief moment and then tough. Why that happens is only partially understood.

On a molecular level, bread is largely made up of intertwined strands of starch, which hold onto droplets of water. The pliable network of starch and water gives bread its tender texture.

Proper toasting causes water to evaporate out of the starch’s embrace, leaving behind a dry, lacy structure where the water used to be, and that crunches pleasingly when we bite into it.

Microwave heating works primarily by targeting water molecules in the food. For many purposes, this works well, since food contains plenty of water. In bread, though, targeting water disrupts that delicate starch network by causing the water bound to the starch strands to rapidly boil in localized spots scattered throughout the slice, even while the surface of the bread is still cool.

Localized boiling dissolves and then redistributes some of the starch molecules, depositing them in thick microscopic formations throughout the bread. These areas have a firm chew while hot, but quickly turn stiff when they cool to room temperature. Tiny pockets of steam also expand within the bread and then collapse, making the slice denser and harder. Within a minute or less, the bread is tough. According to one study that compared microwave versus conventional heating of bread, a slice that was microwaved for 40 seconds was more than twice as tough as a slice that was conventionally toasted for 5 minutes.

The same objectionable-texture problem plagues other starchy foods you might want to reheat in a microwave: pizzas, tortillas, pasta, rice. Eat them straight out of the microwave while they're hot and the texture will be chewy but acceptable, but let them cool down just a bit and you’ve got a starch brick.

The toaster industry is secure for now.