Most people know not to put metal in the microwave. (Watch the above video, but don't try it yourself.) Even the filigree in an old bowl can spark and ignite a kitchen fire. That's because the metal reflects the microwaves. But what's the deal with the occasional vegetable fire in the microwave? After a beet burn in his family's microwave, scientist Sam Westreich investigated and posted his findings to Sharing Science:
1. Many vegetables, including spinach, kale, dark leafy greens — and beets — are high in iron.
2. Because plants take up iron and other minerals from the soil, these elements tend to be most concentrated in the roots.
Beets are a component of a dark, leafy, iron-rich plant (#1), and we're microwaving the roots (#2).
Additionally, we had a couple other contributing factors: we were microwaving small chunks of beet, which meant lots of little air gaps for sparks to jump, and the beets didn't have a sauce or other liquid to absorb the microwave energy.
And it turns out that you don't need a lot of iron to get a spark in the microwave. Just a small bit of iron acts as a focal point, almost like a lightning rod, leading to charge building up in the surrounding material. A little bit of iron on the edge of a cut piece of beet, and the charge will build up until it arcs — causing char and smoke in the surrounding vegetable as that little bit of iron grows very hot.
