I make a lot of kale chips. You might even say I have chipping kale down to an art. But even for a kale connoisseur like me, the crinkly green cruciferous vegetable is still full of surprises. In this case, explosive surprises.
As any kale addict knows, kale chips are deliciously to simple to make. You rub kale in olive oil, sprinkle on some salt and pepper, and cook it in the oven until crispy. But, a few months ago, my oven broke. The Internet assured me that I didn't need to panic: I could still cook my kale in the microwave. So I popped it in, set the timer ... and it exploded.
Well, not exploded, exactly. The veggies started sparking, the power surged, the lights flickered, there was a sort of "whumpf!" and the microwave turned itself off. I opened the door, and smoke drifted out. I stared at the charred (chard?) remains of my kale. Uh, what?
Turns out I'm not alone in my culinary kaleamity. A quick Google search reveals that crackling kale has forced many a chip maker into submission — and it's not just kale. Bell peppers, carrots and green beans can also turn your microwave into a disco ball. But why is this happening?
I did some research and a few very unscientific experiments to find out.
The U.S. Department of Agriculture's microwave safety page says that some foods, like carrots and hot dogs, can spark or "arc" in the microwave. It's the same effect that happens if you accidentally leave a fork in the pasta you're reheating (guilty) or use a plate with metallic edging (also guilty).
But that's a whole fork. Popeye might have taught us that leafy greens are high in iron, but is there really enough metal in veggies to cause a spark? Milt McGiffen, the vegetable crop specialist and plant physiologist at the University of California, Riverside, extension service, was a little dubious at first.
"So," he says, "I heard someone thinks you can set spinach on fire in the microwave?"
After hearing about my kalesplosion and the USDA's blurb, though, McGiffen came around. And he was pretty delighted. "I've never heard of this before — and I study vegetables!"
Plants need iron, he explains, for many of the same reasons we do. It, along with many other metals, plays an important role in how plant cells function. Plants use iron to photosynthesize. They even need it to produce chlorophyll, which is why the leaves of iron-deficient plants start to lose their green pigment and turn yellow.
"As a rule, dark green plants like kale and spinach tend to have more iron," he says.
I didn't have any spinach to microwave, but I did have a lot of kale around. I tried a few different things to see if I could re-create the sparking, and noticed that I only got sparks when I left big chunks of stem attached to the kale leaf.
Did different parts of plants store micronutrients differently? McGiffen wasn't certain about stems, but roots were a different matter. "Plants get iron and metallic micronutrients, like copper and potassium, from the soil. They take it up through their roots." The USDA also called out root veggies as spectacular sparkers.
Sometimes, he explains, plants will even store extra micronutrients in the roots for later, sort of like a squirrel stashing nuts for the winter. (Or like the emergency chocolate shelf in my fridge.) I tried to get some root veggies to go electric — a carrot, a parsnip and a turnip, for the curious — but didn't have any luck.
According to Mark Golkowski, associate professor of electrical engineering at the University of Colorado, Denver, just having metal isn't enough to build up a charge. The shape of the metal matters, too.
"If you just put a big piece of flat, smooth iron in a microwave without any sharp edges, that probably wouldn't have any effect," he explains.
Golkowski is no stranger to electric vegetables. He experienced the sparking first-hand, while heating up some green beans on a plastic plate for his son. There were some sparks, he says, and a little bit of flame. The plate melted.
Golkowski reminisces, "It was very surprising, shocking, and ultimately frustrating, because I was trying to cook my food."
I tried microwaving green beans, but I didn't get any electricity, just some mushy beans.
"The sparking happens because of a local field enhancement," Golkowski says. "A very specific kind of geometry leads to this effect, so you could have one set of beans that does it and one that doesn't."
In a microwave, you have two kinds of fields: electric and magnetic, explains Golkowski. The electric field does the cooking. If a small piece of metal is present in the microwave, it can locally enhance the electric field. He says to think of it like a lightning rod on a house. The rod, because it's both made of a conductive material and has a sharp point, creates a stronger electric field than the air around it.
So basically, you don't need a lot of metal. You just need a difference in the electric properties from one spot to another — say, from a piece of kale with metal to one with no metal, or from a metallic part of kale to the air. The spark needs the air to form, too. "That's why you won't get sparks when you're reheating something in pasta sauce. It's like putting out a fire with a liquid," explains Golkowski.
(I told McGiffen what I had learned from Golkowski. "Whoah!" he said. "Isn't that remarkable?!")
Having lots of pointy edges helps, too. Think back to that smooth bar of iron. Because it's so uniform, there's no difference in electric charge across the bar ... so no sparking. On the other hand, edges in the plant, like the crinkles of kale, can enhance the electric effect.
If you really want to get sparks, says McGiffen, cut up the kale, "leaving sharp edges, and if those sharp edges have concentrations of minerals ... that's your best bet."
It should be noted that Golkowski's idea isn't the only possibility. A few other physicists heard about my investigation, passed it around to their colleagues ("He'll probably get a kick out of this!" one wrote), and emailed me their hypotheses. One physicist pointed out that the salt on kale chips would change the electric potential, too. Most agreed with Golkowski's local field amplification idea.
John Foster, a professor of nuclear engineering and radiological sciences, added that the sparks on the veggies were a sort of "mini-lightning bolt" called a "streamer." It would also reflect energy back into the microwave, which was what caused my power to surge.
Two scientists sent me a video of a grape cut in half in a microwave, with the halves attached by a tiny piece of skin. A ball of plasma — that is, ionized gas — formed above the grape. The two halves, they explained, were acting like antennas or lightning rods and the charge builds up between them.
It looked awesome, but I like my microwave too much to try it.
Ultimately, there's no easy answer to the kale conundrum. Says Golkowski, "Maybe next time somebody's annoyed because they can't cook their food, they can think about all the complicated physics."