What you see: Greenish skin on a potato
What it is: Chlorophyll buildup that signals the presence of solanine, a harmful toxin.
Eat or toss? Is it just a little green? If so, cut or peel away the green area, and eat! But if your potato is starting to look like an emerald, then it’s time to toss.
It’s common knowledge that green potatoes are scary things, but just how scary? And how much anti-green action should a reasonable potato eater take? And how is it that my uncle, who once plunged his hand into a bag of potato chips and defiantly told me that "the green ones" were his favorite, is still alive and I'm pretty sure still eating green potato chips?
Why they're green
The answer lies in the neighborhood around the green potato skin and flesh. That green is harmless chlorophyll, which the potato happily created when it was exposed to natural or artificial light. It’s the kind of thing that might happen in the farmer’s field if there was a crack in the soil, in the grocery store under those bright florescent lights or even in your kitchen if you’ve left them on the counter uncovered (rule: keep your potatoes in the dark as much as possible).
A toxin lurks
The chlorophyll won’t hurt you, but it does indicate that something more sinister is happening inside the tuber. When chlorophyll builds up, so does a substance called solanine. Solanine doesn't need light to develop, but it forms much faster in a bright environment. The potato makes solanine to ward off predators—predators like us hungry humans, I suppose. And it makes sense that this happens when the potato is exposed to light because a potato on the surface of the soil is going to be more visible to potato-eating creatures.
Anyway, solanine, which also accumulates in those sprouting potato “eyes,” can cause some nasty things, including severe gastrointestinal distress, along with vomiting and diarrhea, and even death. Solanine affects the nervous system and can mess with the body's ability to regulate a chemical involved in nerve impulses. Still, a little bit of solanine isn’t going to hurt you—it usually doesn’t make its way into the bloodstream, it is often converted into a less harmful substance by the intestine, and it tends to be excreted pretty quickly. In fact, any time you eat any potato, you're eating trace amounts of solanine. It even contributes to how the potato tastes.
But you need to eat a lot to get sick
While solanine is present in trace amounts in normal-looking potatoes, a 200-pound person would need to eat 20 pounds of not-green potatoes in a single day to reach toxic levels, according a report published by the University of Nebraska - Lincoln Extension. If, writes author Alexander Pavlista, those potatoes had been exposed to light, which can easily increase solanine concentrations by 10, that same person could suffer to some degree after eating two pounds of potatoes. For perspective, a large baked potato can weigh about a pound.
You'll probably taste it before it harms you
Here's a bit of good news — high concentrations of solanine taste so bad that you'll likely notice the bitter flavor before you've consumed enough to make you sick.
Nora Olsen, potato specialist for the University of Idaho, has studied solanine concentration in potatoes, but when she encounters a potato with a bit of green in her kitchen, her first worry is flavor, not sickening people.
“They’d have to be really green to get at levels that are going to cause you issues,” she said.
Olsen said that if a potato is slightly green in one area, she'll just cut that part off. If it's green all over, however, she'll toss it. Worth keeping in mind—solanine tends to concentrate in the peels, so that's an area where you should be especially cautious.
Also important: while cooked potatoes will stop producing solanine, cooking will not eliminate existing solanine.
Potatoes sold in the US are typically low in solanine
One more bit of good news: When it comes to solanine, Olsen says that the potato industry has your back. The potatoes that are bred and distributed are typically selected for their tendency to produce low amounts of solanine.
"There's work in the background going on," she said.
Nora Olsen - Potato Specialist for the University of Idaho
A Review of Important Facts about Potato Glycoalkaloids by Marita Cantwell. Perishables Handling Newsletter.
G1437 Green Potatoes: The Problem and the Solution. Alexander D. Pavlista. University of Nebraska at Lincoln. Jan. 1, 2001.
Idaho Potato Commission. FAQ
Tater Taught: Are Green Potatoes Poisonous? Snopes.com
Healthy Foods that Can Kill You. Cooking Light.
Horrific Tales of Potatoes that Caused Mass Sickness and Even Death. Smithsonian.com
FDA Poisonous Plant Database