Why Do Potatoes Turn Green? And Can You Still Eat Them?

On the evening of September 9, 1969, the students of a South East London day school for boys began feeling ill.

At first the boys complained of nausea and stomach pain. Perhaps something had been off with their lunch—could it have been the steak pie with gravy or the tinned carrots? Maybe it was the custard that was served with the syrup sponge pudding? Soon, however, their symptoms escalated dramatically: One 11-year-old complained that he could no longer see, began talking gibberish, and fell into a stupor. Meanwhile, a 12-year-old boy started rambling incoherently after his skin turned ashen. Two other boys, 13 and 11, began hallucinating and fainting; others developed nosebleeds, muscle spasms, and sores on their bodies. Three of them fell into shock, and one of them became comatose. All of the affected boys experienced spectacular gastrointestinal distress.

In the end, 78 boys became sick, with 17 of them requiring hospital admission. All of them survived after many days of this mystery illness—but for some, symptoms lingered for more than a week.

When investigators were dispatched to the school to determine a cause of the outbreak, they soon found their culprit: old potatoes.

Native to the Americas, potatoes have been cultivated by human beings for thousands of years, and they are now one of the world’s staple crops. In 2020, the United States alone produced 46 billion pounds of potatoes from 914,000 acres of farmland (roughly the size of Rhode Island).

Potatoes, a.k.a. Solanum tuberosum, are members of the vast family of nightshades, which includes tomatoes, peppers, eggplant, and tobacco. It also includes a variety of poisonous plants, including henbane (which some scholars believe inspired the poison that killed Hamlet’s father), belladonna, datura, and mandrake. Like many plants, nightshades also generate defensive compounds to discourage predators, including nicotine (which is toxic to insects), capsaicin (responsible for delivering a chile pepper’s burning heat), and some medically useful tropane alkaloids, including the anticholinergic drugs atropine, hyoscyamine, and scopolamine.

The main defensive agents in potatoes—the glycoalkaloids solanine and chaconine—aren’t usually quite as potent as those of their cousins in, say, datura and belladonna (a.k.a. deadly nightshade, whose hallucinatory qualities may have been responsible for the myth of witches flying through the air on broomsticks during the Middle Ages). In fact, young potato tubers contain little of these two chemicals, which are found in the highest concentration in its shoots and leaves, which should never be eaten.

But when a potato matures or is treated poorly, the story gets a little more complicated.

Surely you’ve seen a bag of new potatoes at a grocery store that doesn’t look all that “new.” Perhaps they’ve started developing eyes (which are just buds waiting to sprout) and a greenish cast to their skin. When I was younger and more naive, I assumed that, like green tomatoes and bananas, green potatoes had simply been harvested too early, and that they would eventually ripen like a fruit.

Not so! The greenish tint is actually a sign that the potatoes have been exposed to too much light—the green is our good friend chlorophyll, which is nontoxic and potentially beneficial for human health. But that friendly greenness signals the arrival of solanine and chaconine, which tend to develop in conjunction with chlorophyll if the potato is exposed to environmental stress, including improper storage conditions or injury. The skin and eyes (and sprouts, if the potatoes are old enough) contain the highest concentration of the alkaloids, although the investigators in the schoolboys’ case also found significant levels of solanine and chaconine in the flesh just beneath the skin as well.

Harold McGee writes in On Food and Cooking: “Most commercial varieties contain 2 to 15 milligrams of solanine and chaconine per quarter-pound (100 grams) of potato.” For comparison, investigators at the London day school found that the potatoes in question contained 47.7 milligrams of alkaloid in their peels alone—plenty to make the boys sick.

Chaconine is more toxic than solanine, but researchers believe that it works synergistically with solanine to produce what is often referred to as solanine poisoning: Symptoms include hypothermia, headache, slow pulse, abdominal pain, vomiting, blurred vision, shock, and, in extreme cases, even death. The onset of symptoms is typically around 2 to 24 hours—which is why some of the schoolboys didn’t feel sick from their lunch until later that evening. (If you suspect solanine poisoning, Mount Sinai warns against attempts to treat symptoms at home. Instead, it advises people to call a poison control hotline or head to the emergency room immediately.)

Current statistics for solanine poisoning are hard to come by, but bad potatoes have certainly been responsible for large outbreaks in the not-so-distant past. As food processing and handling practices have improved, however, and as consumers have become aware of the potential for poisoned potatoes, those large outbreaks have become more rare. But individual poisonings still pop up regularly, such as the case of a Colorado woman who ended up in the ER after eating a bad batch of mashed potatoes in 2020.

The USDA says that you don’t need to discard green potatoes, but you should prepare them properly: “Peel the skins, shoots, and any green color; that is where the solanines concentrate.” Be a little aggressive if the flesh is still green beneath the peel and remove all traces of it.

Keep in mind that, unlike bacteria such as E. coli and salmonella, solanine and chaconine are not destroyed by baking, frying, or boiling. (UC Davis’s Postharvest Technology Center notes: “Glycoalkaloids are heat stable and minimally impacted by cooking.”) Boiling toxic potatoes might cause them to lose some of their alkaloids to diffusion in the pot of water, but it’s wise not to take chances here.

Incidentally, solanine is quite bitter, so even if you don’t see any outward signs of solanine buildup in your potatoes, you will almost certainly taste it if it’s present in significant quantities. McGee notes that the subclinical levels of these alkaloids in normal potatoes “is part of their true flavor” but “strongly bitter potatoes should not be eaten.”

Potatoes are a root cellar vegetable: That means they’re best stored in a cool, dark environment. McGee adds that potatoes “can be stored in the dark for months, during which their flavor intensifies; slow enzyme action generates fatty, fruity, and flowery notes from cell-membrane lipids.”

Temperature-wise, aim for 45º to 50º Fahrenheit. At that temperature, UC Davis advises, “potatoes should have good quality after storage of 3 to 5 weeks,” though “new” or immature potatoes should typically be eaten within 3 weeks. Storing them in colder environments could lead to a buildup of sugars that make them brown excessively during frying.

Of course, it’s always hard to know exactly how long the potatoes you buy in a grocery store have been off the vine; you might have better luck buying directly from a farmer at a local market. And unless you feel like hallucinating while hacking up your insides, keep an eye out for that dreaded greenish tint—and when in doubt, toss ’em out.