Question: I read that a recent Cornell University report revealed that green potatoes could be lethal. I have several questions. Are they literally lethal or do they merely cause illness?
Almost every potato we get at our market is green immediately beneath the skin. Is it safe to eat such potatoes after peeling away the green material or does the presence of green signify that the whole potato is toxic? If one bakes a potato, is there any way to tell if the potato is green?
Answer: Cornell University was reporting on a paper presented at the annual meeting of the Institute of Food Technologists. It covered a study conducted by Cornell graduate student Barry Gosselin and directed by Nell Mondy, professor of nutritional sciences in the New York State College of Human Ecology at Cornell.
According to the Cornell scientists: "Eating large amounts of potatoes cooked without removing the skins may be hazardous to your health because they contain a group of natural chemical compounds toxic to humans.
"Consuming more than 20 milligrams of these compounds--which are called glycoalkaloids--per 100 grams of fresh weight can cause headaches, nausea and diarrhea. And consuming large quantities can even cause death."
In an interview, Mondy noted that "it's difficult to say categorically just how many potatoes someone would have to eat to suffer ill effects, because the amount of this chemical varies from variety to variety. Moreover, the amount also depends on environmental factors such as how much light and heat potatoes receive after harvest."
Glycoalkaloids are concentrated mostly in the peel, the outer layer of potatoes that includes the skin, and cause a bitter flavor. In his study, Gosselin found that about 10% of glycoalkaloids move from the peel to the interior part of the potato when it is boiled or steamed with the skin intact.
Peeling Before Cooking
"Our message, based on our research findings, is that consumers should be aware of the potential danger of the natural toxicant in potatoes and should peel the potatoes before cooking, especially when they are stored for a few months," said Mondy. She recommended removing the peels of potatoes at least 3/16 of an inch deep to eliminate most of the toxicant.
In previous studies, Mondy found that the average glycoalkaloid content in baked potato peels was 20 milligrams per 100 grams of fresh weight--the upper limit considered safe. When fried, however, the content of the chemical more than doubled to 44 milligrams per 100 grams.
"Because the high glycoalkaloid content of fried potato peels could result in possible toxicity, the consumer should be cautious about eating fried potato peels," Mondy said. However, she went on to stress that potatoes are safe to eat if handled properly, and added: "I encourage people to eat potatoes because they are highly nutritious."
Nancy McCarthy, director of consumer relations for the Potato Board, provided us with the following comments from Sorell L. Schwartz Ph.D., professor of pharmacology at Georgetown University, on the above report.
"Toxicity does not imply lethality," Schwartz said. "The mere presence of a pharmacologically active substance does not render a plant poisonous. A poisonous plant is one that contains unsafe levels of the active materials. It is important to note that the human dose of glycoalkaloid for the first signs of toxicity is at the very least about 125 milligrams per 100 pounds of body weight. Under normal conditions, it is unlikely that enough potatoes would be ingested to make someone sick because of the glycoalkaloids."
Prolonged exposure to light causes potatoes to turn green. If the potatoes in your market are consistently green, McCarthy suggests bringing it to the attention of the produce manager. Perhaps they are not handling the potatoes correctly or rotating the stock.
McCarthy added that because the green portion has a bitter flavor, in all likelihood people would not find it palatable enough to eat. If the green portion is peeled away, the rest of the potato is safe. In the case of baked potatoes, any green that might be present will still appear just under the skin after cooking. Once again, if this is cut away the remaining potato is not toxic.