Toxicity of Potato Skins Becomes a Hot Issue : Natural Chemicals in Peels Can Pose Problems If Eaten in Huge Quantities

Times Staff Writer

“Potato Skins Contain Natural Chemicals Toxic to Humans, Cornell Study Says,” read the headline on the university news service release that was picked up by the media coast to coast.

Hey, what’s happening here?

It was just a simple research project measuring the effect of boiling on chemical compounds known as glycoalkaloids and phenols naturally present in the cortex region of potatoes. But when Cornell University graduate student Barry Gosselin reported that toxic chemical compounds found naturally in potatoes may be hazardous to health if eaten in large amounts, the press, it seems, read more in the spud report than the researchers intended. Gosselin authored the Cornell University research project directed by expert potato biochemist Nell Mondy, and reported the findings at a meeting of the Institute of Food Technologists in Las Vegas last Thursday.

The study found that consuming large amounts (more than 20 milligrams per 100 grams of fresh weight) of a group of the chemical compounds naturally present in potatoes could cause headaches, nausea and diarrhea. What the study did not state was the exact amount of potatoes one would need to eat to make a person sick.


When pressed, Gosselin estimated that consuming 10 to 50 potatoes at one sitting would do it. Mondy had noted in an interview reported in the Cornell University release that the variety of the potato, how much light and heat potatoes receive after harvest, weight and size of the potato and body weight of the individual eating the potato made it difficult to determine “categorically” just how many potatoes someone would have to eat in order to suffer ill effects.

“We were simply looking at the effect of boiling on compounds present in potatoes. Some work had been done on baking and frying, but not yet on boiling. Our basic message was to make people aware that toxic compounds exist in certain amounts and that they should peel potatoes before boiling. We had hoped that the press would take it as caution, not an alarm. We’re telling people to tone it down,” said Gosselin when reached by telephone.

Effects of the report’s budding impact on the public brought a quiver or two in the voices of the potato industry persons contacted for comments.

“The story hasn’t affected the potato industry yet, but it could--not that we expect it to,” said Bob Mercer, president of the Potato Board, the promotional arm of the potato industry.

Mercer said that skins of potatoes are no worse or better than several other plant foods in their toxicity potential. “It can become worse when potatoes turn green from exposure to light. Direct light causes chlorophyll buildup. The tuber after all is a modified stem so when light enters, it turns green and builds chlorophyll, a bitter-tasting substance. There is a relationship between light and chlorophyll and it does affect the increase of alkaloid, which is poisonous. But you would have to have quite a buildup to have a negative effect. The bitterness alone will cause anyone--even cows--to spit it out,” Mercer said.

“It makes me very sensitive when the press hits on something as basic as potatoes. That’s the bread and butter for farmers here. It’s our livelihood that’s affected,” said Kathy Johnson, promotion director of the Red River Valley Potato Growers Assn. in East Grand Forks, Me., where 58% of the crop goes into making potato chips, many of them with skins.


Flinched at the News

Even nutritionists, such as registered dietitian Rita Storey, the media spokeswoman for the American Dietetic Assn., flinched at the news. “Oh dear, this news really flies in the face of what we’ve been telling people to do--to eat the skin because it is an excellent source of fiber, complex carbohydrates, vitamin and minerals, not to mention the increased concentration of Vitamin C near the skin.”

Agricultural Extension (UC Davis) vegetable specialist Vincent Rubatzky Ph.D. in the vegetable crops department explained that toxicity may affect people differently. “Some people may get a headache without realizing where it stems from after eating only one bad potato. For others it would take dozens to make them sick. It’s a personal characteristic,” he said.

According to Gosselin, some deaths due to toxic potato consumption were reported in Germany during the ‘20s and ‘30s when victims were said to have eaten heavily sprouted potatoes. Potato sprouts contain high levels of glycoalkaloids. Suffering due to green potato consumption by German soldiers also was reported during World War II.

What’s a body to do?

“I would not recommend peeling potatoes, unless they’re spoiled or green in appearance,” Storey said. “Just wash them vigorously under running water and use them in moderation. The nutritional benefits of potatoes eaten in moderation--say one to three a day--far outweighs the potential risk when identified with as many potatoes as 10 to 50. It goes back to the old adage that Aristotle gave us many years ago: moderation in all things.” After all, pointed out Storey, the presence of toxic elements in potatoes and many other plant foods has been well documented for centuries. “It’s a question of amounts. Mushrooms, rhubarb, spinach and dozens of other plant foods contain elements that are toxic to humans when taken in extremely large amounts,” she said.

Mondy herself stressed that potatoes are safe to eat if handled properly. “I encourage people to eat potatoes because they are highly nutritious. In fact, a diet of only milk and potatoes can supply the human body with all the nutrients it needs.”

According to Gosselin, the motivation for the study was to round out the experiments studying the effect of cooking on the compounds present in potatoes and to call attention to possible hazards of eating too many potato skins now that skins have gained popularity as a snack item in recent years.


“People think the skin is the nutritious part of the potatoes. Glycoalkaloids are a natural constituent of the potatoes. And it doesn’t have to turn green to have glycoalkaloids. They can still synthesize glycoalkaloids without exposure to light,” Gosselin said.

Glycoalkaloids can range from two to 30 milligrams per 100 grams in a single potato. Amounts, however, double and triple when potatoes turn green, usually upon exposure to light.

Gosselin found that 10% of the glycoalkaloids penetrate into the cortex region of the potato, the area between the outside and vascular ring (the dark circle near the skin), but no further.

Gosselin used three cooking methods in his study--boiling in distilled water, steaming and boiling in a 16% salt solution as used for snack potatoes. In all three methods, peeled potatoes had a lower glycoalkaloid and phenolic content, discolored less and were less bitter than unpeeled potatoes.

In previous studies, researchers 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.

According to Mondy’s report, the high glycoalkaloid content of fried potato peels could cause possible toxicity. “These findings are important because fried potato peels have become a popular snack.”


However, if you peel the potato, the level of compound is much reduced. “You can get rid of about 90% of the glycoalkaloid,” Gosselin said.

Potatoes that have turned green should be peeled at least two to three millimeters, about three-sixteenths of an inch deep to eliminate most of the toxicant.

Storing them in an opaque bag or in a cool, dry, dark place is recommended.

Potato sprouts contain high levels of glycoalkaloids. “They measure way off the roof,” Gosselin said, and they should be avoided. Glycoalkaloid levels in the eyes of potatoes are not high, but if sprouting occurs in that area, concentrations of the chemical will be high, according to Gosselin. He recommends removing eyes of potatoes before using for safety’s sake.

According to Gosselin, supermarkets should store potatoes in opaque bags or on the lower shelves and not expose them to light, which will often turn potatoes green, thus raising their toxicity levels.

Red River Valley Potato Growers representative Kathy Johnson heartily concurs. “The biggest enemy of the potato is the produce manager who won’t rotate potatoes. Any potato will turn green when exposed to fluorescent lighting. We advise that potatoes be rotated or stored away from direct light,” Johnson said.