Potato peels contain chemical residues

By Susan Lang

Potato peels harbor significant amounts of chemical residues that may be mildly toxic, adding to the evidence that potato peels could be harmful to people, according to a new Cornell study.

The study shows that chemical sprout inhibitors, which have been shown to have mild toxic effects in animals, remain on the skins in amounts four times greater than government guidelines, said Nell Mondy, professor of nutritional sciences in the College of Human Ecology and an authority on the biochemical and nutritional aspects of fresh and processed potatoes. No human studies on the chemical residues have been reported.

Previous Cornell research, however, has shown that potato peels also contain natural, but highly toxic, chemical substances called glycoalkaloids. "Sprout inhibitors are important chemicals that help prevent shrinkage, blackening, nutrient loss and susceptibility to bruising, and may even reduce the accumulation of some natural toxic chemicals that accompany sprouting," she said.

"Our studies indicate, however, that the peels of dry or moist-cooked potatoes retain up to 20 times more sprout inhibitor residue than the pith (inside) of the potato."

Mondy and graduate student Cyrus Munshi reported their findings at the annual meeting of the Institute of Food Technologists in New Orleans in June. Former graduate student Colleen Wurm is a co-author.

The researchers analyzed the residue of the most widely used and "least offensive" sprout inhibitor, isopropyl N-(3-Chlorophenyl)-carbamate, or CIPC, which is now used on 90-95 percent of stored potatoes in this country. "Only limited data are available on the toxicological effects of CIPC, but animal studies have shown it to have a weak toxic activity," Mondy said. "That raised enough concern for us to look at the residue of CIPC in cooked potatoes."

The nutritional scientists looked at residues in Katahdin and Russet Burbank potatoes that were cooked with both dry heat (baked with and without foil, and microwaved) and moist heat (pressure-cooked and boiled).

They found that in all the methods, the highest levels of residue were found in the peels which contained at least 20 times more of the chemical than the pulp. The peel had more than four times the safety level of 50 ppm (parts per million) designated by the Food and Drug Administration. The pulp, however, was generally below this level. The highest levels in the pulp were found in the boiled potatoes cooked with the peel, apparently due to the migration of CIPC from the peel into the pulp during cooking.

"Peeling potatoes prior to cooking, however, removes most of the problems associated with toxic substances in the peel," said Mondy, who advocates that food processors peel potatoes early as well. "Although most potatoes are baked in the peel, most of the toxicants would remain in the peel and not penetrate to the pith."

That the peel contains the lions' share of nutrients in potatoes is "pure folklore," said Mondy, who points out that the benefit of peeling potatoes far outweighs the loss of fiber and riboflavin in the peel. "In fact, in other studies, we've found that one of the potato's most important vitamins, ascorbic acid (vitamin C), as well as its protein and other nutrients are scattered throughout the potato pulp."

Mondy endorses the peeled potato as one of the most perfect foods with its high quality protein, providing all but one of the essential amino acids, and other important nutrients, including calcium, niacin, several B vitamins, vitamin C, potassium, phosphorus, sulfur, chlorine, magnesium and iron.