That 5-pound sack of Yukon Golds looked fine in the store, but now that they’re home, you notice a couple of them are green at one end. Wait—several of them are. Should they go straight into the compost?
Why Do Potatoes Turn Green?
Let’s start with a review of how potatoes grow. Botanically, a potato is part of the stem of the plant, although it’s evolved its thick, bulbous shape in order to store lots of starch. Although a potato spends most of its life underground, it, like other stems, retains the ability to turn light into energy. When a potato is exposed to light during storage, it starts to form chlorophyll—the green molecule that plants use to harvest sunlight—in and under its skin.
The resulting greenness isn’t intrinsically a problem, but at the same time, exposure to light stimulates the potato to form another, non-pigmented molecule, called solanine. Solanine, which defends the potato from pests in the field, is quite poisonous to humans. (Other plants in the potato family, such as tobacco and deadly nightshade, produce their own potent chemical defenses for similar reasons.) It’s fortunate for us that the visible green pigment forms at the same time as the invisible toxin, as a warning signal.
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Can You Eat Green Potatoes?
Solanine poisoning causes gastrointestinal upset, headache, and dizziness, and in extreme cases can be fatal. Although you can tell from the color whether a potato has formed solanine, you can’t tell how much has formed. A shorter exposure to light, or exposure to certain types of light, may result in only a small amount of the toxin; longer exposures to ultraviolet light sources such as direct sun can form a lot.
The good news is that the toxic layer typically extends no more than an eighth of an inch below the skin, so peeling off the green portion, and removing any eyes and shoots, will make a potato safe to eat.
How Can Potato Greening Be Prevented?
Check potatoes while shopping, and don’t buy the green-tinged ones. After you get them home, even dim light can start the greening response, so store your potatoes in a dark cupboard, dry root cellar, or lidded cardboard box. Avoid an airtight container, since it will cause moisture buildup and eventually mold, which you can’t just peel off.