When potatoes are exposed to light, they turn green and produce toxic compounds called glycoalkaloids. The greening itself is harmless chlorophyll, but the toxins that build up alongside it can cause serious illness if you eat enough. Understanding what’s actually happening inside the potato helps you decide which ones are safe to eat and which belong in the trash.
Why Potatoes Turn Green
Potato skin and the layer just beneath it contain storage cells that normally sit dormant in the dark. When light hits those cells, they transform into structures capable of photosynthesis, the same process that makes leaves green. The cells build internal machinery to capture light energy and begin producing chlorophyll, the green pigment found in all plants.
The greening is cumulative: the longer and more intense the light exposure, the deeper the green color becomes. Blue and white light drive the strongest response, while green light has minimal effect. In retail stores, potatoes can start showing visible greening after as little as half a day of exposure, though most noticeable changes appear over one to five days depending on the variety, light intensity, and temperature. Potatoes stored at warmer temperatures green faster.
The Toxins That Come With It
Chlorophyll is not the problem. The real concern is that light exposure simultaneously triggers the production of steroidal glycoalkaloids, naturally occurring toxic compounds. Over 80 types exist in potatoes, but two of them, alpha-solanine and alpha-chaconine, account for more than 90% of the total. These compounds act as the plant’s chemical defense system, and light ramps up their production through the same signaling pathways that drive greening.
This is why green color serves as a useful warning sign. The genes controlling chlorophyll production and glycoalkaloid production are activated by the same light-triggered signals inside the potato. Blue and white light produce the strongest effect on both processes. A deeply green potato almost certainly contains elevated glycoalkaloid levels, though the reverse is not always true: some potatoes can accumulate glycoalkaloids before visible greening appears, especially in early stages of light exposure.
How These Toxins Affect Your Body
Glycoalkaloids are not destroyed by your digestive system, and they target two areas: the gut and the nervous system. At lower doses, symptoms are gastrointestinal: stomach pain, nausea, vomiting, and diarrhea. These effects are often delayed 8 to 10 hours after eating, which makes it easy to miss the connection to a meal.
At higher doses, neurological symptoms appear. These include drowsiness, confusion, weakness, vision changes, and in rare severe cases, paralysis or loss of consciousness. A toxic dose for humans starts at roughly 1 mg of total glycoalkaloids per kilogram of body weight. For a 70 kg (154 lb) person, that means as little as 70 mg could cause symptoms. A potentially lethal dose ranges from 3 to 6 mg per kilogram of body weight.
For context, commercial potato varieties typically contain under 200 mg of glycoalkaloids per kilogram of potato, and food safety guidelines recommend keeping levels below 100 mg per kilogram. A single light-greened potato with elevated toxin levels, eaten with the skin on, could push past the threshold for mild poisoning, especially for children or smaller adults.
Can You Cook Away the Toxins?
Glycoalkaloids are more heat-stable than most people assume, but cooking does help. Recent research testing boiling, steaming, baking, air frying, and deep frying on potato peels found that all methods reduced glycoalkaloid content by 52% to 84%. Baking at higher temperatures and longer times was most effective (74% to 84% reduction), and deep frying performed similarly well (76% to 83% reduction). Boiling and steaming fell at the lower end of that range.
This means cooking reduces the risk but does not eliminate it entirely. If a potato started with very high glycoalkaloid levels, even an 80% reduction might leave enough to cause symptoms.
How to Handle Green Potatoes
Peeling removes a significant portion of the toxins because glycoalkaloids concentrate in the skin and the first few millimeters beneath it. Studies show peeling reduces solanine levels by 25% to 75%, depending on how deeply you peel and how far the greening has penetrated. You should also cut away any sprouts (eyes) and patches of green flesh, since these areas contain the highest concentrations.
For a potato with light, superficial greening, generous peeling followed by cooking is usually enough to bring glycoalkaloid levels into a safe range. If the greening runs deep into the flesh, or if the potato tastes bitter or causes a burning sensation in your mouth (both signs of high glycoalkaloid content), discard it entirely. The peeled and trimmed potato may still contain enough to cause illness when concentrations are very high.
Preventing Greening at Home
Since greening is driven entirely by light exposure, storage is straightforward: keep potatoes in a dark, cool place. A paper bag, a closed pantry, or a cardboard box all work. Avoid clear plastic bags or countertop bowls near windows. Cooler temperatures (around 7 to 10°C, or 45 to 50°F) slow both greening and sprouting, though refrigerator temperatures below 4°C can cause potatoes to convert starch to sugar, affecting taste and cooking quality.
At grocery stores, potatoes are sometimes displayed under lights for 14 to 24 hours per day, which means greening can begin before you even bring them home. When shopping, choose potatoes from the back or bottom of the display where light exposure is lower, and avoid any with visible green patches already forming.