Potatoes turn green when they’re exposed to light. Light triggers cells near the surface to convert their starch-storing structures into chlorophyll-producing ones, the same green pigment found in leaves. The green color itself is harmless, but it signals that the potato has also been building up toxic compounds called glycoalkaloids, which can make you sick.
What Happens Inside a Green Potato
Potato tubers are packed with tiny structures called amyloplasts, whose job is to store starch. When light hits the outer layers of a potato, those amyloplasts begin transforming into chloroplasts, the same organelles that power photosynthesis in leaves. As chloroplasts develop, they produce chlorophyll a and b, turning the potato’s skin and flesh visibly green.
Blue and red wavelengths of light are the most effective triggers. Blue light (around 450 nm) causes the most intense greening, while green, orange, and red light produce 53% to 75% less. Far-red light barely causes any greening at all. This is why potatoes stored under fluorescent or LED retail lighting, which is rich in blue wavelengths, green so quickly in stores.
The greening process doesn’t need much light to get started. A light intensity of just 750 lux is enough to trigger maximum greening, and increasing the brightness beyond that doesn’t make it worse. For reference, 750 lux is roughly what you’d find on a well-lit store shelf or a bright kitchen counter.
Why Green Potatoes Are Toxic
The real concern isn’t the chlorophyll. It’s a pair of naturally occurring toxins called alpha-solanine and alpha-chaconine. These glycoalkaloids are present in all potatoes at low levels, but the same light exposure that triggers chlorophyll production also activates the genes responsible for making these compounds. Both pathways are switched on by the same blue and red light wavelengths, which is why greening serves as a visible warning that toxin levels are climbing.
That said, the two processes aren’t perfectly linked. Some potato varieties green heavily without accumulating much toxin, while others build dangerous glycoalkaloid levels with only modest color change. The green is a useful indicator, not a precise measurement.
Regulatory agencies in Canada and the United States set the maximum acceptable level of total glycoalkaloids at 200 parts per million (fresh weight). Potatoes exceeding that threshold are considered unsafe for sale. A normal, healthy potato typically stays well below this limit, but greened or sprouted tubers can exceed it quickly.
Symptoms of Glycoalkaloid Poisoning
Glycoalkaloid poisoning is uncommon but not unheard of. The symptoms are primarily gastrointestinal: stomach pain, nausea, vomiting, and diarrhea. These effects are often delayed 8 to 10 hours after eating, which can make it hard to connect the symptoms to the potato.
In cases involving larger amounts, nervous system effects can occur. These include headache, confusion, hallucinations, dilated pupils, vision changes, slowed breathing, and a slow pulse. Severe poisoning can cause loss of sensation, paralysis, or shock, though reaching those levels from store-bought potatoes is extremely rare. You’d need to eat a substantial amount of heavily greened or sprouted potato to get there.
Some Varieties Green Faster Than Others
Skin color makes a significant difference in how quickly a potato greens. Yellow and white-skinned varieties like White Rose and Yukon Gold are the most light-sensitive. White Rose potatoes can see their chlorophyll levels jump 17 to 20 times higher under standard retail lighting. Red and purple-skinned potatoes are more resistant because their pigments (anthocyanins) act as a natural light filter, absorbing some of the wavelengths that trigger greening. Russet-skinned varieties fall in the middle.
Importantly, greening intensity doesn’t always predict toxin levels. One variety studied, Long shu 10, showed pronounced greening but minimal glycoalkaloid accumulation. Other varieties with less visible color change built up far higher toxin concentrations. This means you can’t judge safety by color alone, though it remains the best quick indicator available to consumers.
Does Cooking Destroy the Toxins?
Not reliably. Glycoalkaloids are remarkably heat-stable, resisting breakdown at temperatures up to 180°C (356°F). Boiling, baking, and standard frying don’t reach temperatures high enough, or sustain them long enough, to significantly reduce these compounds. Even at 180°C for 30 minutes, alpha-chaconine dropped by about 60%, while alpha-solanine required even higher temperatures or longer cooking times.
Boiling peeled potatoes does help somewhat, reducing glycoalkaloids by about 39%. The reduction comes partly from the toxins leaching into the cooking water rather than from heat breakdown. Industrial processes like blanching (a hot-water pre-soak used before making french fries) can reduce levels by 17% to 58% depending on the technique and potato variety, but these aren’t practical methods for home cooking.
The bottom line: cooking alone isn’t enough to make a heavily greened potato safe.
How to Handle Green Potatoes
Glycoalkaloids concentrate heavily in the skin. The peel makes up only 2% to 3% of a potato’s weight but contains 30 to 60 milligrams of glycoalkaloids. Concentrations are even higher around the eyes, sprouts, and any damaged or injured areas. Peeling a slightly green potato removes most of the risk, especially if you cut generously past the green layer.
For context, glycoalkaloid levels in the peel can run 5 to 10 times higher than in the peeled flesh. In one analysis, Ranger Russet peels contained 1,694 mg/kg compared to 170 mg/kg in the peeled tuber. Russet Burbank peels hit 1,570 mg/kg versus 290 mg/kg in the flesh. Peeling and then boiling brings levels down substantially.
If a potato is only slightly green, peeling it deeply and cutting away any green flesh is a reasonable approach. If the green extends well into the flesh, or the potato tastes bitter (glycoalkaloids have a distinctly bitter, burning taste), discard it entirely.
Preventing Greening at Home
Since light is the trigger, the simplest prevention is storing potatoes in complete darkness. A paper bag, a closed pantry, or a cardboard box all work. Avoid clear plastic bags or countertop storage near windows.
Temperature matters too, though it involves a tradeoff. Cooler temperatures slow greening, but storing potatoes below about 7°C (45°F) causes a separate problem called cold-induced sweetening, where starches convert to sugars and change the potato’s flavor and cooking properties. A cool, dark spot between 7°C and 10°C (45°F to 50°F) is the sweet spot.
Grocery stores contribute to the problem. Retail displays with bright overhead lighting accelerate greening, and while most stores cover their potato displays overnight, the materials and practices vary widely. Choosing potatoes from the back of the bin or from opaque packaging gives you a better chance of getting tubers that haven’t been light-exposed for days.