Greened and Sprouted Potatoes

Food · §291

The green under a potato's skin isn't a bruise or a look problem. It's the visible tell for a real toxin, solanine, that the tuber builds under the same light and warmth that make it sprout in the bag. Cooking does not fix it: solanine survives boiling, and a bad enough potato can put an adult on the poisoning threshold from a single normal serving. The good news is that spotting the danger costs nothing and the fix is an everyday kitchen habit.

Avoid · As-needed Evidence Moderate Chapter Food

A normal supermarket potato carries 10 to 150 mg/kg of glycoalkaloid in the flesh, and your body clears that with no consequence ¹. The acute-poisoning line is about 1 mg per kilogram of your body weight — roughly 70 mg for a 70 kg adult, a single 200 g serving of a bad tuber ². A heavily greened or sprouted potato can hit 250 mg/kg or more, with the peel and eyes several times higher again. The margin is thin: one affected potato in a normal portion gets you there.

At that dose solanine blocks the nerve off-switch running your gut and heart rate, and tears holes in the cells lining your intestine. So poisoning reads like a stomach bug fused with something neurological: vomiting and diarrhoea, then sweating, a slow pulse, drowsiness. In 1979, 78 boys at one London school ate boiled potatoes stored over the summer; 17 landed in hospital, some comatose with brief convulsions ³.

Two things let it sneak past people. Onset is delayed one to twenty-five hours, peaking at four to eight, so nobody blames dinner ¹. And the mild cases look exactly like a bug you'd wait out.

The whole defence is three sorting rules — plus one override that matters more than any of them.

If it tastes bitter or burns your mouth or throat, stop eating. That sensation starts around 200 mg/kg, already in the danger zone ¹. It's your last warning before the dose is on board. Spit it out, bin the meal, don't finish it "to avoid waste."
Store dark and cool. A paper bag or cupboard away from light, ideally 4 to 10 °C. Not the fridge, and not on the counter beside a warm cooker. Light and warmth are what drive the buildup, so this is the biggest lever ¹.
Trim small offenders generously. For a small green patch or short sprout, cut 3 to 5 mm into the flesh past the green and dig sprouts out at the base. A plain peel already removes most of the toxin, since 30 to 80% sits in the outer millimetre ¹.
Discard the bad ones whole. Green spread into the flesh, several long sprouts, or a soft wrinkled tuber from last season: bin it. That's the range the schoolboys ate. A few cents against a day on the floor.

The fine print — when to skip it, and what people get wrong

"Just cook it well" is wrong. Solanine is heat-stable; boiling cuts the dose by single percent, microwaving ~15%, deep-frying ~40% — none enough to save a bad potato ¹. The 1979 potatoes were boiled.

"Green means solanine" is only half true. The green is chlorophyll, harmless in itself; light just makes both in parallel ¹. A potato sprouting for months in a dark cupboard can carry solanine in its eyes and sprouts with no green at all. Trust the sprouts and the taste as much as the colour.

Pregnancy and small children: on anything visibly green or sprouted, default to discard, not trim. A 20 kg child crosses the threshold from a portion that wouldn't register in an adult. In pregnancy, glycoalkaloids cause neural tube and facial defects in animal embryos, and one large case-control study links frequent sprouted-potato eating around conception to roughly double the odds of a neural tube defect ² ⁴.

References

1Friedman M (2006). Potato glycoalkaloids and metabolites: roles in the plant and in the diet. Journal of Agricultural and Food Chemistry. link
2EFSA (2020). Risk assessment of glycoalkaloids in feed and food, in particular in potatoes and potato-derived products. EFSA Journal. link
3McMillan M, Thompson JC (1979). An outbreak of suspected solanine poisoning in schoolboys: examinations of criteria of solanine poisoning. Quarterly Journal of Medicine. link
4Ni et al. (2018). Maternal periconceptional consumption of sprouted potato and risks of neural tube defects and orofacial clefts. Nutrition Journal. link