Short answer: cut a single green patch out generously, and the rest of the potato is fine to cook as long as the dish does not taste bitter. Throw away potatoes with several green areas, and old, shrivelled or heavily sprouted ones. That is the advice of Germany’s Federal Institute for Risk Assessment (BfR).
The advice rests on a fact few people know: the green color itself is harmless. It is chlorophyll. The danger comes from glycoalkaloids, mainly α-solanine and α-chaconine, and their amount does not always keep pace with the color.
What actually turns green
Light switches on two processes in a tuber. The first makes chlorophyll, which greens the outer layer of the potato, about 3 mm deep. The second makes glycoalkaloids, which help the plant resist pests and disease. They build up in the dark too, but according to the European Food Safety Authority (EFSA), only about a fifth as fast as in light.
The two processes run side by side, yet separate genes control them. As early as 1937, researchers noticed that ultraviolet light at around 300 nm triggers glycoalkaloid synthesis without adding any chlorophyll. So green tells you one thing only: the potato has been exposed to light.
How much solanine it takes to get ill
In 2020, EFSA pulled together poisoning reports and volunteer studies. Its experts took 1 mg of glycoalkaloids per kilogram of body weight per day as the reference point: at that dose, people have already had nausea, vomiting and diarrhea. Reports of severe poisoning mention loss of consciousness, rapid breathing and convulsions. Doses of 3-6 mg/kg are described as potentially lethal, although EFSA calls these estimates uncertain.
Ordinary potatoes sit well below that threshold. EFSA gathered samples from Germany, the Netherlands and Sweden. Their average content was 51.2 mg/kg, and 95% of samples stayed under 116.8 mg/kg. In German monitoring from 2005, 92% of 222 samples were below 100 mg/kg, and only one went above 200.
Let’s do the math. For a 70 kg adult, the threshold works out at 70 mg. With potatoes at 50 mg/kg, that means eating 1.4 kg in one sitting. With potatoes at 236 mg/kg, as in the German case below, about 300 g is enough, which is a normal portion.
A 1992 experiment shows what this looks like in practice. Seven healthy men ate mashed potato containing 200 mg/kg, with each portion sized to deliver exactly 1 mg per kilogram of body weight. Six of the seven showed signs of poisoning, mostly nausea, in one case with diarrhea. The volunteers described an unpleasant aftertaste. In most of them, symptoms started within about 30 minutes and lasted 3-4 hours.
A Dutch study from 2005 added one more detail: the body usually takes more than a day to clear glycoalkaloids, so they may accumulate if eaten daily. Still, EFSA found no evidence that glycoalkaloids from potatoes harm health over the long term.
Why color does not show how much toxin there is
In November 2015, a family in Baden-Württemberg made potatoes boiled in their skins and baked potatoes. Afterward, several family members had vomiting and stomach pain. A state food testing laboratory in Stuttgart tested the potatoes and a potato salad. The tubers looked perfectly ordinary: no damage, no green patches, no sprouts. Microbiological tests found nothing either. The potatoes, however, contained 141 mg/kg of α-solanine and 95 mg/kg of α-chaconine, 236 in total.
After this case, the BfR revisited the old benchmark. Potatoes with up to 200 mg/kg used to be considered safe. The institute now advises that potatoes meant for eating should contain less than 100 mg/kg, and it calls this recommendation provisional. A 2022 European Commission recommendation uses the same figure: where levels exceed 100 mg/kg, EU countries are asked to investigate the cause. It is a monitoring recommendation and does not ban sales.
The reverse happens too. In a 2006 Washington State University study, researchers compared tubers of four varieties at different stages of greening. In two varieties, glycoalkaloids rose along with the green. In the other two, they did not rise in proportion. Averaged across greening levels, skin held 3.4 to 6.8 times more glycoalkaloids than flesh, depending on the variety. Yet the flesh with its attached skin stayed within limits presumed safe at every greening level. The authors stress that this applies to the varieties and greening levels they studied.
Varieties really do respond differently. EFSA gives examples: ‘Maris Piper’ barely reacts to light, while ‘Kestrel’ is very sensitive. You cannot judge the solanine content of a potato by its color.
Bitterness and burning in the mouth
Taste is a more reliable signal. According to EFSA, normal levels under 100 mg/kg usually cannot be tasted. Above 100 mg/kg, a burning feeling builds up on the tongue and at the back of the mouth within 15-30 seconds. Above 200 mg/kg, the burning is immediate. The BfR puts the threshold for bitterness and burning at roughly 140-220 mg/kg.
In the German family, bitterness spared them a further dose: they left the potato salad made from the same potatoes uneaten because it tasted bitter. In Alberta, Canada, in 1983, baked potatoes containing 494 mg/kg made 61 of 109 pupils and staff at a school ill. Of those affected, 44% noticed a bitter or unusual taste, and 18% felt burning in the throat. They recovered in about 3 hours. Several staff members recalled that the potatoes had a slight green tinge.
The BfR’s conclusion is simple: if a potato dish tastes bitter, don’t finish it. No tasting skills are needed, just the habit of taking an odd taste seriously.
What peeling and boiling achieve
Glycoalkaloids are spread unevenly. The skin makes up 2-3% of the tuber but holds 300-640 mg/kg, compared with 12-100 mg/kg in the flesh. Sprouts carry 2000-7300 mg/kg, dozens of times more than the flesh. The BfR lists the places with the highest concentrations: the skin, the layer just beneath it and the eyes where sprouts grow. According to the European Commission, unpeeled potatoes contain more glycoalkaloids than peeled ones, and small potatoes more than large ones.
Here is what peeling and cooking do, according to EFSA’s review:
- peeling removes 25-75% of glycoalkaloids;
- boiling peeled potatoes lowers the content by 5-65%, 33% on average, and a considerable share ends up in the water;
- frying peeled potatoes in oil lowers it by 20-90%;
- oven baking in the skin lowers it by 20-50%, microwaving by 3-45%.
The ranges are wide because they come from different studies, so don’t count on them. Pure glycoalkaloids withstand temperatures up to 302°F (150°C), so boiling leaves them intact and they leach into the water. One study found that when potatoes are boiled in their skins, some glycoalkaloids migrate from the skin into the outer flesh. The BfR advises against reusing potato cooking water and recommends changing frying oil regularly.
Kitchen heat fails against some other toxins as well. Reheating does not destroy cereulide, a toxin made by bacteria in rice, as explained in our piece on reheating rice.
What poisoning cases show
In 1979, 78 boys aged 11-15 fell ill at a British school. Each had eaten two small boiled, peeled potatoes. In 17 of them, vomiting, severe diarrhea and abdominal pain began 7-19 hours after the meal, and they were admitted to the hospital. All were discharged within 6-11 days, and 4-5 weeks later none showed lasting effects.
Suspicion fell on a bag of potatoes left in the stores since the summer term. The content of the boiled, peeled potatoes was estimated indirectly, from enzyme activity, at 250-300 mg/kg. Peeling did not protect the boys.
A British case described in 1959 shows how dose and skin matter. Four family members ate baked potatoes, skin included, on three Sundays in a row, and each time they fell ill about 9 hours later. The potatoes contained 500 mg/kg. Two who had eaten one potato each were only slightly unwell. The third, after two potatoes, was worse, and the fourth, after three, was very ill. A fifth person ate only the flesh and had no symptoms at all. Everyone recovered within 24 hours.
The BfR cites a 1984 review describing more than 2,000 poisonings and 30 deaths. It adds a caveat straight away: the deaths are poorly documented and seem to have involved mainly undernourished people. Few documented poisonings are known from the past hundred years, but the symptoms are non-specific, so many cases may have gone unnoticed.
Children get a bigger dose per kilogram
For a 20 kg child, the threshold is 20 mg. With potatoes at 236 mg/kg, that is less than 100 g. Per kilogram of body weight, toddlers take in more glycoalkaloids from potatoes than adults do. In EFSA’s calculations, the average intake of children aged one to three reached 174 µg per kilogram per day in some surveys. For adults, the highest average was 82.5 µg.
For the younger age groups, EFSA sees a health concern in the surveys with the highest average intake. Among the 5% who eat the most, it sees one in every survey. For adults, the concern applies only to the heaviest eaters, and only in the surveys where their intake is highest. Whether children are more sensitive in themselves, the data cannot say, and EFSA states this openly.
The BfR advises giving small children only peeled potatoes. The green parts of nightshade plants are also rich in glycoalkaloids: potato leaves hold 230-1000 mg/kg, tomato leaves about 975 mg/kg of tomatine. That is why nightshade sprouts are unsuitable as microgreens for children.
How to store potatoes so they don’t turn green
- Keep them somewhere dark and cool, as the BfR, the Netherlands Nutrition Centre and the Swedish Food Agency all advise. The BfR adds: and dry.
- Store them in a paper bag or a net. In one study, potatoes kept in polyethylene bags had higher glycoalkaloid levels.
- Keep damaged tubers out of the light: a wound combined with light raises glycoalkaloid levels around the damaged spot.
- Cold alone does not protect against glycoalkaloids: some varieties built up more at 39°F (4°C) than at 50°F (10°C), others the opposite or no difference.
Ukrainian law makes a best-before date optional on fresh potatoes that are unpeeled and uncut. What the dates on other foods mean is covered in our guide to use-by and best-before dates. With potatoes, you have to judge the tubers yourself.
When to cut and when to throw away
- A single green patch: cut it out generously, deeper than the visible green.
- Eyes and small sprouts: cut them out with a margin.
- Several green areas, or an old, shrivelled or heavily sprouted tuber: throw it away.
- A dish tastes bitter or burns the tongue: stop eating it.
- Eat the skin only on fresh, undamaged potatoes, and skip snacks made from potato skins altogether.
- Give small children peeled potatoes only.
- Don’t reuse potato cooking water.
The logic is similar to mold: sometimes cutting out the affected spot is enough, and sometimes the food is past saving. More on that in our article on mold on food.
Summary
The green color is harmless in itself and only shows that the potato has seen light. The same light drives glycoalkaloid production through a separate pathway, so even a tuber without a single green patch can turn out bitter. Cut green patches and eyes out generously, throw away old and heavily sprouted potatoes, stop eating a bitter dish and give children peeled potatoes.
Another common worry about vegetables is covered in our piece on nitrates. The wider picture is in our food safety hub.
The fridge prohibition for potatoes no longer stands: FSA rewrote its own advice after a COT review. How that fits with the rest of the list is covered separately.
