Why Shouldn't You Eat Potato Sprouts or Green Skin?
― A Potato Guards Its "Children" with Poison
Around the sprouts, and in skin that has turned green in the light, potatoes gather natural poisons called solanine and chaconine. They are a "chemical shield" that protects the next generation from insects and animals. The trouble is that boiling or frying barely breaks these poisons down.
From the back of a kitchen shelf, you find potatoes you bought and forgot. Pale sprouts are poking out, and part of the skin is faintly green.
"Can't I just cut the sprouts off?" "A little green is probably fine, right?" Many people have thought this, peeled the potato roughly, and dropped it in the pot.
Or think of autumn, when children dig up potatoes in the school garden. Even the marble-sized ones seem too good to waste, so let's boil them with the skin on―. This is exactly the scene behind food poisoning cases that happen almost every year.
Just two reasons
A potato is a storehouse of food for growing the next plant. The sprout, in particular, is the "child" itself. The places that would hurt most to lose are where the poison is most concentrated.
When light hits it, the potato takes it to mean "I've ended up above ground" and makes more poison. And at ordinary cooking temperatures, this poison is hardly broken down.
Let's go through them one at a time. At the end, we'll work out from body weight how many grams are dangerous.
The potato guards its "children" with poison
The potato tuber is not a root. It is a stem that swells underground. In spring, sprouts grow from the dents on its surface (the "eyes") and become new plants. So a potato is like a capsule packed with a baby and its lunch box.
The soil is full of insects and mice that gnaw on tubers, and molds that rot them. So the potato makes bitter, harmful substances to protect itself. These are solanine and chaconine.
Look at Figure 1. The poison is not spread evenly through the tuber. It gathers just under the skin, where enemies touch first, and around the sprouts, which matter most. There is only a little in the white center.
Once a sprout starts to grow, the poison around it is thought to increase even more. It is the most generous protection for the growing child. The poison is also known to increase near damage or cut surfaces.
Green means "it has been in the light"
Skin turns green because it makes the same green pigment (chlorophyll) that leaves have. The green pigment itself is not poisonous. But when a potato is lit, it starts making more poison at about the same time as it makes chlorophyll.
A potato that should be underground gets light when it pokes out of the soil and ends up above ground. That is a dangerous situation, where insects, birds and animals can easily find it. The potato senses this through light and strengthens its defences, it is thought. So green is a visible sign that "this potato may have made more poison."
Another nuisance is that the poison resists heat. Ordinary cooking, such as boiling, steaming or microwaving, is thought to break it down hardly at all. "Cook it well and it's safe" holds for food poisoning caused by bacteria, but not for this poison.
How many grams is dangerous depends on body weight
At about 1 milligram of this poison per kilogram of body weight, symptoms such as nausea, stomach pain, diarrhea and headache can appear. Symptoms often show up between several tens of minutes and half a day after eating.
Look at Figure 2. Even if they eat the same potato, a small child reaches that level with far less than an adult. And the more concentrated the poison in the potato, the faster the dangerous amount shrinks.
With the flesh of an ordinary potato, even a child would have to eat about two whole potatoes at once to reach that level. But in potatoes with many sprouts or lots of green skin, the poison can be several times to more than ten times as concentrated. Then eating just a few small potatoes with the skin on can be enough.
This is why food poisoning tends to happen with potatoes from school gardens. Small, freshly dug, immature potatoes are said to have more poison. And as Figure 1 shows, the smaller the potato, the larger the share of skin layer. Small children then eat them with the skin on.
Solanine and chaconine have a strong bitterness and harshness. If your throat or tongue tingles when you eat a potato, or you notice an unusual bitterness, stop eating right there. Many poisonous plants are bitter, and it is thought that this is how they tell animals "don't eat me."
So, what should we do?
- Dig out the sprouts deeply, along with the dent at their baseUse the corner of a knife or the tip of a peeler, and take a slightly wider area around the sprout.
- Peel the green parts thickly and throw away what you cut offIf the green has spread inside, or the potato is small and immature, it is safest to throw it away rather than eat it.
- Store potatoes in a dark, cool placeKeeping light off them is the best way to stop the poison from increasing. Store them in a bag or a box.
If you get nausea, stomach pain, diarrhea, headache, dizziness or the like after eating potatoes, do not wait and see on your own judgment. See a doctor. Telling them what and how much was eaten helps with the examination. Children and older people get dehydrated easily when they vomit or have diarrhea, so give them fluids a little at a time. If the person is limp or not clearly conscious, call the emergency number (119 in Japan) without hesitating.
Summary
Potato sprouts and green skin hold poison that the potato made to protect the next generation. The poison increases in light and is hardly broken down by heat. How many grams is dangerous depends on body weight, so smaller children get symptoms from smaller amounts.
A potato stores poison where it most wants to protect.
So "sprouts, green, and small potatoes" do not become safe just because you cook them.
Natural poisons often cannot be told apart by looks or folklore alone. For mushrooms, see "Can you tell poisonous mushrooms by color or by whether they split lengthwise?"; for food poisoning that heat cannot prevent, see "Why does curry left overnight cause food poisoning?"
- Pick two potatoes from the same bag. Put one in a bright spot by a window and the other in a box in a dark place.
- Each day, at the same time, compare the skin color of the two and take photos with your smartphone. In several days to a week, the skin of the one in the light should turn faintly green.
- Peel a little of the skin thinly off the greening potato and look at how far the green spreads inside.
Do not eat the potatoes you used for observation, especially the one that turned green. The number of days it takes to turn green changes with light strength and season.
For those who want to know more ― Terms, formulas and links to textbooksEach part is labeled from middle-school science up to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology or chemistry
- High school+Advanced high-school material, or textbook column content
- UniversityUniversity specialist subjects not taught in high school (plant physiology, food hygiene, toxicology)
- ResearchNot taught even at university as settled fact; what researchers are investigating now
Middle schoolTerms: this phenomenon has names
- Solanine and chaconine: natural poisons in potatoes. They belong to a family of nitrogen-containing substances with sugars attached.
- Tuber: the potato itself. Not a root, but an underground stem that swells as it stores nutrients.
- Chlorophyll: the green pigment used in photosynthesis. When light hits a potato, it is made near the skin and shows as green.
- Natural toxins: poisons that plants or animals have naturally, not from bacteria. Many cannot be prevented by heating.
Middle schoolHigh schoolChecking with a formula: how many grams does a child need to reach the level?
The harm of a poison is judged by how much has entered per kilogram of body weight (the dose per body weight). Let's calculate the amount of potato that brings a 20 kg child to that level.
| In symbols | M ＝ D × W ÷ c |
| In words | Potato needed to reach the level ＝ level per kg of body weight × body weight ÷ poison per 1 g of potato |
| Where it comes from | It comes from a balance of amounts: the total poison eaten (amount of potato × concentration) equals the level per kg × body weight |
| M | Amount of potato that reaches the symptom level (unit: g) |
| D | Amount of poison per kg of body weight at the symptom level (unit: mg) |
| W | Body weight (unit: kg) |
| c | Amount of poison per 1 g of potato (unit: mg) |
| Symptom level D (per kg of body weight) | Said to be about 1 mg |
| Child's body weight W | 20 kg |
| Concentration in an ordinary potato (per 100 g) | Said to average about 7.5 mg |
| Concentration in a potato with many sprouts or green skin (per 100 g, assumed value) | 50 mg |
| Total poison that reaches the level (mg) | 1 × 20 ＝ 20 |
| Concentration per 1 g of ordinary potato (mg) | 7.5 ÷ 100 ＝ 0.075 |
| Ordinary potato needed to reach the level M (g) | 20 ÷ 0.075 ≒ 267 |
| Concentration per 1 g of poison-rich potato (mg) | 50 ÷ 100 ＝ 0.5 |
| Poison-rich potato needed to reach the level M (g) | 20 ÷ 0.5 ＝ 40 |
| By what factor the dangerous amount shrank | 267 ÷ 40 ≒ 6.7 |
For an ordinary potato it is about 267 g, roughly two medium potatoes. For a poison-rich potato it is about 40 g, the same as two or three potatoes a little bigger than marbles. The dangerous amount falls to about one seventh. For an adult (60 kg) the same formula gives three times the amount, so even from the same pot of potatoes, only the child may fall ill.
High schoolHigh school+Why are smaller potatoes more dangerous?
High schoolSolanine and chaconine are substances with a steroid-like frame, a nitrogen-containing part and a chain of sugars attached. They are called glycoalkaloids. They are thought to bind to components of cell membranes and break them, and to weaken an enzyme that passes on nerve signals.
High school+The poison is said to be abundant in a layer about 1 mm from the skin. The share of a sphere's volume taken by a thin outer layer is roughly "3 × layer thickness ÷ radius." The smaller the radius, the larger the share, so a potato of radius 2 cm has 2.5 times the share of one of radius 5 cm. This is one example of the ratio of surface area to volume (specific surface area) being larger for smaller things.
UniversityHow light and damage increase the poison
Glycoalkaloids are secondary metabolites that a plant makes from cholesterol through several enzymes. Light and damage act as triggers, and the genes involved in this synthesis switch on, increasing the poison. Chlorophyll synthesis by light and poison synthesis occur at the same time but are regulated by separate pathways. So the depth of green is only a rough guide to the amount of poison. Food hygiene treats such plant poisons as "plant natural toxins."
📖 Structure and properties of the substances, and beyond: Solanine (Wikipedia) ／ Alkaloid (Wikipedia)
ResearchWhat is still not clear
- How much does it take for people, and how dangerous is it? The exact level for humans is estimated from past food poisoning records, and it is thought to differ a lot between people.
- How to breed low-poison varieties. Researchers are studying the genes involved in making the poison, aiming for potatoes in which poison does not increase easily even when they sprout.
- How it moves in the body. How long the absorbed poison takes to leave the body, and the effects of eating small amounts repeatedly, are still being studied.
In other words, what this article says is also "an explanation of what is known so far." Treat the numbers as a guide, and if in doubt, choose not to eat it.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Structure and functions of plants" | A potato is a stem; light makes chlorophyll |
| High school | Biology: "Metabolism"; Chemistry: "Organic compounds" | The shape of glycoalkaloids and their action on cell membranes |
| High school+ | Advanced math and physics: "Ratio of surface area to volume" | Why small potatoes have a larger share of skin layer |
| University | Plant physiology, food hygiene, toxicology | Synthesis of secondary metabolites, and handling of plant natural toxins |
| Research | Plant breeding and toxicity research | Breeding low-poison varieties, movement in the body |
| ― | Links to daily life | How to dig out sprouts, how to peel green skin, storing in a dark, cool place |
- Ministry of Agriculture, Forestry and Fisheries (農林水産省), "Information on the natural toxins 'solanine' and 'chaconine' in food"
- Ministry of Health, Labour and Welfare (厚生労働省), "Risk profile of natural toxins: higher plants: potato"
- Solanine (Japanese Wikipedia)
- Friedman, M. (2006). Potato glycoalkaloids and metabolites: roles in the plant and in the diet. Journal of Agricultural and Food Chemistry, 54(23), 8655–8681.
*This article is a popular-science explanation for general readers. The figures given are guides and rough estimates to help you understand how things work. If you feel unwell, see a doctor and follow the instructions of your public health center or local government.