The Green on a Potato Is a Warning

Every September a sack of potatoes comes in from the garden or the farm stand and sits somewhere convenient, by the back door, on the counter, in the light. Two weeks later the top ones have gone green on one side, and the argument starts. One person says cut it off and cook it. Another says throw the whole sack out. Both of them are half right, and the half they are missing is the interesting part. The green is not the poison. The green is the message that the poison has started.

What the green actually is

A potato is a stem, not a root, and like any stem it will make chlorophyll if you give it light. The green that spreads across a potato left on the counter is exactly the green of a leaf, and on its own it is harmless. You could eat a green potato skin and get nothing worse than a bitter mouthful. The trouble is what the light does at the same time.

The potato plant defends its leaves and stems with a family of bitter compounds called glycoalkaloids, mainly solanine and chaconine. Nothing eats potato leaves for the same reason nothing eats tomato leaves, which carry a cousin of the same chemistry. A tuber growing in the dark makes very little of it. A tuber exposed to light, whether it grew too close to the surface or sat in a sunny kitchen, reads the light as a signal that it is above ground and starts making the same defence the leaves have. The chlorophyll and the solanine are two separate responses to the same light, and only one of them is visible.

How much is too much

An ordinary potato carries around 75 milligrams of solanine per kilogram. Commercial growers keep their varieties under 200 milligrams per kilogram, which is the level food-safety agencies treat as the ceiling for a potato that goes to market. Cases of people actually getting sick have been traced to potatoes running from about 100 up to 400 milligrams per kilogram, which is what a badly greened, sprouted, sun-stored potato can reach.

The dose that makes an adult ill works out to roughly two to five milligrams of glycoalkaloid per kilogram of body weight. For a grown person that is a plate of very green, very bitter potatoes eaten with the skins on, not one green patch on one potato. It has happened, mostly in institutions where a whole bad batch was cooked and served at once. The symptoms arrive eight to twelve hours later: nausea, stomach cramps, headache, sometimes a fever. They pass. Deaths are vanishingly rare and involved potatoes nobody should have eaten.

The reason most people never get anywhere near that dose is the taste. Solanine is bitter and it burns at the back of the throat. A potato with enough of it to matter announces itself in the first bite, and almost everyone has spat one out at some point without knowing what they were tasting. That reflex is worth more than any rule on this page.

Where it sits in the potato

Between thirty and eighty percent of the solanine in a potato is in the skin and the first few millimetres underneath it. The sprouts carry the most of anything, and the flesh right around a sprouting eye runs high too. The centre of a potato, even a green one, is usually close to normal.

That geography is why the old kitchen rule works. Peel a green potato deeper than usual, taking the green with the skin. Cut out the eyes and any sprouts with a wedge of flesh around them. Then taste a sliver raw or after cooking. If it is still bitter, the whole potato goes, and so does any other in the sack that looks like it. If a potato is green most of the way around, do not bother peeling it. It is compost.

Why cooking does not fix it

Solanine is stable at kitchen temperatures. Boiling a potato removes a few percent at most, because the compound does not leach into the water in any quantity and does not break down at a hundred degrees. Baking and microwaving do about the same, which is to say almost nothing. Deep frying at a high temperature for ten minutes has been measured to remove around forty percent, which is why chips made from slightly green potatoes are less of a problem than boiled ones. None of that is a plan. The plan is to remove the green and the sprouts before the potato meets the pan, and to keep the rest of the sack from ever getting there.

Storing the harvest so it never greens

The old root cellar had no window for a reason. A potato stored in the dark, cool but not cold, makes almost no solanine and does not sprout for months. In a modern house the nearest thing is a paper sack or a cardboard box in the coolest dark cupboard you have, or an unheated closet, at somewhere between seven and twelve degrees Celsius if you can manage it. Not the fridge: cold turns potato starch to sugar, and a refrigerated potato fries dark and tastes wrong.

Cure a garden harvest first. Dug potatoes need a week or two in the dark at room temperature so the skins thicken and any small cuts heal over. Then sort out anything damaged, which will rot and take its neighbours with it, and put the rest away. Check the sack once a month and pull the sprouters. A potato that has started to sprout in storage is a potato that is making solanine whether or not it has gone green, so treat the sprouted ones the same way as the green ones.

The sack by the back door is the whole problem in one picture. Move it tonight, peel the green ones deep, and the September harvest will still be feeding you in February.