Mulched potato plants
For several years we have had problems with our June-planted, October-harvested potatoes having too many green patches. I’ve been researching what to do, and sorting myth from reality. How poisonous are green potatoes? How can we get fewer green patches on our potatoes? How should we deal with green skin when we get it?
Why do potatoes turn green? The green is chlorophyll, caused by the potatoes being exposed to light. Chlorophyll is not poisonous. But the same conditions that promote chlorophyll production also increase the formation of solanine, which is poisonous. So the green is an indicator of likely trouble, but is not trouble itself.
Potatoes can also have dangerously high levels of poisonous solanine without being green. This can happen if the potatoes are diseased or damaged, or they are stored in warm temperatures, or they experience a spring frost and make only stunted growth as a result.
Solanine is one of the potato plant’s natural defenses against diseases such as late blight, and against pest attacks.
Just discarding all green-skinned potatoes won’t remove all the solanine from our plates. Solanine is a glycoalkaloid found at some level in all nightshade crops.
Apparently the amount of solanine in an average-sized serving of potatoes is easily broken down by the body and excreted. “[S]olanine levels in the blood are low after ingestion due to poor absorption by the gastrointestinal tract. Second, it is removed from the body fairly rapidly in both the urine and the feces, usually within 12 hours, preventing accumulation in the tissues. Third, intestinal bacteria aids in the detoxification by hydrolyzing the glycoside into solanidine (aglycone), which is less toxic than solanine and also poorly absorbed.” Andrew Montario, Cornell University
It takes 2-5 mg of solanine per kilogram of body weight to cause toxic symptoms, and 3-6 mg per kilogram of body weight to cause death.
A regular (not green) potato can contain 8 mg of solanine or 12-20 mg of total glycoalkaloids per kilogram of potato.
So, to get 2 mg solanine per kg of body weight, a 100-lb (45.35 kg) person eating regular (not green) potatoes would have to eat about 90 mg of solanine, or at least 11.25 kg (about 25 lbs) of potatoes, within the 12 hours or so before the compound starts being excreted.
Green potatoes contain 250-280 mg/kg of total glycoalkaloids, 20 times the level of non-green potatoes. The make-you-sick dose of 90 mg of solanine for the 100 lb person could be found in 0.6 kg (about 1.25 lbs) of green tubers. That’s green-all-over potatoes.
Our calculation is backed up by Alexander Pavlista at the University of Nebraska, Lincoln who says that a 100 pound person would have to eat about one pound of fully green potatoes to get sick. That is equivalent to one very large baked potato – diet sites on the internet are full of estimates of weight of potatoes. His report recommends cutting away the green parts.
Green skins contain 1500-2200 mg/kg total glycoalkaloids. That’s just the skin. Don’t eat just green skins!
Various reports give figures of 30-50 mg solanine /100 gm potato; 24 mg/100 gm, 40 gm/100 gm for green-skinned potatoes. See, for example, The Smithsonian article of October 21 2013 by K Annabelle Smith.
Potato shoots (sprouts) are high in solanine. They can contain 2000-4000 mg/kg of glyclakaloids. These figures are from Is It Safe to Eat?: Enjoy Eating and Minimize Food Risks, by Ian Shaw
“Solanine levels above 14mg/100g are bitter in taste. Cultivar[s] with greater than 20mg/100g cause a burning sensation in the throat and mouth.” Andrew Montario, Cornell University
The 'Lenape' potato was developed in the 1960’s for industry to make attractive golden potato chips (it’s hard to make good chips without burning them). But studies showed that Lenape produced a very high level of solanine, and it was pulled from the market in 1974.
The toxic dose varies, depending on the individual’s tolerance as well as the ratio of solanine to the rest of the potato eaten.
The symptoms of solanine poisoning include gastro-intestinal problems, and harder-to-recover-from neurological disorders.
Victims of solanine poisoning usually make a full recovery. People who don’t get treatment, or were undernourished to start with, are the ones most likely to get a fatal dose.
The British Medical Journal of 8 December 1979 reports that there is normally a high concentration-gradient between the peel and the flesh, but this is lost when potatoes are exposed to light or stored in adverse conditions. This means the level of solanine quickly drops as you peel deeper into the potato, unless the potatoes were exposed to light or were stored in a warm place for several weeks or more.
Potato digging by machine
Some studies have shown a link between pregnant women eating potatoes suffering from Late Blight (which increases levels of solanine and other glycoalkaloids) and spina bifida in the fetus. But other studies have found no link at all between eating potatoes and birth defects.
“Solanine is fat-soluble, so deep-frying reduces the danger.” The Department of Animal Science at Cornell University says that solanum-type glycoalkaloids are not destroyed by cooking.
“Solanine is water-soluble, so boiling lowers the levels.” An infamous 1979 case of 78 London school children getting very sick after eating boiled potatoes that had been stored improperly over the summer vacation seems to prove this belief not true. (All made a full recovery.) Results of a study by Takagi, Toyoda, Fujiyama and Saito “confirmed the relatively high stability of CHA [alpha-chaconine, the other main alkaloid in potatoes] and SOL [solanine] in potatoes under normal home cooking conditions.”
The US National Institutes of Health advises never to eat potatoes that are green under the skin. This is ambiguous and has been interpreted to mean either: throw out all potatoes with any green bits, or cut off the green skin and also any green flesh under the skin and eat the rest of the potato. Most people seem to cut off the green bits and use the rest.
“Eating nightshades makes arthritis worse.” This seems to be an entirely different issue, as no source lists arthritis as a symptom of solanine poisoning.
Root cellar storing potatoes
1. When you grow potatoes, try to cover them fully with soil or mulch, so that they are not exposed to light.
2. Give plants enough space so that the developing potatoes are not crowded and pushed up above the soil surface.
3. If mowing to reduce weeds before mechanical harvest, keep the length of time between mowing and harvest to a minimum. For the same reason, harvest soon after removing mulch. Hand digging can be done without removing weeds or mulch first, but there is a limit on how much one person can hand-harvest.
4. When harvesting, minimize damage to the tubers.
5. When sorting potatoes for storage, do not put all the ones showing any green in the same container. Leave the green-skinned potatoes mixed with the others, so that no-one gets a higher amount than average.
Potatoes in the root cellar stored in plastic crates
6. When storing potatoes, keep them in the dark, and cool. Don’t store them for longer than necessary. There seems no need to worry about storage up to one year or so, as generations of potato growers have provided for their family needs this way.
7. Apparently there is no reason to use green potatoes sooner than others. Nor is there apparently any advantage to storing them longer in the hope of de-toxifying them.
8. When preparing potatoes for eating, cut off and compost the green bits. Don’t use all the greened potatoes in the same meal. Reduce the risk by mixing greened and plenty of non-greened potatoes.
9. When eating, spit out any potato that tastes bitter.
10. Enjoy eating your potatoes fried, boiled, mashed, chipped, baked, roasted.
Root cellar Photo by McCune Porter
Potato digger Photo by Twin Oaks Community
Mulched potato plant Photo by Kathryn Simmons
Potato crates in cellar Photo by Nina Gentle
Pam Dawling manages the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MEN Fairs. Pam also writes for Growing for Market magazine. Her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, is available at Sustainable Market Farming, Pam's blog is on her website and also on the Sustainable Market Farming's Facebook Page.
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