For a few years, we have had trouble with too many of our June-planted, October-harvested potatoes having green patches. We’ve been discussing what to do, and trying to separate myth from reality. How can we minimize the amount of green patches on our potatoes? How should we deal with them when they happen? How poisonous are green potatoes?
Here’s what seem to be the facts:
- The green is chlorophyll, caused by the tubers being exposed to light. Chlorophyll is not poisonous. But the same conditions that promote chlorophyll formation also increase the production of solanine, which is poisonous.
- It’s also possible for potatoes to have increased levels of solanine without being green, for example, if diseased or damaged or stored under warm temperatures or after experiencing spring frost (and making stunted growth as a result). Potato sprouts are also high in solanine. An unpleasant bitter taste indicates an increased level of solanine.
- The Lenape potato was developed in the 1960’s to make beautiful golden potato chips (it’s challenging to to reliably make good chips without burning them.) It was pulled from the market in 1974 after studies showed that Lenape produced an extraordinarily high level of solanine.
- Simply removing all green-skinned potatoes won’t remove all solanine from our diets. Solanine is a glycoalkaloid found to some extent in all nightshades. Other commonly-consumed alkaloids include caffeine, nicotine and cocaine.
- Solanine is one of the plant’s natural defenses against pests and diseases, such as late blight.
- The amount of solanine in an average portion 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
- “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
- 2-5 mg of solanine per kilogram of body weight can cause toxic symptoms. 3-6 mg per kilogram of body weight can cause death.
- A normal potato can contain 8 mg of solanine or 12-20 mg of total glycalkaloids per kilogram of potato.
- So to get 2 mg per kg of body weight, a 100 lb (45.35 kg) person would have to eat about 90 mg of solanine, or at least 11.25 kgm (about 25 lbs) of regular potatoes.
- Green tubers contain 250-280 mg/kg of total glycoalkaloids. 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.
- Alexander Pavlista at the University of Nebraska, Lincoln says that a 100 lb person would have to eat about one pound of fully green potatoes to get sick. That is equivalent to one pretty large baked potato. His report recommends cutting away the green parts.
- Green skins contain 1500-2200 mg/kg total glycolkaloids.
- Potato shoots 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
- The toxic dose seems to depend on the individual’s tolerance as well as the ratio of solanine consumed to the rest of the potato. 30-50 mg/100 gm; 24 mg/100 gm, 40 gm/100 gm seems to be the range in green-skinned potatoes, from various reports. See, for example, The Smithsonian article of October 21 2013 by K Annabelle Smith.
- The symptoms of solanine poisoning include gastro-intestinal problems, and, more worryingly, neurological disorders.
- Most victims of solanine poisoning make a full recovery.
- Fatalities are generally restricted to people who don’t get treatment, or were undernourished to start with.
- According to the British Medical Journal 8 December 1979, 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.
Here’s what’s disputed:
- Some studies have shown correlation 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 correlation between birth defects and consumption of potatoes.
- The US National Institutes of Health says never to eat potatoes that are green under the skin.This has been variously interpreted to mean: throw out all potatoes with any green or cut off the green bits and eat the rest or cut off the green skin and also any green flesh under the skin. Most people seem to cut off the green bits and use the rest.
- “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 schoolboys getting very sick after eating boiled potatoes that had been stored improperly for several months, 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.”
- “Eating nightshades makes arthritis worse.” This seems to be an entirely different issue, as no source lists arthritis as a symptom of solanine poisoning.
Here’s what seems wise:
- When growing potatoes, try to cover them adequately with soil and/or mulch, so that they are not exposed to light. Give enough space between plants so that tubers are not crowded and pushed up above the soil surface.
- When mowing to reduce weeds before mechanical harvest, keep to a minimum the length of time between mowing and harvest. Likewise time between removing mulch and mechanical harvest. Manual harvesting can be done without removing weeds or mulch first, but there is a limit on how big of an area one person can hand-harvest well.
- When harvesting, minimize damage.
- When preparing for storage, do not gather all the potatoes showing any green into one container. Leave the green ones distributed among the others, so that no-one gets a higher level than normal. There is apparently no reason to use green potatoes sooner than others. Nor is there apparently any advantage to keeping them in hopes of de-toxifying them.
- When storing, keep potatoes in the dark, and cool. Don’t store them for longer than necessary. Generations of potato growers have provided for their family needs for a whole year from one planting, so there seems no need to worry about storage up to one year or so.
- When preparing potatoes for eating, cut off the green bits. Don’t use all the greened potatoes in the same meal. Reduce the risk by reducing the ratio of greened to non-greened potatoes.
- When eating, spit out any potato that tastes bitter.
- Enjoy eating your potatoes fried, boiled, mashed, chipped, baked, roasted. .