Potatoes, like tomatoes, are a member of the infamous deadly nightshade family. They contain solanine, which is a toxic substance. Now, we are often told that the toxin is only in the potato plant itself, but never in the potato. This is not actually true. In reality, there are large amounts of the toxin in the plant and only small, trace amounts, in the potato, with more being in the skin, and hardly any at all in the flesh. A green tinge on a potato or just under the peel can indicate the presence of more of this toxin.
When potato skins are greenish, this is a signal that there are probably more glycoalkaloids in them, which are toxins similar to that contained in the leaves. So, they pose a bit more danger. And when they are subjected to damage or diseased such as when mishandled or not stored properly, the skins produce this green tint. The green is caused by harmless chlorophyll but it is, again, a signal that there are more of the toxins being developed. An example is a potato being exposed to too much sunlight, which is why you should store your potatoes in a dark place.
Why light? Well, when a potato is exposed to light, it produces this solanine compound as a protective chemical against being eaten by animals. Remember, the potato is a root. It’s purpose is to grow a new plant and it can’t very well do this if it’s being scarfed up by animals like…
Eh..you’re not buying this are you? I thought not. The solanine compound is thought to be a defense again fungi infestation. It is not only produced upon exposure to light, but also in the areas where a potato is cut (wounded), or when subjected to blight; and as the potato ages.
Are Green Potatoes Dangerous?
The chemical solanine is an inhibitor (moderate) of acetylcholine, and this is where we encounter problems with it. But the idea that it is produced to keep us and other animals from eating the root is only speculation. Most plants have a lot more to “fear” from fungi and insects than they do from animals. 1Metcalfe, Dean D., Hugh A. Sampson, and Ronald A. Simon. Food Allergy: Adverse Reactions to Food and Food Additives. Malden, MA: Blackwell Science, 2003. 404-05. 2 Schmidt, Ronald H., and Gary Eugene Rodrick. Food Safety Handbook. Hoboken, NJ: Wiley-Interscience, 2003. 227-28.
Regardless, the bitter taste is there, and it is for that reason that solanine poisoning in humans is rare, except in times of food shortage. At such times people have been forced to consume blighted potatoes, and poisoning has resulted. Normally, the bitter taste should dissuade us and other animals from continued consumption after the first few bites. The upper safe level of glycoalkoloids in potatoes is generally accepted as 100 to 200mgs. Whether the toxins can build up in tissues over time, and thus result in sub-toxicity or chronic toxicity, is still being studied, but this has also been suggested to be possible.
The knowledge that potatoes were in the Nightshade family caused them to take a while to catch on with most of the world. The same thing was true of the tomato. I wrote a bit about that in my post on the history of ketchup.
Although eating large amounts of green potatoes can definitely cause solanine poisoning, you are unlikely to encounter enough of them to pose any real danger, although young children would certainly be in more danger from them. Cooking does not reliably destroy the danger, though, so if I were you I’d avoid green potatoes unless they are peeled. If you peel them, and I mean peel them well, not just the top surface of the peel, you can get rid of most all the bad stuff. Most of the time, however, our potato supply is nice and of the non-green variety.
Are Potato Eyes Poisonous?
The same things are true for any eyes that have developed (little potato sprouts on the skin). These also will build up the toxin and should be removed to well underneath the skin. As long as you remove all the eyes, you will remove most of the toxins. But, again, it is unlikely you will face any real danger from eating sprouted potatoes, unless you eat an obscene amount of them. (Additional source: 3Sizer, Frances Sienkiewicz., Leonard A. Piché, and Eleanor Noss. Whitney. Nutrition: Concepts and Controversies. Toronto: Nelson Education, 2012.)
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