THE POISON GARDEN website       

Thursday 19th April 2012

An email from a visitor to the site set me thinking about the potato. I’ll return to that email later but I thought I’d start by looking at plant names to see how useful they are.

The botanical name for the potato is Solanum tuberosum; in other words it is in the same genus as Solanum dulcamara, woody nightshade, a well-known poisonous perennial. And, though it has a different botanical name, it is very similar to Atropa belladonna, deadly nightshade.

I think that, if we were to start from scratch with plant names, and assuming scientists ruled the process, the deadly nightshade would end up with a name beginning Solanum. It’s possible that the ‘belladonna’ would survive but just as likely that the species would be called something like prunuserasus or some other construction to indicate that the fruit looks a lot like the cherries produced by the Prunus genus.

So the Solanum tuberosum is closely related to some very poisonous plants and, not surprisingly, it is poisonous itself. The principle toxin is solanine, a glycoalkaloid, that is present throughout the plant but in variable quantities.

Like so much information about poisonous plants it is quite difficult to separate proven fact from best available theory, from speculation or from outright nonsense. What follows should not be considered to be proven fact but I hope it is mostly best available theory with a little speculation and no outright nonsense.

Though it is generally acknowledged that the foliage of potato plants contains a potentially harmful concentration of solanine¹ and should never be eaten there is less clear guidance concerning the tubers. It has been said that eating 5kg of potatoes at one sitting would provide a fatal dose of solanine² but others express this in terms of multiples of ‘normal’ daily consumption suggesting that 32 times a normal day’s portion would prove fatal.¹

But those estimates are for ‘normal’ potatoes. Solanine helps to protect potato plants from insect attack and it has been proved that the tubers produce an increased concentration of solanine close to sites of mechanical damage. Happily, they also produce chlorophyll at the same time and the green colour of that chemical makes it easy to identify high concentrations of solanine. The effect of eating a potato with green areas will, of course, depend on the extent of the green but it has been said that one large baked potato would be sufficient to cause illness.³

The question becomes what to do with potatoes that are partially green? The most common advice is to remove any green parts of a potato⁴ but the official advice from the US National Institutes of Health is that you should never eat potatoes that are green under the skin.⁵

Though there are plenty of papers about solanine poisoning⁶, many of them refer to livestock that have ingested the foliage and potato poisoning in humans is not a common occurrence in spite of most people following the ‘remove the green bits and you’ll be fine’ advice rather than discarding the whole potato.

Then there is the question of cooking. I say on the Solanum melongena, aubergine, page of this site that the vegetable must never be served raw but cooking destroys the toxins. As with all things to do with poisonous plants, I have heard of people eating raw aubergine without coming to any noticeable harm but larger amounts eaten raw are not a good idea.

With potatoes, it is suggested that frying destroys any solanine, microwaving reduces it but boiling has no effect.⁴

Incidentally, as well as solanine, Solanum tuberosum produces chaconine, a related glycoalkaloid, but, for some reason solanine seems to be more studied than chaconine. But, even though it has been studied more, there is no strong consensus about what solanine does and why.

There was quite a bit of work up to the 1950s on the toxicity of solanine.⁷ This seems to have focussed on whether it might make a commercially viable pesticide and, once that possibility was found to be impractical, the study of the plant and its poisons seems to have declined.

It’s that lack of detailed research that brings me back to the email that started this line of thought. My correspondent remembered speaking to a PhD student who had been studying solanine in potatoes. Though not the main intention of his research, he had noticed that organic potatoes produced higher levels of solanine that pesticide treated ones. Intuitively, you would expect that, since solanine is produced by the plant to deter insects and pests.

There doesn’t seem to have been any study of this point so it is impossible to say if that makes organic potatoes more of a risk that pesticide treated ones (assuming the potato is thoroughly washed, or peeled, to remove any residue) but, given that most people seem to be aware of the need to remove any green parts, I’m not sure that any increased risk, if it exists, would be converted into increased harm.

References

^1.Nightshade Poisons. Dr Anita Pepi
^2.Top 10 foods to make your skin crawl. 4Food, Channel 4
^3.The Claim: Green Potatoes Are Poisonous. New York Times 3rd July 2007 
^4.What is solanine and is it toxic? Food-info
^5.Potato plant poisoning - green tubers and sprouts. MedlinePlus
^6.The International Poisonous Plants Checklist. D. Jesse Wagstaff, 2008
^7.Factors Affecting the Toxicity of Solanine and Related Alkaloids to Fusarium caeruleum. MCKEE, R. K. (1959). J. gen. Microbial. 20, 686-696 

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