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Watch out for moldy sweet potatoes?!?

At Thanksgiving, a family friend used to warn us to never eat sweet potato skins. This November, when I found myself in the yearly ritual of warning friends of the […]

December 15, 2007

At Thanksgiving, a family friend used to warn us to never eat sweet potato skins. This November, when I found myself in the yearly ritual of warning friends of the dangers of sweet potatoes, I decided it was time to track down this factoid once and for all.

According to Toxicants Occurring Naturally in Foods (National Academy of Sciences, 1973), sweet potatoes can churn out a bunch of nasty chemicals in response to fungi and other foes. I haven’t found any reference to skins, although it makes sense that this would be the front of the fungus versus sweet potato war.

These chemicals include ipomeamorone, which has caused serious liver damage in animal studies, and other toxins that have caused lung problems among herds of animals that have eaten moldy sweet potatoes. But I didn’t find any mention of people having sweet potato problems (maybe we’re less likely to binge on moldy tubers?).

Looking at ipomeamarone levels in sweet potatoes from the supermarket and toxicity data, it looks like one good-lookin’ potato could kill . . . about a seven-pound weakling. (But could liver damage perhaps accumulate over a lifetime of consistent skin-eating? I don’t know.) People are still studying ipomeamarone today.

Apparently baking your sweet potatoes won’t completely rid them of creepy chemicals, so I guess I’ll make sure that any I ingest are free from signs of disease. And I’ll probably skip the skins.

For more info, you can check out the Handbook of Natural Toxins (1983), which is on Google Books.

About the Author

Susannah F. Locke

Susannah Locke holds a B.S. from Haverford College, where she studied molecular biology and psychology and ran the college’s literary magazine. For two years following graduation she played with neurons as a research technician at the University of Pennsylvania and The Children’s Hospital of Philadelphia. After using almost every type of test tube on the market for every conceivable purpose, she removed her gloves to become a journalist and improve the public’s understanding of science.



Marti says:

Wow great article. I’d always wondered if eating potato skins were safe. Thanks for the amusing blog.

Karina says:

Does this hold true for yams too?

Ralph says:

Fascinating! I have noticed a bluish substance — like bread mold? — on broken ends of uncooked sweet potatoes. This confirms my instinct to trim it off.

Susannah F. Locke says:

As far as I can tell, it doesn’t hold true for yams, which are an entirely different species altogether. Everything I’ve read has been “sweet potato, sweet potato, sweet potato.”

“Toxicants Occuring Naturally in Foods” mentions that yams don’t make these chemicals when moldy, but they didn’t include a citation for it.

I don’t even think I’ve ever eaten a true yam, though.

Clara says:

Interesting, especially since the skins are often served.

Ali says:

I’ve always peeled my sweet potatoes, for reasons unknown to man (primal instinct? mom said to one day and I filed it away?). Strange, as I tend to enjoy normal potato skins in my food, and it would follow that potato skins that are delicious in one way would be good in another way, too…but thanks for saving my LIFE in case I ever was going to binge on sweet potatoes. Bagfuls.

Victor says:

In response to Ralph, the Japanese noted that the potato needed to be damaged in order to find the toxin:

“Ipomeamarone 15-hydroxylase activity was not found in fresh tissue of sweet potato roots. However, the activity appeared and increased markedly in response to cut-injury.” This comes from a paper titled “Properties of a Mixed Function Oxygenase Catalyzing Ipomeamarone 15-Hydroxylation in Microsomes from Cut-Injured and Ceratocystis fimbriata-Infected Sweet Potato Root Tissues” written by Masayuki Fujita, Kazuko Ôba, and Ikuzo Uritani from the Laboratory of Biochemistry, Faculty of Agriculture, Nagoya University, Chikusa, Nagoya 464, Japan, and published in the journal Plant Physiology.

Great observation, Ralph.

Louise says:

Great article! I was always told as a child to eat the skins, but (rebellious me, I guess!) I didn’t like them. And since I’ve done my own shopping, I have noticed discoloration, and have continued not eating the skins. Fascinating (and a bit frightening) to know what that discoloration can indicate. Many thanks!!

Mike says:

I suggest you check out the link here that tells exactly why you SHOULD eat sweet potatoes skins.
“three times the antioxidant quantities found in the skin than in the flash of the sweet potatoe”.

i like this website

Andrea Reina says:

I would much rather get antioxidants from already-rich and non-toxic sources like berries than risk toxicity in the pursuit of antioxidants. Thanks for posting the research for us!

Jeffy says:

I’ve been eating a lot of sweet potatoes over the last year. Some moldy some not. I’ve developed some odd rashes that don’t go away. Trying to figure out whether consuming that much sweet potatoes could have something to do with it.

tina says:

I have had 2 sweet potatoes stored in the fridge for over a month. I took them out tonight and noticed mold on the skin. Is it still safe to boil and make make mashed sweet potatoes even if i peel skin??

Shane Kelley says:

Greetings. Every year I collect sweet potatoes from a field after the harvest. This year, there was an incident that caused me to go and search for information and I’d like to share: among the latest sweet potatoes I collected were ones cut from a tractor plowing (to seed the fields with winter wheat) and some of the sweet potatoes had black mold on cut ends. I collected these specimens anyway, and washed, peeled, cut off all damaged parts, and rewashed them, then used them to make sweet potato soup. Also, these sweet potatoes had been lying on the ground in the sunlight for more than a week. My husband (generally healthy) ate 2 bowls and almost immediately complained of strong nausea, and went to bed. At night he woke up and complained of an odd difficulty breathing, which passed. Then the next day I myself ate 2 bowls, and within minutes I too felt extremely nauseous. I worried that perhaps the fields had been sprayed, and contacted someone about that, and apparently there had been no spraying. and them I read about the ability of the sweet potato to produce glycoalkaloids in response to cuts, bruises and molds – and the dangers of these substances. For example, cows have died from eating damaged sweet potatoes and they apparently died from sudden respiratory failure. From what I read, the production of these substances (the glycoalkaloids) occurs in the entire sweet potato, so just removing the ‘bad’ part is not enough. I would strongly advise others to only use sweet potatoes which are whole and mold free!

Joy says:

This is interesting news to me, while I continue personal research into anti-toxins, mold, yeast, and fungus, in general, as it relates to digestive sensitivities and food allergies for me—- and possibly others—particularly, since I have experienced improvements removing my known allergens and sensitivities which coincidentally are known for higher levels of aflatoxin or mold than other foods. I hypothesize that the reason for my improvement is likely due to a decrease in mold content, while being similar to the improvement people experience on the GAPS. Paleo, low-carb, or Caveman diet, and others like it that avoid grains, beans, and potatoes, etc.

Shelly Feller says:

I just cooked a sweet potato in the microwave and when I opened it up, the bottom was greenish dark like an avocado with some areas with white lines and some areas with black lines. What is this?

Ang79 says:

Why in the world are you cooking ANYTHING in the microwave let alone a sweet potato?! Lol – moldy potato skins are the least of your worries ;)

Val Harrison says:

What are your credentials for making these claims? Where is your scientific evidence from?

Jason Waldie says:

I just ate one boiled sweet potato, skin and all , and i tasted somthing bitter in a piece of the skin. 5 mins pater, vomited . fine afterwards. this is likely what we are talking about

Wow what an item delight. It was something that consumed without knowing what was wrong … Thank you for the tip.

James says:

I have home grown sweet potatoes that are unusual because when you cut them they develop this kind of black colour where they been cut quite quickly within a few minutes anyway I harvested the 6 kilos of them. Anyway I ate them and felt quite sick that night, allergic reaction, nausea , difficulty breathing. Gave a break for a few days and then tried them again same thing happened. ones I have been eating have been damaged and I just cut off the kind of moldy bits. From what you say once a bit of mould gets into the potato it’s spreads throughout the whole thing and you cannot use any of it. I can try some of the undamaged potatoes and see if there is no issue but I am wondering if fact they are sweet potatoes at all so can anyone tell me if when you cut the sweet potato it should go a kind of dark color ?

Kevin says:

My partner ate part of a sweet that was at least a month old, I told her I did not think she should eat it due to its age and because it had some black spots. She cut off the skin where the the dark spots were and microwaved it. She said it did not taste great and left most of it, however about 2 hours after eating it she felt ill with stomach pains and nausea, she has now been unwell for about 36 hours and is only improving slowly.. She finished a 5 month course of chemo about 2 months ago, I am sure her immune system is still compromised from that and so more susceptible to bacteria and toxins. However I would say make sure sweet potatoes are fresh and undamaged before eating them. That is true for most foods of course.

Stef says:

I have a sweet potato that has started to go mouldy in the cupboard. I cut it up and put it in the compost bin, but while cutting I noticed a strong smell coming from the potato. Has anyone ever noticed a delightful lychee aroma from their rotten sweet potatoes? I am fascinated that it does not smell bad in any way, and in fact, is quite a delicious smell (but the potato itself is mostly black or has black skin – i’m not about to eat it!).

Phyllis Turk says:

I was about to cook a sweet potato after cutting off the end which was moldy. I’m happy I found and read the comments from people writing in to the “Science Line” website, Newsletter and telling about their experience of becoming ill after eating sweet potatoes that had mold. My sweet potato is now in the garbage. Thank you.
I have another sweet potato in my fridge that looks perfectly fine without mold. After reading about Ipomeamarone toxins in sweet potatoes, I think I’ll skip the sweet potatoes all together.

louise renwick says:

just eaten a month old sweet potato that i cut some mold off of and roasted. IT tasted like roses. (literally).
Feeling fine so far! lets see what happens!

molds yeasts and fungii,,, lil’ fellas have 92% similar DNA to us upright 2 leggers, some of us more reactive to them in chronic lung ailments, GI/stomach skin,urinary tract infections,,goopy itchy eyelids after yard or basement sweeping try getting a 100 year old proven effective treatment for the itchy,swollen eyelids boric acid powder,pharmacists of this age never heard of it,,pharmacy shelves are lined with 45 anti itchy blotchy benedryl cremes lotions,psoriatic sulphur stinky soaps that dehydrate the skin area,,ALL are effing useless avoid eating moldy anything including stinqy french cheeses too,you folks who love eatin their shrooms if you like the stuff have at it source of info? american academy of environmental medicine

Joyce Mayhew March 6, 2021 says:

Great sweet potato info. I just microwaved a sweet potatoe and saw the mold at firsr. Cut it off but interior after bakinng tasted moldy too. Hope I don’t get sick from it, is now in the garbage. so it is true that mold strands can go through veges skin.

Laurie says:

I was looking for any reference at all about sweet potatoes tasting like roses, and the comment from Louise Renwick above is the only mention of it I have seen. I would love to know if she had any long term effects, because my son and I just had a casserole with a giant sweet potato that I trimmed off the bad end, and we were both noticing the rose flavor, and now my whole kitchen smells like it. Hoping for no liver damage or worse.

D. Luna says:

I wonder if the toxins can affect a parrot’s lungs adversely, especially since they are sensitive to environmental toxins. I won’t be eating the skins anymore either, and I definitely won’t offer them to my bird.

K says:

I recently became violently ill (up all night, coming out both ends) after consuming day old leftovers baked sweet potatoes where a few of the skin shreds were left on. Last week my son ate baked sweet potatoes day old leftovers and had the exact same reaction-vomiting, diarrhea. This hasn’t happened before, but what you all are referencing would make sense. I didn’t see any black bits but one of the pieces looked a little greenish. I didn’t inspect the skins carefully before cutting them off either. Just another story for the group.

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