Watch out for moldy sweet potatoes?!?

By | Posted December 15, 2007
Posted in: Life Science Blog

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.

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

    Marti, December 16, 2007 at 1:34 am
  2. Does this hold true for yams too?

    Karina, December 16, 2007 at 3:03 pm
  3. 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.

    Ralph, December 16, 2007 at 6:45 pm
  4. 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.

    Susannah F. Locke, December 16, 2007 at 7:50 pm
  5. Interesting, especially since the skins are often served.

    Clara, December 17, 2007 at 12:31 pm
  6. 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.

    Ali, December 19, 2007 at 3:08 pm
  7. 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.

    Victor, December 23, 2007 at 7:26 am
  8. 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!!

    Louise, December 23, 2007 at 9:25 am
  9. 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”.

    Mike, October 8, 2008 at 2:32 am
  10. i like this website

    sheldriana hardy, March 25, 2010 at 9:51 am
  11. 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!

    Andrea Reina, January 9, 2011 at 9:23 pm
  12. 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.

    Jeffy, March 25, 2011 at 5:48 am
  13. 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??

    tina, January 17, 2012 at 7:02 pm
  14. 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!

    Shane Kelley, December 17, 2013 at 2:24 pm
  15. 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.

    Joy, January 15, 2015 at 3:33 pm
  16. 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?

    Shelly Feller, April 5, 2017 at 2:20 pm
  17. 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 ;)

    Ang79, July 24, 2017 at 7:34 pm
  18. What are your credentials for making these claims? Where is your scientific evidence from?

    Val Harrison, November 15, 2017 at 1:02 pm
  19. 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

    Jason Waldie, January 29, 2018 at 12:18 am
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