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I had many potatoes I wanted to preserve for a long time. Usually the potatoes I keep in my kitchen get rotten after a month or two (and I don't have a cellar to keep them cool), I decided to dehydrate them. So I grinded them in a food-processor and put in a food-dehydrator in 65 Celsius. After several hours they became dry but also got colored black - a strong black color on the (previously) white potatoes and a weaker color on the red potatoes.

Is there anything useful I can cook with these potatoes that turned black?

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  • Did you peel them before grinding them?
    – GdD
    Commented Aug 27, 2014 at 8:16
  • @GdD yes, I peeled them. Commented Aug 27, 2014 at 10:25

6 Answers 6

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Throw them away. I would not expect potatoes to go black in the dehydration process, and although they may be safe to eat I would not expect them to be palatable, or inviting in any way. You cannot be sure they aren't toxic, and you wouldn't put them on a guest's plate, so chuck them.

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  • Indeed. They do go black after a long exposure to air in room temperature so any other indicators of foulness are void. Commented Aug 27, 2014 at 22:01
  • The potatoes sat on the shelf for a long time and my wife didn't want to touch them. So you are probably right... I will have to throw them away. Commented Sep 10, 2014 at 7:15
  • Hey, it was worth a try.
    – GdD
    Commented Sep 10, 2014 at 7:29
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The color on the potatoes is attributable to the oxidation that's a natural degradation process. The main cause is the direct exposure with open air but other factors can accelerate it (even the metal on the knife or the food processor's blades in your case). Some vegetables are more susceptible than others (for example, avocados turn black in a matter of minutes). There are some methods to slow this transformation like a bath in water mixed with some acid (lemon, vinegar ...) or baking soda. Now the good news: the result of oxidation is not toxic, the worst thing is that the more the process goes on the more vitamins the food lose. In your particular case, I think the food-dehydrator contributed to the oxidation. Since now they're dehydrated you sould regenerate them with a liquid and then you can try to cook them (maybe some puree) but you won't get back the nice clear color, they'll keep looking ... bad.

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  • 1
    Oxidation is indeed common in potatoes, but it is a warm brownish colour. I doubt that somebody will describe it as "black".
    – rumtscho
    Commented Aug 26, 2014 at 12:17
  • @rumtscho I agree with your comment. I don't know, but wondered if the low temperature of the dehydrator over a period of several hours could have possibly made them unsafe to eat. Any ideas on this?
    – Cindy
    Commented Aug 26, 2014 at 13:34
  • @CindyAskew the danger zone starts at 60 Celsius, so it is just high enough not to be unsafe. Some unusual handling could allow bacterial growth though, or the potatoes could have caught some culture which is heat resistant. 60 Celsius doesn't stop all bacteria from multiplying, just the common disease causing ones. Frankly, I have no idea what the cause is based on that description, chemical and biological is both possible - but simple oxidation sounds very unlikely.
    – rumtscho
    Commented Aug 26, 2014 at 14:00
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    I think this extract from a book could help understand the problem a little more or at least, since there's a color picture, try to understand if it the same color @Erel-Segal-Halevi is referring. It's the description of a homemade test, so you won't find lab measurements or particular references to papers or pubblications, sorry about that. Commented Aug 27, 2014 at 7:51
  • @StefanoDriussi the black color on my potatoes is similar to the black color in the top of the two cups. Commented Aug 27, 2014 at 12:10
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I just dehydrated things for the first time yesterday, mainly russets. I saw something in the instructions about preparing potatoes but neglected to follow through. They were reeeeaaally black. I mean, they brought to mind black mold. But I knew it couldn't be that. They taste fine, but aren't attractive. I'm going to use them as snacks over the coming week. I ain't scared. It's not like I'm serving them to guests, which I wouldn't do. Next time I'm blanching, as strongly recommended by the following website. http://www.gettystewart.com/dehydrating-potatoes/

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I dehydrate potatoes all of the time, you need to slice them then boil them for 6to 8 minutes drain the water good then you can dehydrate them, if not they will turn black.

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  • This does not answer the question, but another one: 'How can I dehydrate potatoes?'
    – user34961
    Commented Aug 31, 2017 at 6:31
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TL;DR: Blackening is a thing with potato. Grinding makes it worse!

The blackening/browning of potatoes when exposed to air is caused by a type of enzyme known as a Polyphenol Oxidase (PPO). These are incredibly common enzymes found throughout plant and animal life; they are what causes avocados and apples to brown as well. They are at the highest concentration in potatoes in the vascular ring a couple of millimetres below the skin and in the medullary rays within the flesh of the potato. You can see these structures in freshly cut potatoes - the rays that come out from a centre point are the medullary rays and the vascular ring is a slightly darker line than runs around the potato just below the skin. They are normally there as a defense mechanism against damage to the potato, and you need to break the cells to expose the enzyme to oxygen for it to activate.

PPOs cause browning or blackening of potatoes depending on the variety of the potato and how much of the enzyme you have present, and some varieties it is quite high and results in a blackish colour, as seen in the following image for D6 (second row) from reference1:

Wang et al., 2020

and this one from reference2:

Chi et al., 2014

Note that for both of these they are only looking at the cut surface of the potato, they haven't ground the potato and then very gently cooked it. In the question, OP suggests that the ground the potato and cooked at 65 C in a dehydrator. Reference3 in figure 4 suggest that PPOs are inactivated after 10 min incubation at 65 C, with a temperature optimum of 30 - 40 C in potato juice (PJ) and purified enzyme (ppPPO). Based on my use of dehydrators, I would have thought that it is unlikely that the potato reached 65 C quickly and had plenty of time to oxidise, especially as the potato was ground. We'd need to know the variety of potato used and have a temperature profile to work out if that was really the problem, and not something else, but given that the potatoes were ground I think it highly likely.

I don't believe it alters the taste of the potato too much, but it certainly looks unpalatable.

References:

  1. WANG, L. & WANG, W. & ZENG, L. & SUO, H.C. & LI, C.C. & SHAN, J.W. & LIU, J.T. & LUO, H.M. & LI, X.B. & Xiong, Xingyao. (2020). Characteristics and differences of polyphenol oxidase, peroxidase activities and polyphenol content in different potato (Solanum tuberosum) tubers. Applied Ecology and Environmental Research. 18. 8171-8187. https://doi.org/10.15666/aeer/1806_81718187.

  2. Chi, M., Bhagwat, B., Lane, W.D. et al. Reduced polyphenol oxidase gene expression and enzymatic browning in potato (Solanum tuberosum L.) with artificial microRNAs. BMC Plant Biol 14, 62 (2014). https://doi.org/10.1186/1471-2229-14-62

  3. Bøjer Rasmussen C, Enghild JJ, Scavenius C. Identification of polyphenol oxidases in potato tuber (Solanum tuberosum) and purification and characterization of the major polyphenol oxidases. Food Chem. 2021 Dec 15;365:130454. doi: 10.1016/j.foodchem.2021.130454. Epub 2021 Jun 24. PMID: 34256230.

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The black is from not being blanched before the dehydrating process. They should be perfectly fine to use, just not attractive to look at.

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  • There's already another answer with the same info, just different wording... Please avoid repeating existing answers if it doesn't add anything new.
    – Luciano
    Commented May 16 at 12:42

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