I just sliced an onion and found that there is a large section of one of the layers that's black. What is the black substance? Is it safe to eat? If not, is the rest of the onion safe to eat?

The onion in question:

enter image description here

4 Answers 4


This is either mold or bacteria-caused rot. Don't eat it - the microorganisms which feed on raw plants are rarely pathogenic for humans, but will frequently create toxic metabolic byproducts. I would throw out the whole onion, molds can be toxic in very low doses, just from contamination between layers.

  • 3
    What if it's on the outside layer? That happens all the time.
    – Cascabel
    Commented May 26, 2014 at 22:02
  • 1
    @Cascabel normally eat the onion when it's only on the outside - the yellow papery skin is quite dense, and besides, if there are no signs on the inside, I figure it probably wasn't feeding on that particular onion spreading invisible colonies within it, but came to stick on the outside only. I am not sure this is not risky though.
    – rumtscho
    Commented May 26, 2014 at 22:37
  • @rumtscho Do you have a link available? I would like to read more about it Commented May 27, 2014 at 19:14
  • @belisarius no, no link. I don't even remember where I got this information from - years of reading books, I guess.
    – rumtscho
    Commented May 27, 2014 at 21:34
  • @rumtscho OK! no problem, I'll search for it. Thanks! Commented May 27, 2014 at 22:16

Quoting from Can you use onions with black mold?:

Black mold on onion is caused by aspergillus niger, a common fungus in soil. To discourage mold growth, store onions in the refrigerator up to two months. Rinse off small amounts of the black mold on the outer scales of the onion under cool, running tap water or cut off the affected layers. The unaffected part can be used. Persons known to be allergic to Aspergillus niger should not use onions with black mold.

But if the mold penetrates the inner shell of the onion, I recommend throwing it away. Only use the above mentioned advice if the black mold is on the outside.


Do not be afraid like these people who have answered above. Yes, this is black mold. Since onion is layered and layered quite densely as that, you just need to remove the portion which has mold on it.

If the mold is topical, that is, within the layers of the skin, then all you need to do is wash thoroughly and soak in water adding a little (about 1 tsp.) of vinegar to it. You can go ahead and use it without any fear.

In this case too (as in the photo, I mean), you can use every portion of the onion that has not got any mold on it. Just repeat the rinsing method that I've suggested.

Please don't waste food. It is my humble request. Vinegar, ACV, Salt, lemon, turmeric are really effective ingredients to cleanse food. There's also baking soda water to use for rinsing the outer layers of any fruit or vegetable.

If you are still unsure about it and want to throw it away, then wash and discard it in a pot and let it decompose and make itself useful.

What grows among us, is not necessarily poisonous. So please don't waste food. Just make sure you find ways of cleansing it and/or re purposing it.

Thanks for the question. Hope my answer helps. Love and blessings from India.

  • 2
    The black stuff is the mold spores, which are growing in the air space between layers. The actual mold is in the layers themselves.
    – Sneftel
    Commented Aug 22, 2019 at 14:22
  • I just can’t imagine a situation where vinegar, lemon, turmeric, or your time is less valuable than the value of a single onion.
    – Preston
    Commented Mar 19, 2022 at 3:55

If there is just a thin line of mold inside the onion, I discard the black and some of the area around it. rinse the rest chop it up and freeze it.Then use it in soups etc where it's cooked before use. Have done this for 40 years.

  • Welcome to SA! Some coaching on giving good answers. First, it's rarely useful to answer a question which had an accepted answer 7 years ago. Second, you're supplying an answer which duplicates another one already supplied. Please try to avoid those two things. Thanks!
    – FuzzyChef
    Commented Mar 7, 2022 at 3:54

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