What you see: Powdery black lines, clusters, dots or smudges on your onion. You may see black sooty stuff on the paper-like exterior and/or the inner layers.
What it is: Mold!
Eat or toss: Do not eat moldy areas or use them for stock; discard them. If the mold is isolated, trim around it, make sure none of the remaining onion tissue shows signs of decay, and the rest should be fine. Those with respiratory allergies to mold may want to be more careful–keep reading.
Can you use onions that have mold on them?
Sometimes it seems like every other onion at my grocery store has this sooty, dirt-like stuff clinging to it. Unfortunately the black substance isn’t simply dirt. It’s mold.
The sooty stuff is a collection of spores and filaments. You don’t want to eat them, and if you have allergies you don’t want them wafting up your nose. But unafflicted areas of the onion are perfectly edible.
The black sooty stuff on onions is a very common mold that you interact with every day
Did you garden today? Did you go outside? Did you breathe? Then, you’ve likely interacted with this mold in the last 24 hours.
The molds that typically cause black lines and spots on onions are members of a group known as black aspergilli. They are very common in air and soil; researchers have estimated that each of us inhales hundreds of spores from the aspergillus genus every day. Plunge your hands into some soil and odds are good you’ll touch the same types of spores you see on your onion.
So, there’s no need to put on a hazmat suit to interact with it. Still, you don’t want to eat the mold.
Don’t eat moldy parts of the onion
Some molds produce toxins to keep away other hungry creatures. The black molds that commonly grow on onions are capable of producing toxins, though researchers’ findings have been mixed on whether those toxins actually land in the onion’s flesh where you’d be at risk of consuming it if you didn’t trim the onion. The toxins associated with this mold are less potent than some vicious mold toxins out there.
“They’re not making the toxins to kill us,” said David Miller, a research professor at Carleton University who specializes in how fungi impact human health. “They’re making the toxins to tell the competition to bugger off and let me eat.”
Cut off, clean off or peel off the affected areas
If the mold is obviously superficial, you can peel or cut off the affected areas. The U.S. Department of Agriculture says you’re good to rinse off small amounts under cool, running tap water. As long as the rest of the onion is crisp and not showing signs of rot, it should be fine to use.
If the onion is covered in mold, compost it.
Use more caution if you have serious respiratory allergies
The types of mold that grow on onions can trigger allergic reactions when microscopic spores and other mold bits are knocked airborne and inhaled by sensitive people. So if you have serious allergies or asthma, you may want to be careful handling and eating affected onions.
“If you are careless with stuff that has a lot of visible mold, you could get a snoot full of spores,” Miller wrote in an email. “Depending on your allergic status, this might matter. Otherwise probably not.”
Even among sensitive people responses vary widely, explained Miller, who has worked with officials in a number of countries to develop guidelines for managing fungi.
“Small amounts of mold are always present,” Miller wrote. “Just don’t try and save food that is covered with mold.”
Why do black aspergilli molds grow on onions?
This mold likes to eat dead plant material; that’s why it’s pretty comfortable on the dried outer layers of your onion. But, if the onion is bruised, if it’s stored in a too warm, too moist environment (room temperature is warmer than ideal), or if it’s taken from cold storage into a warmer space and finds itself covered in condensation, newly available moisture and nutrients could empower the fungus to sink its filaments into the onion flesh.
When an onion plant matures, its leafy above-ground foliage flops over. This creates a wound that this mold finds rather inviting, and is why you often see black dots near the onion’s neck. As the fungus advances on the onion, you’ll likely see it in lines on the “veins” on the outside of the onion. An onion can even look fine, but have black mold on some of its inner layers.
Some onions may be past the point of no return
Leave an onion to black aspergilli’s devices and it will eventually shrivel. It might also become a squishy mess; according to the Postharvest Center at the University of California, a soft rot bacteria often joins the microbial party.
What’s the best way to store onions to prevent mold growth?
Onions will last longest and be less likely to develop black mold if they are kept cool and dry. To make sure fresh onions stay dry, don’t shroud them in plastic wrap or bags. You have several options for where to stash them:
- Fridge. FoodKeeper, an online resource operated by the U.S. government, suggests storing onions in the fridge for up to two months. Keep them in a low-humidity drawer, and make sure your fridge temperature doesn’t fluctuate too much. I actually don’t store fresh onions in my fridge because it occasionally dips below freezing; that can damage onion cells and make them more susceptible to microbial infection when they return to normal refrigeration temperatures.
- Pantry. FoodKeeper only estimates one month of shelf life in the pantry, but it’s the best option for me, given my fridge’s tendency to create onion-cicles. If you go the pantry route, the National Onion Association advises against storing onions with potatoes, as the tubers could release moisture, which, as we know, is just what encourages that black mold to grow. You’ll also want to keep onions in a dark place; that prevents green patches from appearing.
- Freezer (for chopped onions). While freezing can damage onion cells and change their texture, that’s not a problem if you plan to cook them as soon as you take them out of the freezer. So, if you don’t plan to eat your onions fresh and want to extend their shelf life, chop them up and stash them in a container in the freezer.
- J. David Miller. Distinguished Research Professor. Carleton University. Helped draft guidelines for mold and dampness published by Health Canada. Co-editor of World Health Organization report, “Mycotoxin Control in Low- and Middle-Income Countries.” Interview and email correspondence Sept. 2022.
- Silva, Josué José et al. “Black aspergilli in Brazilian onions: From field to market.” International journal of food microbiology vol. 337 (2021): 108958. doi:10.1016/j.ijfoodmicro.2020.108958
- Onions. Vegetable Produce Facts. Trevor Suslow. Department of Plant Sciences, University of California, Davis. Postharvest Center University of California.
- Can you use an onion with black mold? AskUSDA. U.S. Department of Agriculture. July 17, 2019.
- Black mold of an onion. Vegetable pathology – Long Island Horticultural Research and Extension Center. College of Agriculture and Life Sciences. Cornell University.
- Storage And Handling. National Onion Association.
- Onion: Black mold. Plant Health Clinic Newsletter. University of Arkansas – Division of Agriculture, Research and Extension. Issue 14, 2021.
- Gherbawy, Youssuf et al. “Molecular characterization of black Aspergillus species from onion and their potential for ochratoxin A and fumonisin B2 production.” Foodborne pathogens and disease vol. 12,5 (2015): 414-23.
- Black mould of onion: aspergillus niger. Plantwise Knowledge Bank. Centre for Agriculture and Bioscience International.
- Aspergillosis. Fungal diseases. Centers for Disease Control and Prevention.