Black Coating on Onions: Doctor Warns of Storage Mistakes and Health Risks

Home cooks across India have noticed a strange black layer forming on onions, prompting questions about safety. Dr. Nandita Iyer holds an MBBS degree from Bombay University, a Fellowship in Applied Nutrition from Medvarsity in Apollo Hospitals, and a Diploma in Community Mental Health from the NIMHANS Digital Academy. She is a doctor, health and lifestyle coach, and health educator. Recently, she addressed a particular health issue in a Reels video on her Instagram account, @saffrontrail.

She said, “Have you noticed that onions these days are looking like they've been covered in some black soot? And no matter where you buy them from, they turn out to be the same quality. That's not mud, it's Aspergillus niger, a fungus that thrives in warm, humid spaces. It’s found on onions, fruits and even on your bathroom walls.”

The rise in black-coated onions is linked to environmental and storage conditions. Dr. Iyer explains: “It’s because of the humid weather, then there's poor ventilation in storage, longer shelf times in the warehouse and the musty dark stores of the quick commerce brands.” These conditions create an ideal environment for Aspergillus niger spores to grow, which can affect onions’ outer layers.

Aspergillus niger, a filamentous fungus, is commonly found in soil and decaying plant matter. While it plays a role in the natural decomposition process, it can also affect stored produce, including onions. Infections typically occur through wounds on the bulb, often during harvesting or storage. The fungus thrives under warm, humid conditions, leading to the development of black mold on onions.

While Aspergillus niger is generally not harmful when present on the outer layers of onions, certain strains can produce mycotoxins, such as ochratoxin A. These toxins are associated with health issues, including kidney damage, when consumed in large amounts over time.

Not all black spots are a cause for alarm. As Dr. Iyer advises: “If the black stuff is only on the outer papery skin, peel it, wash it and then cook it. But if you see these dark streaks inside the fleshy layers, then keep peeling it until you hit a clean pink-white flesh.”

Onions with a musty smell or slimy texture should be discarded. Certain strains of the fungus can produce mycotoxins, linked to nausea and, in rare cases, liver issues.

Dr. Iyer emphasizes hygiene and storage: “Always wash your hands and your chopping board with soap after cutting these kinds of black onions because these spores can hitchhike to other foods and trigger allergies. Store onions in mesh baskets for better ventilation and never plastic bags or closed boxes. Buy fresh, not in bulk, and buy it personally from a store.”

Interestingly, she notes the resilience of these spores: “Fun fact, Niger spores are so hardy, they've been found in Egyptian tombs. So your kitchen doesn't need to be their next colony.”

References:

1. Pel, Harmen J., Jeroen H. de Winde, David B. Archer, Peter S. Dyer, Gerhard Hofmann, Peter J. Schaap, Graham Turner, René P. de Vries, Ralf Albang, and others. "Genome Sequencing and Analysis of the Versatile Cell Factory Aspergillus niger CBS 513.88." Nature Biotechnology 25, no. 2 (2007): 221–31. https://pmc.ncbi.nlm.nih.gov/articles/PMC8314004/

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