You bring home a bag of onions, ready to cook, and as you peel one, you find fine black powder between the layers. Your first instinct is probably to throw or wash it away. But is it really dangerous? Experts say that this black dust is actually a fungus called Aspergillus niger and its health impact depends on how much of it you ingest and your immunity.
What exactly is the black mould?
Dip Narayan Mukherjee, senior consultant, clinical microbiology and infectious diseases, CMRI Kolkata, explained, “The black mould on onion is basically a mycelial fungus called Aspergillus niger. It is present in the soil and appears black because of spore formation. This fungus contaminates vegetables like onions, especially post-harvest, during storage in warm or humid conditions.”
He added that this is essentially a superficial colonisation of the onion skin. The fungus grows on the surface, forming the black sooty appearance we see. In most healthy individuals, it is harmless and can be washed off, but there are exceptions.
When can it harm you?
For healthy people, washing or peeling off the outer layers is often enough to make the onion safe to eat. But not everyone should take it lightly. “Patients who are immunocompromised, such as cancer patients undergoing chemotherapy, or those with chronic respiratory conditions like COPD, asthma, or cystic fibrosis, need to be cautious. These spores can be inhaled and may lead to allergic reactions or even invasive fungal diseases like aspergillosis,” said Mukherjee.
Symptoms after consuming mould-contaminated food can include nausea, vomiting, headache, stomach ache, or diarrhoea. In severe cases, it can trigger life-threatening infections in vulnerable individuals.
What nutrition experts say
Payel Kumar Roy, HOD of Dietetics Dept & Critical Care Nutritionist of Techno India Dama Hospital, noted, “Black mould on onions appears as a dry, sooty coating on the outer skin. People with allergies or respiratory conditions should be cautious. If the mould has spread to the inner, fleshy part of the onion, or if the onion is soft or discoloured, it is best to discard it entirely to avoid potential infections or allergic responses.”
How to prevent and store onions safely
Experts stress that proper storage is the best prevention. Onions should be kept in a cool, dry, well-ventilated space with no moisture, as dampness encourages fungal growth. Always inspect them before buying, and discard those with heavy mould growth to avoid cross-contamination with other vegetables.
How to clean and use mould-affected onions
If the contamination is only on the outer layers and the inner parts of the onion are firm and clean, you can safely use them. Peel away the affected layers, wash the onion thoroughly under running water, and dry before cooking. Make sure no visible black mould remains.
