Is the black powder on onions dangerous? Signs to look for before using

Onions are a staple in most Indian kitchens – adding depth, flavour, and aroma to a wide range of dishes. But what if you pick up an onion and notice patches of black, dust-like powder on its surface? Should you toss it out? Is it still safe to use?

These black patches are often caused by a common soil-borne fungus called Aspergillus niger. It’s not unusual to find this fungus on onions, especially those that have been stored in humid environments with poor ventilation. While its appearance can be alarming, the health risks depend on several factors, including your overall health and the condition of the onion.

What causes the black powder?
Aspergillus niger thrives in warm, moist conditions. When onions are stored in damp or poorly ventilated areas, such as sacks or closed containers, the fungus can start to grow on the outer layers, forming a black powdery residue. This is more likely to occur if the onion’s outer skin is damaged, allowing the spores to settle in.

Is it safe to eat?
For most healthy individuals, onions with minor fungal contamination on the surface can be used safely, provided they are thoroughly washed and peeled before cooking. Studies suggest that small traces of Aspergillus niger on the surface are unlikely to cause harm when removed properly.

However, it’s a different story for people with compromised immune systems, asthma, or other respiratory conditions. In such cases, even mild exposure to fungal spores can trigger allergic reactions or respiratory issues. So if you or someone in your household falls into this category, it’s best to be cautious.

When should you discard an onion?
Even if the black powder seems superficial, inspect the onion carefully:

• If the inner layers show mould, rot, or a musty smell, discard it entirely.
• If the onion feels soft, damp, or slimy, it's not safe to consume.
• Onions that are completely dry inside and only have slight external black powder may be used after peeling and washing.

Tip: Always store onions in a cool, dry, and well-ventilated space to prevent fungal growth.

Source: Agricultural Officer, Kerala State Land Use Board