When buying onions, you might occasionally notice dark spots or a sooty, black discoloration on the dry, papery outer skins. This common post-harvest issue is often mistaken for dirt or minor bruising, but it is actually a sign of a fungal presence. These black patches affect the onion’s cosmetic appeal and can raise questions about its freshness. The fungal growth often begins while the onions are still in the field or during the curing process, posing a challenge in bulb storage.
What Causes Black Spots
The black spots are caused by the fungus Aspergillus niger, commonly known as black mold, which is naturally abundant in soil. This mold typically manifests as dry, powdery, black streaks or patches concentrated on the outer, dry scales or near the neck of the onion. The black color comes from the dense collection of spores produced by the fungus. The fungus is opportunistic and primarily gains entry through wounds, such as those made when the tops are cut at harvest or any physical damage. Aspergillus niger thrives in warm, humid conditions, which is why infections often develop during transport or storage if temperatures are high and ventilation is poor.
Is It Safe to Eat
When considering food safety, the location and extent of the black mold determine whether the onion can be salvaged. Aspergillus niger is generally considered a surface mold, preferring to grow on the dry outer layers and rarely penetrating the fleshy inner scales immediately. If the discoloration is strictly confined to the dry outer skin, the onion is safe to use after the affected layers are thoroughly peeled and the underlying flesh is firm and white.
Mycotoxin Risk
However, the safety concern increases because some strains of Aspergillus niger can produce mycotoxins, such as ochratoxin A, which can be harmful if consumed in large quantities. These toxins are resistant to heat, meaning cooking will not destroy them. If the black mold has penetrated the inner fleshy layers, caused the onion to soften, or resulted in a foul odor, the entire bulb should be discarded. This is important for individuals who have weakened immune systems, as they are at a greater risk from spoiled produce.
Preventing Black Spots Through Proper Storage
Controlling the environment after purchase is the most effective way to prevent black mold from developing and spreading. Since the fungus flourishes in moisture, the primary goal of storage is to maintain a low-humidity environment with adequate air circulation. Onions should be stored in a cool, dry, and dark location, but they should not be kept in the refrigerator, as high humidity encourages fungal growth.
Storage Methods
It is best to store onions in mesh bags, baskets, or open crates, avoiding plastic bags or sealed containers that trap moisture and inhibit airflow. Keeping onions separated from produce that releases moisture, such as potatoes, also helps maintain dry conditions. Before placing onions into storage, inspect them for any cuts, bruises, or soft spots, as damaged bulbs provide easy entry points for the fungus and should be used promptly.