Dear Lisa: Why do onions sometimes have black mold inside? Is it ok to just wash it off or use the part of the onion that is unaffected? -- G.G.
Dear G.G.: Black mold in onions is caused by a fungus that can be present in the soil where onions are grown, or it can infect the onion seeds before planting. The mold has an opportunity to grow if the onion is bruised or damaged during harvest or transportation, or if the onion is stored improperly.
When purchasing onions, make sure the neck is tightly closed. Avoid any with visible damage or soft or moist spots.
Onions are best kept in a single layer in a dry, dark, cool and well-ventilated place. That's the best way to store potatoes, too. But you should avoid storing onions with potatoes. Potatoes give off moisture, which can cause onions to rot.
Mold is not something you want to handle or ingest. Even if you wash it off, microscopic spores remain. It's best to discard any onions found to contain mold.