Onions are powerfully pungent vegetables that, once they spoil, produce an acrid odor. In addition, bad onions may appear to have discolored spots, or even be mushy and slimy. Ultimately, these are signs that the onion has lost its water content and is beginning to degrade.
A good onion should have crisp, dry outer skins and feel firm when squeezed. It should also have a vibrant color of either red, yellow, or white and a fresh appearance overall. Onions that have developed a gray, green, or black mold are no longer good and should be discarded.
The odor of a good onion is quite strong, but not overpowering or unpleasant. A bad onion will smell putrid, with a sharp, sour scent that can cause your eyes to water or your nose to run. A bad onion will also be mushy, slimy, or seeping, which is a sign that the vegetable has lost a lot of its water and is beginning to rot.
The most common way to tell if an onion is bad is to do the sniff test. If an onion smells bad, it is definitely no good and should be discarded. Onions that are mushy, slimy, or showing signs of mold should also be thrown away. In some cases, an onion that shows signs of rot on the outside will still be good inside. However, this is not always the case and it’s best to be safe than sorry by avoiding onions that show any kind of rot or mold on the outside or inside.