Properly harvested, cured and stored onion bulbs keep for a long time, often for three months or more. Improper storage conditions cause the bulbs to decline in quality and spoil more quickly. Onions don't require refrigeration until after they are cut. The moist, cool conditions in the fridge can speed spoilage and lead to waste. Recognising the signs of an onion that has gone bad ensures you only use fresh, top quality bulbs during meal preparation.
Dispose of onions with soft spots, as these indicate rot. If the rot only penetrates the top layer of skin, peel this layer from the onions and use the remaining onion immediately.
Throw out onions that sprout. Sprouting alters the flavour of the onion and the remaining bulb quickly declines after the onion produces a green shoot.
Smell the onion. Fresh onions have a slightly sweet, mild onion scent. Old onions and those beginning to spoil become more pungent, lose their sweet aroma and may smell slightly rotten.
Cut into the onion with a knife. Onions past their prime have a soft layers instead of the crisp layers of a fresh onion.
Store onions in a cool, dark and dry location to prevent rotting and sprouting. Moisture, warm temperatures and bright lights cause onion spoilage or sprouting. Diced onions freeze well, but keep them in a tightly sealed container or bag to prevent freezer burn.
Onions absorb odours from other food and spread their odour to other items stored nearby. Avoid cutting the onion until you are ready to use it to minimise spoilage and odour transfer.