The onions we use for cooking and relishes come to us in their own skins. Depending on the amount of sugar in the bulb, most onions keep well in a cool place for days, weeks or even months. The food safety of a chopped onion depends on how it has been handled and refrigerated.
Keeping Onions
Uncut onions keep for one to two weeks in a pantry and two months in a refrigerator. Sweet onions contain more sugar and less sulfur and are more susceptible to bruising and rot during storage, making them a more perishable crop. Refrigeration slows the enzymes that begin to break down plant tissue once it has been harvested. Freezing stops the process, but it also changes the texture of onions by crystallizing the fluid that fills their tissues, causing their walls to collapse. Cooked onions have already undergone this process and can be frozen without risk of losing their flavor. Begin by purchasing onions that are uniformly colored and that have no blackish bruising. Purchase sweet onions, whose skins are lighter, in season to avoid spoilage that develops during storage.
Refrigerators
Store whole onions in a mesh bag and whole sweet onions in a single layer in the vegetable bin. Protect them from freezing, which will make them squishy and tough to cut. Refrigerators, even the most expensive ones, respond to temperature and humidity in the environment around them, cranking up the cold on hot days. Interior temperature must be kept between 34 and 40 degrees to keep food from freezing and retard growth of bacteria and mold, which grows fastest on surfaces covered in moisture and carbohydrates, such as cover the edges of cut onions.
Cutting Onions
The first step in proper handling is to ensure that everything that comes into contact with the onion's flesh --- hands, knives, cutting boards -- is washed in hot water with soap to minimize the presence of bacteria such as salmonella and Toxoplasmic gondii. Onions, however, should not be washed. Even rinsing with water can carry surface bacteria between layers to the onion's flesh. Peel the protective skins and cut the ends. Use a clean knife or food processor blade to chop the onion. Cut onions with a sharp knife to avoid bruising.
Keeping Cut Onions
Extra onions or onions prepared ahead for a family picnic or cooking keep safely in zippered bags for the next day if they have been handled and stored safely. Squeeze as much air out as possible before sealing the first bag and seal it in a second bag to contain the onion's smell. Precut onions are best used within two days. Never keep leftovers in the refrigerator for more than three to four days.
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