Curing Onions for Storage

19 July 2013, written by Barbara Pleasant

Of all the articles I have written here, the one with the longest list of comments is The Art of Harvesting Onions, which appeared three years ago this week. More than 80 of you have contributed thoughts and ideas on the subject, so clearly there is much to discuss.

Picking up where I left off, this time around I will focus on the fine points of how to cure onions, which you will need to do if you expect your onions to store well. Fresh from the garden, onions are full of sugary juices, and they wear only a light covering of skin. As onions cure, the skins dry into papery wrappers, pungent compounds replace sugars, and the necks at the top of the bulb come together to seal out moisture and microorganisms.

Of course, not all onions need to be cured. Gardeners in warm climates who grow mostly short-day onions need only dry the harvested onions for a few days before trimming them and moving them to the refrigerator. I do the same with some of my heirloom onions, most notably my red Italian torpedo onions, which don’t store worth a flip even when perfectly cured. Instead of going to the trouble of curing sweet onions like these, I think it’s best to let them dry in warm shade for less than a week, and then put them in the fridge and start eating them.

How to Cure Onions

As for full-season onions with good storage potential, commercial onions are typically cured at very warm temperatures for six weeks, and then gradually cooled down to refrigerator temperatures. However, recent research indicates that curing onions naturally, by keeping them in a warm, dry place for a month, will work just as well. Thinking it might save energy, a team of UK researchers set onions to cure at various temperatures, and found that 68°F (20°C ) worked almost as well as 82°F (28°C), the standard recommended temperature for this process.

This does not surprise me, but it barely touches the surface of how to cure onions at home. In my experience, newly pulled onions benefit from being dried immediately, but not in direct sun, which can cause uneven drying. After trimming off shriveled leaves and tickling clods of soil from the roots, I lay my newly pulled onions in a single layer in a covered shed, where temperatures range between 60°F (15°C) at night and 80°F (27°C) during the day. There they rest for about a week, and I try not to touch them. Uncured onions bruise easily, so it’s important to handle them like eggs.

Opinions differ on whether it is best to lop off most of the green onion leaves or leave them intact. Recent research from Sweden indicates that the onions don’t really care. Once the necks have thinned to the point of breaking, the bulbs are no longer receiving nutrition from the leaves. If you want to let your onions finish curing in a braid, trim off all but the three newest leaves, and use the remainder for weaving, or for tying together small bunches of onions.

I like to top back onions gradually, leaving a longish stub of stem that will help protect the neck of the onion as it slowly shrinks until the opening is closed and the onion within is protected. During the four to six week curing period, I usually trim back the tops two or three times, but wait until the curing onions are ready to be brought indoors before clipping the necks off altogether.

Commercially-grown onions have their roots clipped off at harvest, but I like to let them dry naturally for a week or so, and then use scissors to trim the roots back to one-fourth inch (6mm). This small bit of patience gives the onion’s basal plate time to dry, signaling the onion to enter a state of deep dormancy.

How to Store Onions

Onion varieties vary in their natural dormancy periods. Some are ready to start regrowing after two or three months, while others will keep for six months or more in a cool, dry place. Refrigeration will keep short-dormancy onions quiet longer, but it’s still best to eat them fresh and save your storage efforts for bulb onions and long-storing shallots.

After a month or more of curing outdoors, you are ready to sort through your collection and select the most perfect onions for storage. At this point I take out obvious doubles, in which two bulbs are joined together, because doubles often develop a soft spot in the middle that lead to spoilage. I also set aside onions with split scales, because split wrapper scales cannot protect the onions from moisture loss or invasion by fungi and bacteria. I use these imperfect onions in canning and freezing, or we eat them fresh.

As for the keepers, they are allowed to cure one last time indoors, where it is consistently cool and they go deeper into dormancy, getting ready for winter. By the time winter actually comes, the basement is cool enough for long-term storage.

Sometimes I put onions in mesh bags for hanging, but more often they go into small baskets no more than three onions deep. In addition to allowing some air circulation, the shelves or containers you use for storing onions should be easy to check, because good onions can go bad unexpectedly. Each time I bring up onions from the basement, I take a minute to sniff around for signs of trouble. When a stored onion goes bad, you will smell it long before you see it.

By Barbara Pleasant