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Onion
Onions preserve themselves if you let them. The step everyone skips is curing — the two or three weeks of drying that seals the neck and turns a soft, wet-necked bulb into something that will hang in a braid until March. Get that right and there is very little else to do.
The season
Lifted in August and early September when the tops fall over, then cured for two to three weeks before they go into store.
Acidity decides the method
Onion is low-acid at roughly pH 5.3–5.8, which is why adding a generous quantity of it to a tomato sauce changes what that jar needs.
This is a reference guide, not a recipe authority and not medical advice. The one organism the safety rules here are written against is Clostridium botulinum, and the one way to be protected against it is a tested recipe followed exactly — substitutions of the acid, the sugar or the salt are not cosmetic. Where the national food authority publishes advice for a specific food (Baltic fish, wild mushrooms), that advice outranks this page.
Method by method
| Method | Verdict | The note |
|---|---|---|
| Freezing | Good | Chop and freeze raw, no blanching needed. They go soft, which does not matter for cooking. |
| Drying | Good | Slice thin and dry at 57–63 °C. Powerful smell — dry them apart from anything sweet. |
| Fermenting | OK | Ferments well in a 3 % brine and makes a good condiment, though it is rarely the main event. |
| Pickling in vinegar | Best | Small whole onions in a tested vinegar brine are one of the oldest pickles there is. |
| Jam and sugar preserves | Good | Onion marmalade is a real and excellent preserve; it lives in the fridge unless the recipe was tested for canning. |
| Water-bath canning | OK | Only as a properly acidified vinegar pickle. |
| Pressure canning | OK | Possible for plain onions; rarely worth the jar. |
| Curing and salting | Good | Curing here means drying the neck after lifting, and it is the whole basis of onion storage. |
| Smoking | Poor | Flavour only. |
| Root cellaring | Best | Cured, then stored cold and dry: 0–5 °C at 60–70 % humidity, netted or braided, with air around them. |
The classic preparation: Curing and braiding
Lift the onions when the tops have flopped over and yellowed, on a dry day, and leave them somewhere warm, dry and airy with the leaves still attached — a shed, a greenhouse bench, a covered porch — for two to three weeks. The neck shrinks and seals, the outer skins go papery, and the bulb stops losing water. Only then trim the roots and either cut the tops to a couple of centimetres or braid them. Store cold and dry, in a net or a braid with air circulating, and check for soft necks every few weeks. Onions that never got their curing period will rot from the neck down within a month, and no amount of cold will save them.
Preparation that matters
Sort before storing: thick-necked bulbs, split ones and anything that stayed green never keep, so eat those first. Curing is dryness, not warmth — a humid shed is worse than a cool airy one. For freezing, chop straight into bags without blanching; onions have little of the enzyme that spoils other frozen vegetables, and they will be cooked anyway. Freeze them flat so you can break off a portion.
What the preserve is worth
Onions are eaten for flavour rather than nutrients, and their preserved forms lose little that matters. The exception is onion marmalade, where the sugar dominates: it is a sweet condiment and should be counted as one. Dried onion is intensely concentrated and a teaspoon replaces a good deal of fresh onion, so it is worth remembering that the weight on the jar is not the weight it started as.
The ways this goes wrong
- Storing onions in a damp cellar, where they rot at the neck. They want the cold, dry corner, not the sand box.
- Storing onions and potatoes together: potatoes give off moisture and onions do not tolerate it.
- Skipping the curing weeks.
- Adding a large quantity of raw onion to a water-bath-canned sauce, which shifts the whole jar to low-acid.
Safety rules, temperatures, blanching times, headspace and the pH 4.6 line: USDA Complete Guide to Home Canning and the National Center for Home Food Preservation guidance (US Government / land-grant publications, public domain). Acidity classes: the FDA's published approximate-pH tables. Jam composition: Directive 2001/113/EC. Fish freezing before raw or lightly cured use: Regulation (EC) No 853/2004, Annex III. Checked 2026-09-06.
Sources for this food
Frequently asked questions
Why do my stored onions sprout or rot?
Usually because they were not cured, or because the store is too humid. Onions want cold and dry — 0–5 °C at around two-thirds humidity — which is the opposite of what carrots and cabbage want.
Do onions need blanching before freezing?
No. Onions, peppers and herbs are the standard exceptions to blanching. Chop and freeze.
Can I add extra onion to a canned tomato sauce?
Not to a boiling-water-canned one, unless the tested recipe includes that quantity. Onion is a low-acid vegetable and enough of it moves the jar past the pH 4.6 line.
Related pages
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The full table, season by season, with the classic preparation for each.
Nutrition figures
The published USDA row for this food, per 100 grams.
When to sow and harvest it
The sowing calendar entry for this crop in this climate.
How long it keeps fresh
Fridge, freezer and pantry days for this food before it is preserved.
From the garden to the log
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