How to Dehydrate Onions in Canada (and Make Onion Powder)

Onions dry easily, grind into the most useful powder in the cupboard, and stink out your house while they do it. The method below is short. The two things worth reading are the section on what dried onion must never go into, and the one on where to put the dehydrator.

Onions are also the crop this site has been pointing at for a year without having anywhere to send you. They show up in season on what’s ready now and they are named as the low-acid vegetable in almost forty sentences about why you cannot water-bath a sauce. This is the page that was missing (when to harvest onions).

Which onions

Storage onions, which is what a Canadian autumn produces anyway. Copra, Cortland and Stuttgarter are the keeper varieties named on our root cellaring guide, and the qualities that make an onion keep — dense flesh, low water, papery skin — are the same ones that make it dry fast and grind clean.

• Sweet onions (Walla Walla, Vidalia-type) dry perfectly well but hold more water and more sugar, so they take longer at the same temperature and brown more. Not a problem, just a slower tray.
• Red onions dry to a pale mauve and make an attractive powder. The colour fades in storage; the flavour does not.
• Green onions and scallions are a different job entirely — they dry in an hour or two and behave like a herb. Treat them the way you would dried herbs.

What you need

• A dehydrator with a temperature control, or an oven that will hold a low temperature with the door propped open (how to dry without a dehydrator)
• A sharp knife or a mandoline — even thickness is the whole game
• A spice grinder, high-speed blender or food processor — only if you want powder
• Airtight glass jars and a food-safe silica gel packet

Method

Step 1: Chill, then cut

Put the onions in the fridge for an hour before you start. A cold onion gives off less of the volatile sulphur that makes you cry, which means less of it leaves the slices — you keep flavour and your eyes.

Trim root and neck, peel off the skin and any tough outer layer, halve pole to pole, and slice 3 to 5 mm. Separate the slices into rings.

Step 2: Load

Single layer. Rings may touch, but any overlap traps moisture between the two faces and those pieces will still be pliable when everything else is done.

Step 3: Dry at 50 to 55 °C

Then stop watching the clock. Ours usually run 6 to 12 hours, and that range is this site’s own — no Canadian authority publishes a drying chart, and the honest reason a range that wide is useful is that it depends on your machine, your slice thickness, your onion and the humidity in the room.

The finish test is the actual instruction: a finished piece shatters. Take one out, let it cool for a minute, and bend it. If it snaps and crumbles, the tray is done. If it folds, bends, or feels cool and leathery, it is still holding water and goes back in. A piece that feels dry while warm can still be pliable once cool, which is why you cool it before testing.

Step 4: Cool, then jar or grind

Cool fully on the trays — at least half an hour. For slices, jar them. For powder, grind in batches for 30 to 60 seconds, sift, and re-grind whatever the sieve holds back.

⚠ Dried onion is not dried herbs

This is the one part of the page that can hurt someone, and it is a distinction people get wrong in a specific, predictable way.

Bernardin’s tested Tomato Sauce lists its ingredients as tomatoes, bottled lemon juice or citric acid, optional salt, and optional dried herbs to taste. Dried herbs are allowed in that jar. Onion is not — and it is not on the list in any form.

The difference is not dried versus fresh. It is teaspoons versus cups. Dried herbs go into a jar by the teaspoon and add no meaningful mass or water. Onion goes in by the cup, and an onion is a low-acid vegetable however you dried it. Drying removes water. It does not add acid, and it does not move the pH of what ends up in the jar once the onion rehydrates in the sauce.

So: if a tested recipe does not list onion, dried onion is not a permitted substitute for the onion it does not list. The same goes for onion powder, which is the version most likely to feel like a harmless seasoning rather than a vegetable.

Where dried onion belongs is in cooking you eat now, or in a jar that was never going to be shelf-stable on acid alone — soups, rubs, stocks, dips, anything you make and refrigerate. If you want onions in a jar on the shelf, use a tested recipe built for them.

Curing is not dehydrating

Two different operations, easy to conflate because both dry an onion.

	Curing	Dehydrating
What it is	Drying the outer skin and neck of a whole onion	Removing the water from cut onion
Result	A whole onion that keeps in storage	Shatter-crisp slices or powder
Where	Single layer, greens attached, 2 to 3 weeks	Dehydrator trays, hours
Then what	Cool, dry storage — 6 to 9 months	Airtight jar, cupboard

Curing is covered on the root cellaring guide, which gives the conditions, the timing and the varieties. If your onions are whole and sound and you want them to last the winter, cure them and store them — dehydrating is for the ones with a soft spot, the ones you have too many of, or the ones you want as powder.

The smell, and where to put the dehydrator

Warm air over cut onion for eight hours puts onion into every soft surface in the room. This is the most common complaint about drying onions and nothing is going wrong.

Run it behind a door you can close. A garage in spring or autumn, an unfinished basement, a covered porch, a spare room with the window open. Not the kitchen, and not next to a bedroom overnight. A garage clears in about a day; fabric takes several.

Storage

• Airtight glass, with a food-safe silica gel packet
• Cool, dark and dry, away from the stove and the sink
• Best quality about 12 to 18 months — and that figure is this site’s convention, not a published one. No Canadian authority we can reach publishes a storage window for home-dried vegetables, so treat it as a quality guide rather than a date.
• Never shake the jar over a simmering pot. The steam goes straight up into it.

Onion powder takes up moisture faster than dried tomato or mushroom. If it cakes, it has not gone off — break it up and carry on. If it smells musty or shows any spot of mould, the batch went into the jar with water still in it; discard it and dry the next one to the shatter test.

Troubleshooting

• Some pieces bend, some shatter. Uneven slicing, or overlapping rings. Put the bendy ones back on a tray and finish them.
• The slices went brown. Temperature too high, or a sweet onion at a storage-onion setting. Harmless, but drop to 50 °C next time.
• The powder caked solid. Ground while warm, or jarred without a desiccant. Still usable.
• The jar smells musty. Under-dried. Discard it — this is the one failure that is not cosmetic.
• It clouded up the inside of the jar. Residual moisture. Get it back on a tray now, before it moulds.

Next steps

• How to make tomato powder — same grinder, same jars, more umami
• How to make tomato powder — the third of the three powders worth keeping
• What can you dehydrate? — the whole list
• Root cellaring 101 — curing and storing whole onions instead
• Why you cannot water-bath spaghetti sauce — the low-acid onion rule in full