Dry Bulb Onions

Onion harvest begins when the tops fall over and start to dry down, signalling that bulb growth has finished. The bulbs are undercut or lifted from the soil and left with the tops attached at first, because the drying foliage is part of what the bulb draws on as the neck closes. Lifting too early gives a bulb with a thick, wet neck that will not cure properly; leaving the crop too long risks skins splitting and the bulb starting to regrow roots.

The onion is non-climacteric. Unlike a tomato it will not develop or improve after being detached — there is no ripening process to complete. The bulb is a dormant storage organ, and everything post-harvest handling can do is preserve the condition it was lifted in and coax it into dormancy. Nothing later in the chain will make a poorly filled or immature bulb into a good one.

Curing is the drying of the onion’s neck and outer scales after lifting. Two things are achieved at once. The neck — the wound left where the tops are or will be removed — dries and seals, closing the most direct route by which rots enter the bulb. The outermost fleshy scales dry into the thin papery skins that armour the bulb against water loss and mechanical damage in handling. Until both have happened, an onion is a perishable vegetable; once they have, it is a storage commodity.

Curing is done in the field, in windrows or under the tops, where climate is dry and reliable; or in forced-air rooms and barns where it is not, which gives control at a cost in energy and handling. Either route can produce a well-cured bulb. What matters is that the neck is genuinely dry through, not merely dry on the surface — a bulb that looks cured but has a soft neck will rot in store, and the failure appears weeks after the point where it was caused.

Cured onions are graded on firmness, size and uniformity, skin retention and colour, neck condition, and freedom from defects. Skin retention deserves particular attention because it is both a quality attribute in its own right — buyers want a bright, intact, well-covered bulb — and the bulb’s physical protection, so a lot that has shed its skins in handling is both less saleable and more fragile than the same lot with skins on. Onions are also traded by type and colour, since red, white, and yellow onions and the milder sweet types serve different markets.

• Thick or soft necks — from immature lifting or incomplete curing, and the usual route for rot
• Neck rot and bacterial soft rots — often established before storage and only visible later
• Sprouting and rooting — dormancy breaking in store
• Bruising and skin loss from rough handling
• Doubles, splits, and bolted bulbs
• Black mould and other surface moulds on damaged or poorly cured bulbs

A recurring theme in onion grading is delay: most of the serious defects are set at lifting or during curing, but present themselves in store or on arrival. A lot can grade well out of the field and fail months later. This is why the curing record matters commercially and why long-store lots are selected on how they were handled, not only on how they look.

A well-cured onion can be held for months, and the object of storage is to keep the bulb dormant and dry. Onions are not chilling-sensitive, so unlike tomatoes or cucumbers they tolerate genuinely cold holding; the enemies are sprouting, rooting, and rots, and these are managed with temperature, low humidity, and continuous airflow through the bulk. Onion storage is distinctive in wanting dry air — most fresh produce is held humid to stop water loss, while onions are held dry to keep skins and necks sound.

Storage life also depends on the onion itself: cultivars differ substantially in dormancy, and pungent storage types generally hold far longer than mild sweet types, which are traded as a short-season commodity. Onions move in bags, nets, bins, and cartons, ventilated rather than sealed, and the same airflow logic that governs the store governs the load.