Onion Store

A carrot or a potato has no protective structure of its own worth speaking of: its skin is thin, water leaves through it readily, and the storage problem is to slow that loss by keeping the surrounding air near saturation. An onion has been given a different solution by its own biology. During curing, the outer scales dry into papery layers and the neck — the point where the leaves met the bulb, and the obvious route for infection — dries and closes. Those dry tissues are a barrier that rot organisms cannot easily cross.

A barrier made of dryness only works while it stays dry. Raise the humidity around a cured onion and its scales take up moisture, soften, and cease to be papery; the neck rewets and stops being sealed. Worse, free water on the surface — from condensation, from a humid ventilating airstream — is what rot organisms need to germinate and infect. The bulb has not changed, but its defences have been dissolved. This is why humidity control in an onion store points downward while in almost every neighbouring store it points upward, and why the two crops cannot share a store even though they are both field vegetables lifted in the same season.

A bulk of onions is a bulk of living organs, each respiring: consuming sugars and oxygen, releasing carbon dioxide, heat, and water vapour. That water vapour is released into a store whose entire purpose is to be dry, and it is released continuously for as long as the crop is stored. Ventilation is how it is removed. Air driven through the bulk picks up the moisture and the heat of respiration and carries both out of the store, and it does so at every point in the mass rather than merely at the surface — which matters because the neck of a bulb buried in the middle of a pile is exactly where a rot will start unnoticed.

Ventilation is also how the crop is brought down to and held at a temperature at which it stays dormant. But the two aims constrain the air that can be used: air that is cool enough to cool the bulk may be too humid to dry it, and in a maritime climate cool night air is frequently saturated. Ventilating with such air delivers exactly what the store exists to remove. So onion store control is a decision about the ambient air’s temperature and humidity together, and where ambient air cannot satisfy both, refrigerated and dehumidified air is used instead. Target temperatures, humidities, and airflows are commodity- and climate-specific and are set by the applicable national storage guidance.

Everything an onion store does depends on the bulbs arriving cured. Curing is a distinct operation — holding the harvested crop under warm, moving air until the necks dry and seal and the outer scales become papery — and it happens in the field, in a shed, or in the store operated deliberately in curing mode, before storage conditions are applied. A bulb whose neck has not sealed goes into store with an open route to its interior, and no amount of careful ventilation afterwards closes it.

This is why neck rot is the classic onion storage failure and why it is usually a curing failure rather than a storage one. The infection is frequently established in the field before lifting and develops in store, and inadequate curing is what lets it in. The same applies to mechanical damage: bruised, cut, and skinned bulbs are entry points, and a store full of damaged crop will fail regardless of how it is run. What the store controls is whether the condition curing established is maintained. What went into it decides whether there was anything worth maintaining.

Versus onion and garlic curing

Curing is the conditioning step that dries the necks and scales. An onion store maintains the resulting condition. Some buildings do both, in sequence, under different settings — but they are different operations with opposite temperature and airflow objectives.

Versus potato storage

Same-looking building, opposite humidity target. Potatoes are held humid to prevent shrivel; onions are held dry because dryness is their defence. The crops cannot share a store.

Versus a root cellar

A cellar is cool and humid — which is precisely wrong for onions. Traditional onion storage in dry climates uses airy, ventilated structures, not earth-contact ones.

Versus cold storage

Refrigerated onion storage exists and is standard in humid climates, but it is cold storage run dry rather than run humid, which is unusual among refrigerated produce stores.