Curing and storing onions: how bin choice protects the crop

Onions are stored, not just shipped. Get curing and airflow right and they hold for months; get it wrong and you watch the crop soften, sprout and rot in the shed before it’s sold. The bin you cure and store in is not a detail — it decides whether dry air reaches the middle of the stack or stops at the outer skin. This guide covers why onions cure, the airflow curing and storage each need, what poor storage costs, and how to choose bins that breathe.

Why does onion bin choice matter so much?

Because an onion is still alive in the bin and the bin either helps it breathe or smothers it. Freshly lifted onions respire — giving off heat, moisture and ethylene for the entire time they’re stored. A genuinely vented bin lets that load shed heat and dry down; a solid box traps exactly the warmth and humidity that rot a neck. With a crop that sits in store for months, the container is doing real work every day, not just on the trip in.

The difference shows up where you can’t see it. In a sealed or barely-vented box the outside layer looks fine while the centre of the load sweats, warms and softens — the first soft bulbs are usually buried three layers deep before anyone notices. Vented walls and an open base keep the whole column even in temperature and humidity, so the crop you tip out in spring matches the crop you put away at harvest.

Why do onions have to be cured first?

Curing dries the necks and outer skins down so the bulb seals itself against disease before long storage. A freshly pulled onion has a soft, green neck — an open wound that botrytis neck rot exploits. Drive warm, moving, dry air around every bulb for a week or two and the neck tightens, the outer scales go papery, and the onion is ready to hold. Skip or rush curing and you store the crop with the door left open to rot.

This is where bin choice earns its keep, because curing happens in the bin on most Australian operations — there’s no separate step if the bin breathes. Bins that trap moisture hold precisely the dampness that seeds a rotten neck; bins that move air cure the crop in place with no extra handling. As the United Nations’ Food and Agriculture Organization puts it, around 14 percent of the world’s food is lost… between harvest and retail, and post-harvest handling and storage are where much of that vanishes (FAO, The State of Food and Agriculture 2019 — fao.org). For onions, the cure is the first and cheapest defence against ending up in that 14 percent.

For bulk onion, a full-height vented Australian-Standard box cures and stores in one bin: dry air moves through the vented walls and base, the food-grade HDPE hoses down between crops, and it folds to roughly a third of its height to free shed space once the season ends. At ~750 L it holds around 410 kg of onions on the standard 1165 footprint, so it palletises and racks the same as your other crops.

What airflow do curing and storage actually need?

They need opposite settings through the same bin: curing wants warm, fast, drying air; long storage wants cool, gentle, steady air. The bin’s job in both phases is identical — present an open path so the air your fans push actually reaches the centre of every stack rather than skating around solid walls. The table below sets out the broad targets growers work to and what each phase demands of the container. Treat the figures as planning ranges; your agronomist, variety and climate set the exact numbers.

The thread through every row is the same: a bin that only vents through a few wall slots leaves the core of a deep load starved of air in the phase that matters most. For onions, the base venting does as much work as the walls, because warm, moist air sinks and has to be able to leave through the floor of the stack — not pool on a solid pan under the bottom layer.

How much does poor onion storage really cost?

More than most growers tally, because the loss is spread across a long holding season instead of landing as one invoice. Shrink (moisture loss), neck rot and premature sprouting all accelerate when heat and humidity build in a stack — and they compound the longer the crop is held. The FAO’s ~14% harvest-to-retail loss figure above is the global backdrop; for a crop that can sit in store from autumn harvest through to the following spring, the storage window is exactly where a grower wins or loses margin.

Australia treats that waste as a national priority. Through the National Food Waste Strategy, the Australian Government has committed to halving food waste by 2030 (Department of Agriculture, Fisheries and Forestry — agriculture.gov.au). On-farm and packhouse storage sits squarely inside that target: every bin of onions that ships instead of softening is product sold and waste avoided. Breathable bins don’t make the crop, but they protect the percentage of it that survives the months between lifting and loading.

Where onions are graded by hand, sold in smaller lots, or you want to limit how deep the load sits during curing, a low vented half bin keeps the same 1165-class footprint but drops the fill depth. A shallower column cures faster and more evenly — there’s simply less crop for the dry air to push through — and at ~470 L it holds around 260 kg of onions, which is easier to lift, tip and rotate through a packing line.

Why vented plastic over timber field bins?

Because timber works against an onion crop in three ways plastic doesn’t: it absorbs moisture, splinters, and carries disease between seasons. A timber bin soaks up the very dampness you’re trying to drive out of the necks, its rough and nail-studded surfaces bruise skins that then break down faster, and porous grain harbours neck-rot spores that reinfect next year’s crop. Food-grade HDPE has none of those failings — smooth, sealed, non-absorbent and washable.

• Non-absorbent. HDPE doesn’t wick moisture into the load or hold it against the bottom layer the way damp timber does.
• Smooth, radiused mouldings. No splinters, nails or staples to graze skins or end up in graded product — which matters on any HACCP-aligned line.
• Cleans between crops. A sealed plastic surface hot-washes and sanitises; raw timber can’t be reliably decontaminated, so it carries pathogens forward.
• Holds shape and lasts. Quality HDPE survives many seasons of outdoor and cold-store use without rotting, warping or shedding fasteners.

The hygiene case is the decisive one for a stored crop: rot you can’t wash out of a bin is rot you re-introduce every harvest. For the full material breakdown, read food-grade plastic explained, and see why the same logic applies across the fresh-produce range.

How do you choose bins for onions?

Start from the cure and the cool store, then match the spec. The bin has to breathe enough to cure a deep load, fit your handling, and wash clean between crops. Get those three right and the rest is detail.

• Genuine ventilation. Favour vented walls and an open or vented base — not a few token slots. For onions the base venting is critical, because warm moist air has to be able to leave through the floor of the stack.
• Food-grade HDPE. Smooth, non-absorbent and hot-washable, so the bin doesn’t hold moisture or carry neck rot forward.
• Fold or nest empty. A folding bin that drops to ~280 mm frees shed and cold-store space out of season instead of stacking air.
• Fill depth. A ~470 L half bin cures faster and suits hand-grading; ~750 L full bins cut lifts per tonne for bulk onion.
• Stack rating. Check the static (stacking) load against your store’s stack height — a bin that holds the crop still has to carry the bins above it.
• Standard footprint. 1165 × 1165 mm keeps one forklift, one racking pattern and one truck layout across all your crops.

Browse the full bulk containers category, compare vented volumes and load ratings in the product finder, or — if you also run grapes or other fruit through the same shed — see the vented picking-bin options used in winemaking and harvest. Not sure which fill depth suits your cure? Answer a few questions and we’ll recommend the right bin for your variety and store.

How do you keep onion bins clean between crops?

Wash and sanitise them — which is exactly what food-grade HDPE is built for and timber isn’t. A hot wash at roughly 60–80 °C with an approved detergent lifts soil, skins and spores off the smooth mouldings; a sanitiser rinse finishes the job, and the bin dries fast because nothing soaks in. That break in the disease cycle is the whole point: neck rot left in the grain of a timber bin reinfects next season, while a washed plastic bin starts each harvest clean.

Doing this well is a documented SOP, not a hose-and-hope — water temperature, detergent, contact time and drying all matter for a HACCP-aligned operation, and the open, smooth design of a good vented bin is what makes it quick. When the bins are ready to replace, send your variety, volume and store type for a spec-backed quote, or read the companion guide on why potatoes and onions need vented bins for the wider produce picture.

Common questions

Why do onions need to be cured before storage?

Curing dries the necks and outer skins down so they seal against disease before the crop goes into long storage. A green, undried neck is an open wound that botrytis (neck rot) walks straight into. Curing needs warm, moving, low-humidity air around every bulb — which a vented bin delivers in place, without rehandling the crop.

Can you store onions and potatoes in the same room?

No. Onions cure and store in dry, moving air and give off ethylene; potatoes want cool, slightly humid conditions and are sensitive to ethylene, which breaks dormancy and drives sprouting. The same vented bulk-bin design suits both crops, but they belong in separate stores tuned to each. See why potatoes and onions need vented bins.

How many kilograms of onions fit in a bulk bin?

It depends on the bin volume and onion bulk density — about 550 kg per cubic metre, since whole bulbs leave void space. A ~750 L vented bin holds roughly 410 kg of onions, and a ~470 L half bin around 260 kg. Potatoes pack denser (~650 kg/m³), so the same bin holds more. Our produce bin sizing chart has the full crop-by-crop maths.

Are plastic onion bins food-grade and hygienic?

Quality vented bins are moulded from food-grade HDPE — smooth, non-absorbent surfaces that hot-wash and sanitise clean between crops, with no nails or splinters to end up in product. In Australia and New Zealand, food-contact materials sit under the FSANZ framework. That makes them a far better fit for a HACCP-aligned packhouse than absorbent timber field bins that carry neck rot from one season to the next.

What is the standard Australian onion bin size?

1165 × 1165 mm is the common footprint for Australian field and storage bulk bins. It palletises neatly, squares up in the cool store and on the truck, and works the same forklift and racking as your other crops. Volumes step up on that base — a ~470 L half bin for hand-filling and gentler grades, and ~750 L full bins for bulk onion.

Sources: FAO Platform on Food Loss and Waste and The State of Food and Agriculture 2019 (~14% of food lost between harvest and retail); Department of Agriculture, Fisheries and Forestry, National Food Waste Strategy (halve food waste by 2030); Food Standards Australia New Zealand (food-contact materials). Curing and storage targets are indicative planning ranges; onion fill weights are calculated from a bulk density of ~550 kg/m³ and vary with variety, grade, moisture and fill method. Curing and storage outcomes depend on your variety, climate, cold chain and shed — treat this as general guidance, not a quote.