Onion Storage in India: Ventilated Chawl vs Low-Temperature Cold Room

Most fresh produce is stored at 85–95% RH to stop moisture loss. A cured onion is different: dry outer scales and a closed neck already seal the bulb, and dry air keeps it dormant. In humid air, onions put out roots, neck rot and black mould spread across the scales, and bacterial soft rot starts in bruised bulbs. That is why the HyperCold temperature chart lists onion at 65–75% RH, next to garlic, and why an onion store should be designed as a dry room that happens to be cold — the opposite of a potato cold storage, which needs 90–95% RH.

Temperature works differently too. Onions sprout fastest at intermediate temperatures, so storage works at either end of the scale:

Some weight loss happens in every method; the aim is to keep it to moisture loss from the outer scales rather than rot. For context, the NABCONS 2022 study cited in Lok Sabha Unstarred Question 1773 (5 December 2024) puts post-harvest losses for vegetables as a group at 4.9–11.6%, and storage is the stage a store owner controls.

Curing dries the neck and outer scales so the bulb seals itself. Only mature, properly cured bulbs belong in any store:

1. 1Harvest when most tops have fallen over; immature, thick-necked bulbs store poorly.
2. 2Field-cure in windrows for a few days, with the leaves of one row covering the bulbs of the next to prevent sun scald.
3. 3Cut the tops leaving a short neck, then shade-cure until necks are tight and outer skins are dry and papery, usually one to several weeks depending on weather and variety.
4. 4Grade out doubles, bolters and bruised, sprouted or diseased bulbs before bagging.
5. 5Load only dry bulbs, never straight after rain or on a humid morning.

Rabi onion, harvested in the dry season, stores far better than kharif onion, which matures in the wet season and is mostly sold fresh. In the major onion belts of Maharashtra, Karnataka and Madhya Pradesh, storage therefore means holding rabi stock from the summer harvest into the lean months before the next kharif crop arrives.

A well-built chawl with a raised, slatted floor and ventilated sides holds rabi onion at low capital cost and almost no running cost, and it remains the default for farmers and FPOs. Controlled cold storage costs far more to build and run, so it only pays when the stored onion earns that back. It tends to make sense when:

• the buyer needs onion on fixed dates through the monsoon, such as exporters, processors, dehydration units and retail chains;
• the lot is high value (export grade, sized and graded) and a monsoon rot loss would be expensive;
• humid weather in the region makes chawl losses unpredictable from year to year;
• the chamber can hold other low-humidity crops, such as garlic, in the remaining months.

It usually does not pay for kharif or poorly cured onion, when the price gap between harvest and the lean season is thin, or when stock cannot be sold within days of coming out, because cold-stored onions sprout soon after removal. A chawl with fans that push air through the stack when outside air is dry is a middle step between the two. These are qualitative guides; test your own price spread, holding period and power tariff in the ROI calculator before committing.

At 0–2 °C the evaporator coil runs below the room dew point, so the refrigeration removes moisture on its own, but only while the compressor runs. During off-cycles and defrost, and whenever humid outside air leaks in, RH climbs. An onion room therefore needs a humidity sensor and controller, an evaporator selected for a larger coil-to-room temperature difference than a high-humidity produce room, defrost water drained out of the chamber, and, at humid or coastal sites, a desiccant dehumidifier. The humidifier that belongs in potato and fruit rooms has no place here. Dehumidifiers are not on HyperCold's standard rate card and are priced after the site survey.

Airflow has to pass through the stock, not only around it. Store onions in mesh (leno) bags or ventilated crates on pallets, with gaps between rows along the air path. Bags piled on the floor without air channels create warm, humid pockets in the middle of the stack, which is where sprouting and rot begin.

A bulb at 1 °C carried into monsoon or post-monsoon air is far below that air's dew point (air at 30 °C and 80% RH has a dew point of about 26 °C), so water condenses on the scales within minutes. Wet scales at warm temperatures suit mould and soft rot, and cold-stored onions also come out of dormancy quickly, so rooting and sprouting follow.

The answer is a controlled warm-up:

• Move dispatch lots to an ante room or tempering chamber and raise the temperature in steps over a few days, with dry air moving through the bags.
• Keep the bulb surface dry throughout; if condensation appears, slow the warm-up.
• Plan removal against confirmed orders, so warmed onions are sold and moved within days.
• Never return warmed lots to the cold chamber.

An ante room also protects the cold chamber, because every door opening straight to humid ambient pushes moisture into a room that is trying to stay dry. On the HyperCold FY2026-27 rate card (GST extra), an ante chamber is ₹89,250 per unit and a PVC strip curtain ₹8,925 per door.

HyperCold's configurator counts onion at 550 kg/m³ as stacked (bulbs plus bag or crate) and uses 0.62 of gross volume in a palletised room, after aisles, 100–150 mm wall clearance, a 300–450 mm ceiling gap and the evaporator air path. Capacity (MT) = gross m³ × 0.62 × 550 ÷ 1,000, or about 9.7 kg per cubic foot of gross volume.

Onion stacks should not be packed as tightly as potato: keep the row gaps described above and avoid stacking so high that the bottom bulbs bruise. Keep onion in its own chamber, because its odour taints eggs, dairy and fruit stored nearby.

Because an onion room is a dry room, check that the quotation covers the humidity side as carefully as the cooling side:

• the setpoint and RH target written into the specification (0–2 °C, 65–75% RH);
• the dehumidification method, and whether the evaporator was selected for low-RH duty;
• a humidity sensor with logging and a high-RH alarm, not just a thermostat;
• defrost drainage out of the chamber and an insulated floor with vapour barrier;
• an ante room or tempering chamber for warm-up before dispatch;
• no humidifier in the scope.

Rate-card references (HyperCold FY2026-27, GST extra): 100 mm PPGL PUF panel ₹2,032 per m², 100 mm insulated door ₹31,080, Premium Digital HMI (IoT) controller ₹36,540, and automation with remote monitoring, alerts and data logging ₹78,750 per system. Final pricing depends on the site survey; build a line-item BOQ in the quotation configurator.