# Why do onions sweat? Condensation after coming out of storage

Onions sweat when colder than the surrounding air's dew point. We show a dew point table and how to release and receive onions so the skin stays dry.

An onion "sweats" when its surface is colder than the dew point of the surrounding air. A cold onion coming out of storage meets warmer, humid air, and water vapour condenses on the skin and between the layers. If the skin stays wet for a matter of hours, mould can take hold (black mould needs only 6-12 hours of moisture), and over time the skin darkens and the onion sprouts sooner. Below we show exactly when an onion sweats and how to release and receive stock so it does not happen.

## Dew point: when water condenses on an onion

The dew point is the temperature at which air of a given humidity starts giving up its water. If an onion is colder than the dew point of the air around it, the air next to its surface cools and water settles on the skin. It works the same way as a bottle taken out of the fridge on a warm day.

The dew point can be worked out from air temperature and humidity. Below are values calculated with the Magnus formula. They match the dew point tables found in storage guides:

| Air temperature | 60% humidity | 70% humidity | 80% humidity |
| --- | --- | --- | --- |
| 10°C | dew point 2.6°C | dew point 4.8°C | dew point 6.7°C |
| 15°C | dew point 7.3°C | dew point 9.6°C | dew point 11.6°C |
| 20°C | dew point 12.0°C | dew point 14.4°C | dew point 16.4°C |

How to read it: an onion sweats if it is colder than the dew point in the relevant cell of the table. Take an onion at 2°C, straight out of a cold store. In every one of the nine situations in the table, it will get covered in dew. In air at 20°C it takes only around 30 percent humidity or above, and in air at 10°C only around 58 percent or above.

Dew is not only a risk in warm conditions. At 5°C outside and humidity above around 81 percent, a typical foggy, late-autumn day, an onion at 2°C will also get wet.

## What water does on the skin

Dew on its own is not spoilage. The problem starts when the water stays for hours:

- **Storage diseases.** Temperature swings trigger condensation on the onion and between dry layers, which, according to the University of California's IPM programme (UC IPM), favours black mould. We write about this disease in our post on [onion black mould](/blog/czarna-plesn-cebuli).
- **Skin colour.** The USDA's storage handbook (Agriculture Handbook 66) states that condensation favours rot and changes the colour of dry skin. Onions with blotchy, darker skin look worse in a sack.
- **Roots and shoots.** Water on the surface of an onion speeds up sprouting. We write about what wakes an onion from dormancy in our post on [onion sprouting in storage](/blog/kielkowanie-cebuli-w-przechowalni).
- **Packaging.** Wet stock dries slowly in a sack, and paper packaging weakens from moisture.

## How we release onions dry

We run a store of 13 000 tonnes and release onions from it from autumn through spring, so releasing a cold batch into warm air is routine for us. Before winter dispatch, we let a batch come up to close to ambient temperature for 1-2 days and release it dry. We describe the whole temperature curve, from curing through cooling to release, in our post [at what temperature to store onions](/blog/w-jakiej-temperaturze-przechowywac-cebule).

Warming has to be slow. The rule from American onion storage guidelines (PNW 277) says you can only ventilate with air whose dew point is lower than the temperature of the coldest onions in the pile. If we let warm, humid air into the chamber to warm the batch faster, we would cause exactly what we are trying to avoid: dew would settle on the coldest onions in the middle of the pile.

{% captionedImage src="/images/video/zaladunek-magazyn.webp" alt="Onions being received into storage: onions moving from a trailer onto a conveyor in front of the warehouse" caption="In storage, temperature and ventilation steer the batch. The same rule applies on release." /%}

## What the receiving side can do

Condensation can also appear after delivery: in the trailer, on the loading ramp or in a packer's hall. The most common situations and what to do to limit the risk:

| Situation | What happens | What to do |
| --- | --- | --- |
| Cold onions enter a warm, humid hall | dew on the skin right after unloading | move the batch to a cooler, unheated zone and warm it gradually; ventilate a sweaty batch until it dries |
| A pallet wrapped in film over its full height | moisture stays trapped inside, sacks get wet | secure the pallet with net wrap instead of full film |
| Onions go straight from cold storage onto the line and into sacks | water trapped inside the packaging | let the batch equalise to ambient temperature before packing |
| A warehouse with large temperature swings | repeated cycles of dew | hold a stable temperature and ventilate only with air whose dew point is lower than the temperature of the onions |

A hygrometer that shows the dew point, and a thermometer with a probe pushed into an onion in the middle of a sack, are both useful here. If the dew point of the air in the hall is higher than the temperature of the onions, the stock will sweat. We write about how we protect a load in transit in our post on [onion transport and palletising](/blog/transport-i-paletyzacja-cebuli).

{% callout tone="warning" %}
Do not pack sweaty onions straight into sealed packaging. Let them dry out in moving air first. Water trapped in a sack stays on the skin the longest.
{% /callout %}

## Onions from storage, autumn through spring

We cure our onions for 2-4 days after harvest and sort them on the DOWNS CropVision optical sorter. Thanks to our store, we sell onions for most of the year, not just in autumn. We describe the whole process in our guide on [storing onions](/blog/przechowywanie-cebuli). If you are planning deliveries for winter and spring, take a look at our [onion offer](/oferta/cebula) and get in touch through the [contact form](/kontakt). We calculate the price based on current market rates.

## Frequently asked questions

### Is a sweaty onion spoiled?

Not straight away. Dew on the skin on its own does not spoil an onion. The problem starts when the water stays for hours, because that is when mould takes hold, the skin darkens and the onion sprouts sooner.

### Why do onions sweat even on a cool day?

Because what matters is the dew point, not just the air temperature. At 5°C and humidity above around 81 percent, the dew point is higher than 2°C, so an onion straight out of a cold store will get covered in dew.

### Does wrapping film protect onions in transport?

No. Film wrapped over the full height of a pallet traps the moisture the onions give off and favours sweating. Onions need packaging that lets air move through.

### How quickly can onions be warmed before dispatch?

Slowly. Before winter dispatch, we give a batch 1-2 days to come up to close to ambient temperature. Warming it quickly with warm, humid air triggers dew on the coldest onions.