Onions [German version]

Table of contents

Product information
  Container transport
  Cargo securing

Risk factors and loss prevention:
Temperature Odor
Humidity/Moisture Contamination
Ventilation Mechanical influences
Biotic activity Toxicity / Hazards to health
Gases Shrinkage/Shortage
Self-heating / Spontaneous combustion Insect infestation / Diseases

Product information

Product name

German Zwiebeln
English Onions
French Oignons
Spanish Cebollas
Scientific Allium cepa
CN/HS number * 0703 10 ff.

(* EU Combined Nomenclature/Harmonized System)

Product description

The edible onion belongs to the lily family (Liliaceae) and comes originally from Central Asia.

The onion (bulb) is composed of a highly compressed basal plate, which gives rise to the roots, and the main shoot apex, around which the thickened onion scale leaves are arranged. These end in the onion neck, from which the above-ground shoot or peduncle emerges. The outermost scale leaves are dry and protect the onion from external influences.

The ripening process starts when the onion bulb proper forms. Bulb formation is determined by day length and varies with the variety of onion.

Postharvest, the onion undergoes a post-ripening process. This post-ripening does not consist merely of the purely physical process of drying of the onions but also of the attainment of a certain physiological stage known as sprout or growth dormancy. During this process, the roots, peduncle and outermost scale leaves dry; the latter change color and lie tightly around the onion neck, providing protection against evaporation and attack from microorganisms. This process is further assisted by subsequent post-drying.

Quality / Duration of storage

When inspecting the cargo to be loaded, it is essential to pay attention to the following points:

the onions must not be wet or covered with condensation; instead, they must be dry and parchment-like, which may be discerned from the „lovely rustling sound“ the bag makes when lifted.
the onions must be fully developed and well ripened, they should be round and not flat and must not be soft to the touch.
the onion neck must be closed and not thick, which would be an indication of inadequate ripeness.
no peduncle residues must be left on the onions: these must be twisted off, not cut off, as there is otherwise a risk of onion neck rot.
the roots must be dry and free of soil, which contains rot pathogens.
the onions should where possible not have sprouted or have done so only slightly, since sprouting is an indication of improper storage and the risk of rot.
mechanically damaged, squashed onions must always be rejected, as they cause rapid spoilage due to more intense respiration. This effect is also caused by contamination with leaf residues.
loose-skinned onions are fit for shipping provided that they are dry and not damaged.
the onions must be sound and free from rodents.

Maximum storage duration is stated in [1] as three months.

Where controlled atmosphere transport is used, the transport and storage duration of onions may be extended. The following parameters apply in such a case [16]:

Designation Temperature Rel. humidity O2 CO2 Suitability for controlled atmosphere
Onions, dry 0.6 – 1.7°C 65 – 75% 1 – 2% 0% good
Onions, green 0.6 – 1.7°C 95 – 100% 2 – 4% 10 – 20% good

Intended use

Onions are mainly used for seasoning and in cooked dishes and salads. They are also processed to produce powdered onion, onion rings for frying and pickled pearl or cocktail onions.


(Click on the individual Figures to enlarge them.)

Photo, onions

Figure 1
Drawing, onion

Figure 2
Photo, onions

Figure 3
Photo, onions

Figure 4
Photo, onions

Figure 5
Photo, onions

Figure 6
Photo, onions

Figure 7
Photo, onions

Figure 8
Drawing, onion

Figure 9

Countries of origin

This Table shows only a selection of the most important countries of origin and should not be thought of as exhaustive.

Europe Netherlands, Italy, Poland, Spain, France, Hungary, Turkey, Israel
Africa Egypt, South Africa
America USA, Chile, Argentina, Canada
Australia Australia, New Zealand, Tasmania

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Onions are transported in wide-meshed jute fabric or plastic bags, crates, boxes, cartons, baskets or fruit crates.

Small-meshed bags are unsuitable for transport due to their low permeability to air.

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Symbol, general cargo

General cargo
Symbol, temperature-controlled


Means of transport

Ship, aircraft, truck, railroad

Container transport

As chilled goods: refrigerated containers with fresh air supply or controlled atmosphere.

As general cargo: actively ventilated containers, open-sided containers, open-top containers, flatracks (for loading below deck)

Cargo handling

Since onions are highly sensitive to impact and pressure, they must be handled with appropriate care.

Damage to onions, caused by setting them down roughly for example, results in greater respiration intensity and self-heating.

The required refrigeration temperature must always be maintained, even during cargo handling.

In damp weather (rain, snow), the cargo must be protected from moisture, since this may lead to self-heating, sprouting, root growth and packaging decay.

Stowage factor

2.40 m3/t (jute fabric bags, 25 kg) [1]
2.14 – 2.50 m3/t (crates, 27 kg) [1]
2.23 – 2.29 m3/t (cartons and fruit crates) [11]
2.60 – 3.80 m3/t (bags) [11]
2.37 – 3.40 m3/t (bags) [14]

Stowage space requirements

Cool, dry, good ventilation, dark (due to risk of sprouting)


Fiber rope, thin fiber nets, matting, jute coverings

Cargo securing

Because of its considerable impact- and pressure-sensitivity, packages of this cargo must be secured in such a way that they are prevented from damaging each other. Spaces between packages or pallets must be filled, to prevent slippage or tipping. By selecting the correct packaging size or cargo unit (area module or area module multiple), holds can be tightly loaded (without spaces).

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Risk factors and loss prevention

RF Temperature

Onions require particular temperature, humidity/moisture and ventilation conditions (SC VII) (storage climate conditions).

A written cooling order must be obtained from the consignor before loading is begun. This order must always be complied with during the entire transport chain.

The following Table merely constitutes an estimate of appropriate temperature ranges. Temperatures may deviate from these values, depending on the particular transport conditions.

Designation Temperature range Source
Travel temperature 2 – 4°C [1]
0 – 4°C [4]
0°C [5]

Where journey times are relatively long or a relatively long storage life is desirable, onions must be transported under temperature-controlled conditions.

The freezing point of onions lies between -1.59 and -1.90°C. They are able to withstand gentle freezing to -3°C, as they are able to resorb completely the cell water expelled by freezing when they thaw. However, they must not be moved when frozen (risk: rough seas), since this would cause bruises, which lead immediately to rotting. Freezing injury manifests itself in glassiness and watery, grayish tinged spots. The travel temperature should therefore not fall below 0°C.

If the temperature rises from 0°C to 20°C, a rapid increase in respiration intensity ensues. Temperatures of up to 30°C are admissible in the short term, but from 40°C the onions undergo physiological changes which are known in practice as „scalding“ (physiological heat injury) and the symptoms of which include rot, spoilage, sprouting and self-heating. The onions acquire a glassy appearance, become mushy and exude a foul odor.

In the event of an increase in temperature accompanied by an increase in respiration intensity, the essential oils evaporate to a certain degree (increase in odor intensity in the hold), so causing a reduction in the seasoning action of the onions. Since the vitamin content is also reduced, 30°C is considered the maximum admissible temperature.

If the surfaces of a cargo stack cool down too sharply, a wet, rapidly heating, rotting, sprouting boundary layer, which causes the packaging material to decay, may form as a result of a marked difference in temperature relative to the rising heat and water vapor released by the onions from below, leading to total loss of the cargo, while the 0.5 – 1 m thick surface layer appears intact.

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RF Humidity/Moisture

Onions require particular temperature, humidity/moisture and ventilation conditions (SC VII) (storage climate conditions).

Designation Humidity/water content Source
Relative humidity 75 – 80% at -1 – +1°C [1]
70% at 8 – 10°C [1]
65% at 15°C [1]
65 – 70% [5]
Water content 82 – 88% [1]
Maximum equilibrium moisture content 65% [1]

Onions require a lower, temperature-dependent equilibrium moisture content than other vegetable cargoes with a high water content.

Their particular relative humidity requirements stem from the protective function of the outermost scale leaves, which need to be kept dry. High relative humidities encourage rot, root growth, premature sprouting and self-heating. Since it is not always possible to comply with the values stated above when onions are loaded conventionally without a refrigeration installation, an attempt must be made to get as close as possible to these values using suitable ventilation measures. The temperature must also be lowered to the temperature prevailing at the port of discharge.

During the voyage, onions release large amounts of water vapor. Evaporation losses entail additional losses of essential oils, sugar and vitamins.

If the scale leaves of the onions are dry and parchment-like, penetration of microorganisms is largely prevented. Moist scale leaves, on the other hand, lose their resistance and offer favorable living conditions to adherent microbes.

The first consignments of onions from a new harvest are more susceptible to injury than warehoused goods due to their higher water content. They must be post-ripened or dried for at least 2 weeks prior to transport (sprout/growth dormancy).

If the holds are washed prior to transport, the tank top ceiling (if present) must be completely dry again before loading (max. water content 15%), as otherwise the cargo may spoil.

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RF Ventilation

Onions require particular temperature, humidity/moisture and ventilation conditions (SC VII) (storage climate conditions).

Recommended ventilation conditions: 60 – 80 air circulations/hour with continuous supply of fresh air, the circulating fans being operated constantly at full power.

If the onions are transported as standard general cargo rather than as chilled goods, 25 air circulations/h with constant supply of fresh air will suffice.

The fresh air supply must be controlled in such a way that the CO2 content of the circulating hold air does not exceed 0.5 vol.%.

If the onions are stacked too high and ventilation is inadequate, the undissipated intrinsic heat may cause heat injury.

Where holds or containers are opened for ventilation purposes, care must be taken to ensure that the onions are protected from the light, to prevent any risk of sprouting.

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RF Biotic activity

Onions display 2nd order biotic activity.

They are living organs in which respiration processes predominate, because their supply of new nutrients has been cut off by separation from the parent plant.

Care of the cargo during the voyage must be aimed at controlling respiration processes (release of CO2, water vapor, ethylene and heat) in such a way that the cargo is at the desired stage of ripeness on reaching its destination. Inadequate ventilation may result in fermentation and rotting of the cargo as a result of increased CO2 levels and inadequate supply of atmospheric oxygen (see Ventilation).

In addition, onions must be protected from light during the voyage, as there is otherwise an increased risk of sprouting.

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RF Gases

CO2 evolution Respiration processes may lead to an increased CO2 concentration during storage, associated with a simultaneous O2 shortage. Onions are known to consume large amounts of oxygen, as a result of their high respiration intensity.
Upper limit of permissible CO2 content 10 vol.%
Ethylene evolution  
Active behavior The rate of ethylene production is very low, being below 0.1 µl/kg*h [16].
Passive behavior Dry onions exhibit low sensitivity to ethylene, while green onions are moderately sensitive to it [16] (allelopathy). Onions should not as a rule be loaded together with apples or pears.

An increase in the CO2 content of the hold air from 0.03 vol.% (i.e. the normal content in inhaled air) to 5 – 10% does not cause any injury to onions. 12 vol.% and above causes the onions to convert to anaerobic respiration, resulting in relatively rapid spoilage.

If ventilation has been inadequate (frost) or has failed owing to a defect, life-threatening CO2 concentrations or O2 shortages may arise. Therefore, before anybody enters the hold, it must be ventilated and a gas measurement carried out.

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RF Self-heating / Spontaneous combustion

Oil content: 0.0037 – 0.055% allyl sulfide (essential oil)

The respiration process in onions may give rise to a tendency to self-heating and a high CO2 concentration and high O2 consumption, resulting in „scalding“ (see RF Temperature).

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RF Odor

Active behavior Onions have an unpleasant, pungent odor. An increase in odor levels indicates self-heating, which is associated with the loss of essential oils, sugar and vitamins.
Passive behavior Onions are highly sensitive to odor-emitting goods and should not be stowed in a hold together with hides, furs, bones or other animal products.

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RF Contamination

Active behavior Onions produce dust.
Passive behavior Onions are sensitive to dirt, fats and oils. They may turn brown, especially if the previous cargo was ammonia (NH3).

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RF Mechanical influences

Since onions are highly sensitive to impact and pressure, they must be handled with appropriate care.

Damage to onions, caused by setting them down roughly for example, results in greater respiration intensity and self-heating.

No more than 12 bags should be stowed on top of one another, as too great a stack height leads to bruising, excessive heating and rot. 12 – 16 bags causes distortion and losses within usual limits, while more than 16 bags should not lie on top of one another.

Onions intended for use as seed must not be stacked more than 8 bags high and lower stack heights (6 bags) would be even more favorable.

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RF Toxicity / Hazards to health

If ventilation has been inadequate (frost) or has failed owing to a defect, life-threatening CO2 concentrations or O2 shortages may arise. Therefore, before anybody enters the hold, it must be ventilated and a gas measurement carried out. The TLV for CO2 concentration is 0.49 vol.%.

In marine transport, it is important to inform the crew of the hazardous nature of cargoes of onions. No access is permitted to the holds without breathing apparatus or until they have been ventilated for a long enough period for the normal composition of the air to be reestablished and a gas measurement has been carried out using a gas detector.

The following accident provides a telling example of the hazardous nature of cargoes of onions due to CO2 formation and of the ignorance of those involved:

When a ship containing 5000 bags of onions was to be unloaded, the first dock worker climbed down into the hatch and immediately collapsed unconscious. Exactly the same happened to the second dock worker, when he came to the aid of the first. The ship’s doctor and an officer then climbed into the hold and the ship’s doctor also collapsed unconscious. The officer was able with his last ounce of strength to carry the first unconscious man out. Another member of the crew climbed into the hold and also collapsed unconscious. Only then did someone put on a breathing apparatus, rescue the unconscious men and take them to a hospital, where two people were confirmed dead. An investigation gave as the cause of the accident an excessively high CO2 concentration in the hold.

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RF Shrinkage/Shortage

A loss of volume of up to 0.2% of the cargo caused by torn bags may be deemed normal. The risk of decay of the jute bags must be taken into consideration. The normal water content of jute bags is approx. 13.75%; if this is exceeded, they decay quickly and further losses are sustained.

Weight loss may also occur due to water vapor release during the voyage.

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RF Insect infestation / Diseases

Losses most frequently result from onion neck rot, the cause of which may be found in inadequate post-ripening or post-drying or excessive humidity or inadequate air supply during transport.

Onion neck rot is a gray mold rot caused by the fungus Botrytis allii. The fungus enters the onion neck and attacks the fleshy scales. Brown, somewhat sunken spots on the outermost scale leaves, followed by sinking and softening of the head and neck parts are the typical signs of this disease. The flesh of the onion becomes glassy and a layer of gray spores spreads between the onion scales.

The application of thumb pressure to the onion neck allows timely detection of the sunken parts.

The mold may still develop even at temperatures of around 0°C.

Bacterial soft rot is another dangerous disease of onions.

The quarantine regulations of the country of destination must be complied with and a phytosanitary certificate may have to be enclosed with the shipping documents. Information may be obtained from the phytosanitary authorities of the countries concerned.

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