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Onion, Allium cepa / Liliaceae (Alliaceae)
Postharvest Atmosphere Management
Onion
The onion requires several treatments previous to its storage. The first step is to dry it, in order to improve its conservation. The bulbs are also treated to avoid their sprouting; this ii done with chemical products or irradiation. Conservation is done at 65-75% of relative humidity and temperatures around 0ºC.

Before onions are put in storage rooms, it is important to dry or cure them. The aim is to dry the neck and the outer layers to protect them from possible diseases. The moisture content must be reduced in 10%, which is equivalent to 3 to 10% loss of weight. The drying process may take place in the field if the environmental conditions allow to do so (dry climate). If not, they use artificial drying systems, like forced air, either environmental temperature or hot air.

During storage, it is important to avoid the bulb sprouting and the secondary roots. For this purpose it is important to keep low temperatures and modified atmospheres. However, in some cases other methods are used, like the application of chemical products and irradiation. Among the chemical products used there is the maleic hydracid, that is applied shortly before harvesting. Irradiation consists of exposing the bulbs to ionizing gamma ray radiation to inhibit sprouting. For this process they use very low doses.

After the harvesting it is not usual to subject onions to pre-cooling. It is only recommended in case of tender onions, since a fast decrease of temperature improves their conservation.

The storage time depends on the variety. Early onions cannot be stored more than 4 months. The bulbs are stored in bulk in piles, in sacks on pallets, in field boxes or palots. In any case, ventilation in all the produce surface must be allowed.

Storage can be done in different ways. One can use traditional storage facilities. This method is suitable for cold areas and when we do not want to store the bulbs for a long time.

Storage under forced ventilation is also used. For this treatment they use fans, that renew the store room’s air, removing the heat produced by the bulbs.

Another storage system is under controlled conditions. The onions are not sensitive to chilling injuries, so one must take advantage of this characteristic for its storage. Thanks to this system, some varieties are preserved up to 32 weeks. The moisture levels must be kept between 65 and 75%, and temperatures should not be lower than -0,8ºC. However, for short storage up to 5 weeks, the temperature recommended is 15ºC. The use of modified atmospheres is not recommended, because it gives contradictory results.
Distribution
The onions must not be transported at temperatures over 20ºC, and they must be lowered in relation to the length of the journey.

The onions are transported at temperatures between -1 and 20ºC, according to the distance of the trip. Relative humidity must be kept between 65 and 75%. Onions give off their smell to other products; thus, they must not be transported in mixed loads.

There must be a suitable ventilation during the transport. For transoceanic trips they use containers with apertures. By means of a fan the air circulates through them. In case of palletized onions, the air must be distributed through the palets.

Distribution is due to be at temperatures between 5 and 20ºC, with average moisture and ventilation levels.
Postharvest Problems
Onions have different problems in their storage. There appear several physiological alterations like translucent scales, sun burns and greening. They can also suffer diseases caused by Botrytis, Fusarium, Sclerotinia, Aspergillus, Penicillium and Colletotrichum or by the bacteria Erwinia and Pseudomonas.

Among the alterations we find the following ones.

Translucent scales: it consists of a paling of the scales colour, that later on turn brown. Although the reasons for these changes are not clear, it is thought that high temperatures before the harvesting favour them.

Sun burn: it takes place during the drying, and it affects mainly the onions with few protective dry scales.

Greening: the outer scales acquire a greenish colour, and there appear unpleasant tastes. This phenomenon takes place if the bulbs have been exposed to the sun for too much time.

Some of the diseases that affect the onions during their storage are:

Botrytis rot, caused by the Botrytis fungus. It is one of the main postharvest alterations. It is characterized by a wet rotting in the outer tunics of the affected area, covered with very short gray hairs. In order to control this disease it is important to harvest the bulbs in their maturation, in dry wheather, and to let them dry appropriately.

Rotting in the base of the bulb. This alteration appears already in the field, and it fully developed in the warehouse. It is caused by the Fusarium oxysporum fungus, that infects the bulb through the roots, reaching the base of the bulb and producing the rot.

White rot. Caused by the fungus Sclerotinia sclerotiorum, and characterized by the presence of a white down on the affected areas; there develop a kind of small black spots of 1-2mm of diameter, similar to a pin’s tip.

Black mouldy rot. This disease originates from the fungus Aspergillus niger. This organism penetrates through damages, normally in the top area, producing black efflorescences and rotting in the tissues affected.

Another fungus belonging to the same genus, Aspergillus alliaceus, has the same characteristics but it produces a yellow efflorescence.

Blue mould. This disease is caused by the Penicillium fungus, and it is characterized by a rotting on which a blue down grows.

Coletotrichum circinans is a fungus that produces anthracnose or grime. This alteration is characterized by the presence of circular black spots on the external tunics of the onion.

Onion Mildew. The fungus Peronospora schleidenii attacks the onion during the culture cycle, and it is lodged in the apex of the bulb, producing blossom-end-rot if there is enough moisture.

Wet rots. They are caused by bacteria of the genus Erwinia and Pseudomonas. They cause the tissues softening and wet rots that gives off a foul-smelling.
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