Onions

Infobox on Onions
Example of Onions
Freshness facts
Optimum carrying temperature	0°C (dry/green)
Highest freezing point	Approx. -0,8°C (see text)
Acceptable product temp. at loading into containers	Max. 2°C above carrying temperature
Optimum humidity	65%-75% (dry) >95% (green)
Ventilation setting for containers	10 m³/hr
Storage life	6-9 months (dry) 3-4 weeks (green)
Climacteric / non-climacteric	Non-climacteric
Ethylene production	Very low
Ethylene sensitivity	Low
Modified / controlled atmosphere	2% O2; 5% CO2
Potential benefits	See text
Availability
Australia/New-Zealand South Africa Egypt Argentina/Chile	April - August February - July March - May February - April

Onions

Harvesting and Handling

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.

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 colour and lie tightly around the onion neck, providing protection against evaporation and attack from micro-organisms. This process is further assisted by subsequent post-drying.

High quality onions should have mature bulbs with good firmness and compactness of fleshy scales. The size, shape and colour of the dry skin should be typical for the variety. They should be free of mechanical or insect damage, decay, sunscald injury, greening of fleshy scales, sprouting, bruising, doubles, bottlenecks (onions which have abnormally thick necks with only fairly well developed bulbs) and other defects.

Quality green onions have a thin, white shank or neck at least 5 to 7.5 cm in length. Green onions should be well-formed (at most slightly curved or angular), uniform in shape, thin-necked, turgid, bright in colour, well cleaned, and free from excessive roots, decay, insect-injury, mechanical damage, broken or crushed leaves, or dehydrated clipped-ends. Green onions are eaten for their immature bulb and green foliage.

Cooling and Storage

In order to maintain high quality, bunched green onions should be pre-cooled to <4°C within 4 to 6 hours of harvest. Dry onion bulbs for long-term storage should be pre-cooled to 0°C immediately after drying. Rapid pre-cooling inhibits rooting and sprouting during storage.

Bunched green onions can be stored 3 to 4 weeks at 0°C with 95% to 98% RH. Storage-life decreases to 1 week if the temperature is 5°C, and rapid yellowing and decay of leaves occurs at higher temperatures.

Pungent, dry onions can be stored for 6 to 8 months at 0°C with 65% to 75% RH. High RH induces root growth, while high temperature induces sprouting. A combination of high temperature and high RH increases rotting and decreases quality.

After harvest, onion bulbs enter a state of rest for a period of 4 to 6 weeks, depending on cultivar and weather conditions during growth. Maleic hydrazide, a sprouting inhibitor, is often used to prevent root growth and sprouting during long-term storage. It is applied 2 weeks before harvest, when bulbs are mature and 50% of tops are down.

Onions intended for storage should be dried well and cured in the field, under sheds, or in storage. After 2 weeks of field drying, onions can be transferred to storage rooms for final drying and curing. Drying is complete when the onion neck is tight, outer scales are dry and make a rustling noise when touched, and the skin colour is uniform. After drying and curing, the temperature should be lowered gradually to the normal seasonal temperature, or bulbs can be pre-cooled in cold storage at 0°C. In either case, condensation should be avoided as it encourages rot and changes the colour of the dry skin.

For cold storage, onions are usually packed in crates or containers. Air circulation must be sufficient to maintain a constant temperature and remove moisture from inside storage containers. Onions packed in sacks can only be stored for a limited period of time, since air movement through sacks is insufficient to maintain proper storage conditions.

Mild and sweet onions can be stored for only 1 to 4 months, even in optimal cold storage. CA may extend the storage period.

Onions tolerate storage at 30°C to 35°C for short periods before marketing or processing, but their quality and external colour is less attractive than cold-stored onions.

When stored below -1°C to -2°C, onions should be thawed at 5°C for 1 to 2 weeks before they are removed from storage. Rapid thawing damages onion bulbs. Storage at <-4°C may cause freezing injury, which is manifested by soft, water-soaked fleshy scales and rapid decay after transfer from cold storage to higher temperature, which results in microbial growth.

Controlled atmosphere considerations

Bunched green onions can be stored for 6 to 8 weeks in 2% O₂ and 5% CO₂ at 0°C. Low O₂ atmospheres reduce respiration and extend storage-life, while elevated CO₂ reduces sprouting and root growth.

CA is unlikely to be economical for traditional cultivar dry onions because they can be easily stored for up to 9 months under refrigeration.

Storage disorders

Bacterial soft rot, Basal rot, Black mould rot, Blue mould, Brown stain, Bruising, Freezing injury, Grey mould rot, Sclerotium rot, Smut, Water staining.