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.
At 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 colour and lie tightly around the onion neck, providing protection against evaporation and attack from microorganisms. This process is further assisted by subsequent post-drying.
Onions should be grown on friable soils, which contain high amounts of organic matter and have good water-infiltration rates and good moisture-holding capacity. The soil should not be compacted, and the pH should be 5.8 to 6.6. Sandy loams and muck soils are often used for onion production. For sweet Spanish onion production, soils with low sulphur levels (below 40 ppm) are recommended.
Land Preparation and Formation of Beds
Prepare the field by 2 ploughings and 2 harrowing. Level and pulverize the soil to a fine texture to facilitate formation of beds. Raising beds are constructed at 0.5-meter-wide by 20 m long (maximum) by 15 cm high. Double row beds will be prepared with a distance of 20-25 cm between hills and with a hole depth of at least 3.5 cm. http://cagayandeoro.da.gov.ph/wp-content/uploads/2013/04/PRODUCTION-GUIDE-ON-BULB-ONION.pdf
- Red Creole
- Red Pinoy
- Yellow Granex (Hybrid)
Short day variety
Short-day onion plants require about 12 hours of daylight each day to form bulbs. Grow short-day onions during fall and winter in USDA zones 6 through 10. Plant them when temperatures range from 55 to 75 F. Short-day varieties include:
- ‘Red Burgundy’ (Allium cepa‘Red Burgundy’) produces sweet, mild, red, cooking onion bulbs that are ready to harvest 95 days after planting. The bulbs are 4 inches in diameter with red skin and white, red-tinted flesh. They stay fresh in cool storage for months, which is unusual for red onions.
- ‘Yellow Granex’, ‘Maui’, ‘Noonday’ or ‘Georgia Sweet’ (Allium cepa‘Yellow Granex’, ‘Maui’, ‘Noonday’ or ‘Georgia Sweet’) are sweet yellow onions that are similar to ‘Vidalia’ onions (Allium cepa ‘Vidalia’) but are ready to harvest sooner.
- ‘Yellow Granex’ onion bulbs are ready to harvest after 80 to 109 days while ‘Vidalia’ onion bulbs need about 160 days. ‘Yellow Granex’ bulbs appear flattened and are 3 to 4 inches in diameter.
- ‘White Granex’ or ‘Texas Sweet’ (Allium cepa‘White Granex’ or ‘Texas Sweet) produces sweet, mild, salad onion bulbs that are ready for harvest in 100 days.
- ‘Crystal White Wax’ (Allium cepa‘Crystal White Wax’) has pearl onion bulbs that can be pickled or put in soup or stew. They are ready for harvest after 60 days when they are 1 inch wide.
Long day variety
Long day onion plants require roughly 14 to 15 hours of daylight each day to form bulbs. Grow long-day onion plants in spring and early summer in USDA zones 3 through 7. Sow their seeds when the soil warms to 50 F. Plant sets two to four weeks prior to your area’s last average annual hard frost date in spring. Long-day varieties include:
- ‘Red Wethersfield’ (Allium cepa‘Red Wethersfield’) onion bulbs are 4 to 6 inches in diameter with a deep-maroon skin and pale, pink flesh. They are sweet and can be added to salads or used in cooking. They are ready for harvest 100 to 115 days after planting and store well.
- ‘Yellow Ebenezer’ (Allium cepa‘Yellow Ebenezer’) onion bulbs are 2 ½ to 3 inches in diameter, pale yellow, mild-flavored with crisp flesh and used in cooking. They are ready for harvest after 100 days and store well.
- ‘Snow White’ (Allium cepa‘Snow White’) produces sweet, white bulbs that grow in long- or short-day climates. They are ready for harvest after 90 days.
Seedbed Establishment and Seed Sowing (Nursery)
Seedbed should be located in a well-drained, friable soil with good water holding capacity and high organic matter content. If soil pH is lower than 5.8, lime application is necessary at the rate of 3 tons/ha applied one month before transplanting. Land should be prepared by thorough ploughings and harrowing. Level and pulverize the soil to facilitate formation of beds (1-meter-wide and 20 m long). Prior to seed sowing, sterilize the beds by burning rice straw on top or by pouring boiling water to prevent pest and disease infestation.
Broadcast chicken manure or compost at the rate of 10 t/ha combined with 10 bags 14-14-14. Space 10-15 cm between rows and sow seeds evenly in a row at 5-6 seeds/inch with seeding depth of ½ inch. Approximately 25 grams of seeds is needed per square meter. Irrigation should be applied adequately in the field right after seed sowing. Seeds will germinate at about 7-10 days after sowing and are ready for transplanting 45 days after sowing. http://cagayandeoro.da.gov.ph/wp-content/uploads/2013/04/PRODUCTION-GUIDE-ON-BULB-ONION.pdf
Bulbing of onions is primarily induced by photoperiod or day-length. Both long day and intermediate day onion varieties are recommended for in the north eastern United States. In addition, onions are classified by skin colour (red, white, brown, or yellow), taste (sweet or pungent), and shape of the bulb (round, flat, or globe).
|Recommended onion varieties for growers in the north eastern United States.|
|Planting Method||Type||Variety||Days to Maturity|
|*indicates hybrid variety|
|Sets||N/A||Early Yellow Globe||90|
|Sets||N/A||Southport Red Globe||90|
|Seeds/Transplants||Storage||Spartan Banner 80*||115|
|Seeds/Transplants||Sweet Spanish||Alisa Craig||95|
|Seeds/Transplants||Sweet Spanish||Spanish Medallion||110|
|Seeds/Transplants||Sweet Spanish||Red Sky* (red bulb)||103|
|Seeds/Transplants||Sweet Spanish||Redwing* (red bulb)||115|
|Seeds/Transplants||Sweet Spanish||Mercury* (red bulb)||115|
Climatic and Soil requirement
Onion is a cool season crop. It is grown during winter and harvested before the real hot season begins. Onion can be grown under a wide range of climatic condition. But it succeeds best in mild season without extremes of heat and cold. According to Rao and Purewal (1954), it can be grown as a rained crop even at elevation of 1500 to 2000 m between April and August, Onion produce bulbs more rapidly at warm (210 C to 260 C) than at cool 150 C to 210 C) temperature.
Soil and its Preparation
Onion can be grown on various soils. But sandy loam, silly loam and deep friable soils are best suited for onion crop. The land is prepared by giving 5-6 ploughings. The optimum pH range is between 5.8 and 6.5. http://agriinfo.in/default.aspx?page=topic&superid=2&topicid=935
Environmental Requirements for Onion Seed Production
Onion seed production is influenced not only by genetic factor but also environmental factors such as temperature, rainfall, soil conditions and presence of beneficial insects.
Cultivars: Seed production potential also depends on cultivars. Some onion types produce few flower stalks in the country due to their higher chilling requirement. So, producers should exercise seed production on adapted onion varieties like Bombay red, Adama red, etc. Likewise, in selecting onion varieties for seed production, emphasis should be given to the most popular varieties under production. Currently, Bombay red is the most widely grown onion variety under irrigation in the country due to its higher bulb yield and earliness. This variety is not suitable for production under rain fed as it easily rots in the field if encounters rain during maturity stage. Yields up to 400 qt for Bombay red were observed on farmer’s fields in CRV areas, which is mainly due to its tolerance to higher plant population (can successfully produce good size bulbs at spacing as low as 4cm between plants). On the other hand, Adama red can produce good size bulbs only at plant spacing greater than 6cm. Unlike the former, Adama red can be produced under rain fed conditions as it tolerates rotting due to rain effects during maturity stage. Thus, when selecting a variety for its seed.
In this regard, it is paramount important that the government extension agents and producers’ knowledge and skill be improved in the areas of seed production technology. True to type bulb selection, seed production practices, harvesting, seed processing and storage are the underlying principles in the production and supply of quality onion seed. Thus, this simplified manual that encompasses the main processes in onion seed production technology is prepared in order that higher and quality seed yields are produced and supplied to users. It also plays significant role in the maximization of profits from the intervention in seed production the yield and area coverage of that variety should also be taken in to account.
Most of the onion varieties are adapted to low and mid altitude areas (700-1800 m.a.s.l), even though onion can grow up to 2000 m.a.s.l. The ideal temperature for mother bulb production is 180 c-240 c day and 10-120 c night temperature. For bulb production it can go higher beyond these ranges. However, it is major factor for flower stalk development and seed set. Higher temperature can prevent flowering. After bulb develops, cool weather with ample moisture supply is required for flower stalk initiation. Then, drier conditions with good sunshine are required for seed maturity, harvesting and processing. It is also important to know specific requirement of the crop varieties. For instance, Bombay and Adama red can flower and produce higher seed yield under relatively lower chilling temperature while variety like Red Creole needs very low temperature and cannot produce sufficient seeds like the other varieties in the CRV and similar areas.
High temperature during flowering also results in flower abortions and hence lower seed yield. So, selection of appropriate months in a given locality is crucial in onion seed production venture. Studies and experiences show that onion seed production in the country is best if mother bulbs are planted in September and October for flowering to take place in the months of January and February- in cooler and drier months. This is the best period for getting higher and quality seed yields. This is particularly true in the Central Rift Valley (CRV) areas where this experience was developed.
During flowering, seed development and maturity excessive rainfall and very cool condition is undesirable as they lead to disease development and poor seed setting. Good sun shine at the time of full blooming stage will facilitate the activity of beneficial insects for higher rate of cross pollination and seed set. The relative humidity should be lower at the time of seed development.
Light soil with good fertility and drainage and pH of 6.0-8.0 is preferred for onion production. Loam or clay loam soils are best suited for seed production. http://coin.fao.org/coin-static/cms/media/7/13029380384160/onion_seed.pdf
As the onion is a shallow-rooted plant, care must be taken not to injure the roots by deep tillage. When hard rains incrust the soil before seeds have germinated, light raking or rolling will be an advantage. Horse cultivators are sometimes employed, especially in the heavier soils, which are difficult to work by hand. However, the additional spacing between rows, required for horse tillage, necessarily limits the yield. If ground has been properly prepared, there will be few instances when the rows should be more than 1 foot apart. Hand wheel hoes must then be employed in cultivating. Both single and double wheel types are in common use, but it is more economical to use double than single wheel hoes when the plants are small. Many growers prefer the single wheels at all stages of growth. Straight rows and uniform spacing are a great advantage in the operation of wheel hoes. The vertical shovels or teeth are most useful in heavy soils, while the horizontal sweeps are most serviceable in light soils. The latter attachments may be used without danger of covering the small plants. It is necessary to cultivate from 8 to 15 times during the season.
Hand weeding and thinning are required. This work is often done by boys and girls. The thinning is usually performed at the first weeding, when 8 or 10 plants are allowed to the linear foot of row, but in very good soils and when large bulbs are desired, the plants should stand about 2 inches apart. Special hand weedier are in common use. Both weeding and thinning should be avoided as much as possible by the proper preparation of the soil and the adjustment of drills. The figure below shows a well-managed onion field on Long Island.
In many sections of the West and the Southwest onions cannot be grown without irrigation, the ground being too dry to supply the moisture necessary. All of the Bermuda onions of the Southwest are grown under irrigation. The land is flooded before planting and afterwards at intervals of a week or 10 days until the bulbs are full sized, when water is withheld to induce ripening. An increasing number of growers in various parts of the country are employing the overhead system of irrigation, which is ideal when applied to this crop. Sprinkling before or after planting prevents the blowing of muck and sandy soils and the accompanying disastrous results in young plantations. Irrigation increases yields and insures the crop against loss from drought. http://bookdome.com/gardening/Vegetable-Gardening/538-Cultivation-Of-Onion.html#.WcUIIsiGPIU
Methods of Onion Seed Production
There are two methods of onion seed production. Seed to seed and bulb to seed method; both can be used in seed production. But the bulb to seed is the most commonly used method in Ethiopia. This method has a number of merits; options of selection of bulbs of good size, uniform, typical colour, free from diseases and physical damages. It produces several stalks per bulbs hence give higher seed yield. This is in conformity with CDMDP experiences where seed yields up to 20 qt/ha was obtained on farmer’s field for the variety Bombay red. The method is also good to maintain the variety identity. However, in this method it takes 10-11 1/2months to produce seed (4 – 4 1/2 months for bulb production and 6-7 months for seed set and maturity).
The seed to seed method lacks the above mentioned merits. It also produces less flower stalk per bulb. But it takes 7-8 months to produce seed.
Bulb to Seed Method of Onion Seed Production
Mother Bulb Production
The following recommended practices should be followed in preparing beds and raising good quality seedlings.
Nursery site: site should be near water and with good soil condition, not planted with crops like onion, garlic, tomato etc. in the previous two seasons.
Seed bed: soil should be well prepared and seed bed be levelled; bed size to be of 1m width x5m length (most commonly used bed size). Bed size canalso be of 1mx10m. Types of bed can be raised, flat or sunken bed depending on soil type and moisture condition of an area. Raised bed is the most commonly used in CRV areas as it avoids risks of bed over moistening.
Seed treatment: treating onion seed with fungicide like Apron Star is recommended particularly in CRV areas to avoid fungal diseases like damping off.
Seed rate: 80-100gm/1mx5m bed (for seeds >90% germination), 3.5-4 kg/ha.
Spacing: seeds are drilled on the rows opened at 10cm space and 0.5-1cm depth against the bed length and slightly covered with soil.
Fertilizer: 100 gm Urea per bed (1mx5m) or manure should be applied during sowing.
Bed covering/Mulching: bed should be covered with grass (dry) at 3-5cm thickness; green or non-dried grass and broad leaved materials should never be used to cover the beds.
Grass cover removal: the grass cover is removed when seedlings emerge; best time is in the morning or late afternoon.
Bed Irrigation: the bed should be irrigated with watering-can in the morning or afternoon from sowing until two weeks after seedling emergence, then if possible better to continue same till transplanting or use furrow irrigation with care.
Pest control: better to apply registered fungicides like Ridomil Gold within 3-4 days after grass cover removal, then after fungicides/insecticides application depends on disease or insect appearance.
Preconditions and Transplanting of Seedlings
The following preparations and practices should be followed for transplanting seedlings from seed bed to fields.
Seedling stage: Age of the seedlings is important for establishment and higher final bulb yield. Seedlings are ready for transplanting at 45-55 days after sowing or when 3-4 true leaves emerge; this is just before bulb formation starts (Figure 3). If seedlings overstay on beds for more than 60 days after sowing, bulb formation starts and potential for bulb size development reduced with consequent significant yield reduction.
Bed irrigation: irrigate slightly seedling bed before uprooting for transplanting to avoid damage to seedlings.
Trimming: root or shoot trimming is not recommended when seedling is transplanted at the appropriate stage. The practice reduces the final bulb yield. Producers usually trim shoots when it gets older and taller, and also lightly trim roots to facilitate planting operation. If practiced, better to trim shoots part only or root parts very lightly; not good to trim both at one time as it reduces yield significantly.
Furrow/ridge preparation: the field should be prepared well and furrows opened at 40 cm distance including ridge.
Field pre-irrigation: better to irrigate the ridges/field just immediately before transplanting to facilitate planting operation and reduce seedling damage.
Transplanting/spacing: This depends up on the variety. For the variety Bombay red, strong and healthy seedlings selected is planted at 5 cm space between plants on the rows and 20 cm space between the two rows on a ridge while for variety Adama red the spacing between plants should be 6- 8cm. It is better to transplant in the morning or in the afternoon for better establishment.
Fertilization: The rate depends on the soil fertility of an area. 200 kg DAP all at transplanting and 100 kg Urea in split, half at transplanting and the other half at 30-45 days after transplanting is recommended for CRV areas on sandy loam soil. http://coin.fao.org/coin-static/cms/media/7/13029380384160/onion_seed.pdf
Pest and Disease Management
|Post-harvest black discoloration at neck, lesions on outer scale||Wash hands after coming into contact with fungus||Fungus||Seeds to be treated with appropriate fungicide to reduce rot in mature bulbs, storing at temperature below 15 degree Celsius reduces mold from spreading, avoid bruising bulbs|
|Botrytis leaf blight
|Small white lesions with light green halos which may slightly expand as they age||Disease emergence favours high humidity and warm temperatures, older leaves are susceptible to blighting than younger leaves||Fungus||Allow at least 30 cm spacing between plants to promote good air circulation and quick drying of foliage after rain, appropriate fungicide sprays also required|
|Pale and elongates patches on leaves, leaves turning pale then yellow||Disease emergence favoured by cool temperature and leave wetness||Fungus||Plant in well-draining areas and don’t over crowd plants, avoid infected plant sets, apply appropriate fungicide and destroy infected crops|
|Fusarium basal plate rot
|Necrosis begins at leaf tips and move downwards, Wilting plants, infected bulbs may be brown and watery with rot spreading from stem plate to basal leaves||Disease emergence favours moderate to high temperatures||Fungus||Rotate with non-susceptible crops for at least 4 years, plant resistant onion varieties|
|Pink roots which darken and turn purple, roots become transparent and water soaked||Fungus colonizes through root tips, fungus can survive in soil down to a depth of 17.7 inches||Fungus||A rotation of 3 to 6 years is preferred, plant more resistant varieties, fumigation can help reduce the pathogen in the soil|
|Severely infected foliage may die, large lesions may coalesce and girdle leaf, killing any tissues between the lesions and leaf tip||Disease emergence favoured by wet foliage, with sporulation occurring in night period on high humidity||Fungus||Fungicides are effective at controlling the diseases but should be rotated for optimal control, Cultural controls includes long rotation and the reduction of leaf wetness by planting in well – drained soil and timing irrigation|
|Pythium seed rot
|Seeds water soaked and decomposing, seedlings that have already emerged prior to infection collapse and die||Disease emergence favours high soil moisture and cool temperatures||Fungus||Can be controlled by minimizing moisture, treat seeds with appropriate fungicides prior to planting|
|Rotting seeds, discoloured root tips which may be pink, tan , yellow , red or black||Fungus survives in soil and emergence is favoured by moist to wet soil||Fungus||Plant only disease free seed, treat seed with fungicide, rotate crops to reduce the levels of pathogen in soil|
|Older leaves yellowing, stunned growth, fluffy white growth on base of bulb which spreads up bulb to storage leaves||Fungus can survive in soil for 20 years and is one of the most damaging diseases of Allium crops worldwide||Fungus||Fungicides along with cultural methods recommended, treat seeds with hot water prior to planting, use long term rotation with non-allium crops|
|Dark, thickened areas on cotyledons which may become large and causes leaves to bend downwards, lesion mature and become covered in black powdery fungal masses, plant growth stunted||Smut can persist in soil for many years and is mainly introduced through infected seeds and transplants||Fungus||No resistance to disease known in onion, plant only healthy seeds and transplants – if smut is present in the soil they will not become infected|
|Leaf streak and bulb rot
|Dark green oval lesions or streaks on leaves, reddish brown discoloration of inner scales, rot developing in ring like pattern||Greatest damage occurs during winter, rapid spread of disease on infected plants is promoted by rainfall||Bacterium||Avoid fertilizing during winter, apply appropriate bactericidal sprays|
|Onion yellow dwarf
Onion yellow dwarf virus ( OYDV)
|Yellow streaks on bases of first leaves, leaves may be flattened or crinkled, flower stalks yellow and twisted||Transmitted by several aphids, including peach aphid, virus is not spread via seed or pollen||Virus||Plant more tolerant varieties, remove infected plants, to certain extent insecticides can be used to control aphids, planting sets or transplants which were produced in area free of virus|
|Distorted tissues, scarring of leaves, infected plants may have a slivery appearance||Most damaging at the initial stage of bulb development||Insect||Predatory mite, pirate bugs and lacewings are natural enemies, avoid planting onion in close proximity, apply appropriate insecticides, overhead irrigation can helps to reduce thrips|
|Small white flecks on leaves and stems which develop into circular or elongated orange pustules, severe infestations can cause leaves to yellow and die||Favours high humidity and low rainfall||Fungus||No resistance, apply appropriate protective fungicide|
|White and winding trails on leaves, early infestation can cause yield to be reduced||Mature larvae drop from leaves into soil to pupate, entire lifecycle can take less than 2 weeks in warm weather, insect may go through 7 to 10 generations per year||Insects||Check for leafminers before planting, remove plants from the soil after harvest, use appropriate insecticide whenever required.|
|If infestation occurs during bulbing, bulbs will be deformed and susceptible to storage rots after harvest, stunted or wilting seedlings, adult insect lay eggs around the base of plant and the larvae that emerge are tiny and bore into onion plant||Females can lay hundreds of eggs during their 2 to 4 week life span||Insect||Can be managed by good sanitation and appropriate insecticide.|
|Stunted plant growth, bulbs rotting, pest is a cream – white, bulbous mite which resemble pearl with legs||Damage by bulb mites allow secondary invasion by other pathogens and causes bulb rots||Arachnid||Don’t plant successive crops of onion or garlic in same location, allow fields to fallow to ensure that any residual organic matter decomposes completely because crop residues harbor mite population|
Harvest and Postharvest Handling
When the bulbs are to be stored they will keep better if allowed to become fully ripe before pulling. Figure 93 illustrates a field of onions in ideal condition for harvesting; the tops are dead and shrivelled and the outer skin of the bulbs dry. While full ripeness is highly desirable, other factors should be considered: There is danger of second growth, especially if there is much rain; better prices for the early crop may be an inducement to gather part or all of the crop sooner than if the bulbs are to be stored; when there are large areas to harvest it is necessary to start in ample time in order to complete the work while weather conditions are favourable and before there is loss from rain. Harvesting is often begun when most of the tops have merely turned yellow. Early pulling in the North is especially important for bulbs of foreign types. August and September are the busy harvesting months in the North, and March and April for the Bermuda crop in the South.
It is the universal custom to partially dry or cure the crop in the field. After removing the bulbs by hand or with a plough, if they are covered with soil, 8 or 10 rows of onions are thrown together into windrows, allowed to remain undisturbed for a few days and then stirred occasionally with a wooden rake to facilitate drying. White bulbs are quickly injured by exposure to sun and rain, so that these must be cured under some kind of cover. Topping is usually done in the field after the bulbs are ready for storage, the tops being twisted off by hand or cut with sheep shears. Extensive growers sometimes use topping machines, which also grade and deliver the bulbs in bags or crates. The curing process is continued in sheds, cribs or other suitable houses until the bulbs are ready for permanent storage.
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 as three months.
Where controlled atmosphere transport is used, the transport and storage duration of onions may be extended.
|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|
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.
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|
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.
Means of transport
Ship, aircraft, truck, railroad
As chilled goods: refrigerated containers with fresh air supply or controlled atmosphere.
As general cargo: actively ventilated containers, open-sided containers, open-top containers, flat racks(for loading below deck)
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.
- 40 m3/t (jute fabric bags, 25 kg)
- 14 – 2.50 m3/t (crates, 27 kg)
- 23 – 2.29 m3/t (cartons and fruit crates)
- 60 – 3.80 m3/t (bags)
- 37 – 3.40 m3/t (bags)
Storagee space requirements
Cool, dry, good ventilation, dark (due to risk of sprouting)
Fiber rope, thin fiber nets, matting, jute coverings
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).
Risk factors and loss prevention
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.
|Travel temperature||2 – 4°C|||
|0 – 4°C|||
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 odour.
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 odour 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.
Onions require particular temperature, humidity/moisture and ventilation conditions (SC VII) (storage climate conditions).
|Relative humidity||75 – 80% at -1 – +1°C|||
|70% at 8 – 10°C|||
|65% at 15°C|||
|65 – 70%|||
|Water content||82 – 88%|||
|Maximum equilibrium moisture content||65%|||
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 humidity 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 vapour. 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 favourable 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.
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.
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 vapour, 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.
In addition, onions must be protected from light during the voyage, as there is otherwise an increased risk of sprouting.
|CO2evolution||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 CO2content||10 vol.%|
|Active behavior||The rate of ethylene production is very low, being below 0.1 µl/kg*h .|
|Passive behavior||Dry onions exhibit low sensitivity to ethylene, while green onions are moderately sensitive to it (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 CO2concentrations or O2 shortages may arise. Therefore, before anybody enters the hold, it must be ventilated and a gas measurement carried out.
RF Self-heating / Spontaneous combustion
Oil content: 0.0037 – 0.055% allyl sulphide (essential oil). The respiration process in onions may give rise to a tendency to self-heating and a high CO2concentration and high O2 consumption, resulting in “scalding” (see RF Temperature).
|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.|
|Active behaviour||Onions produce dust.|
|Passive behaviour||Onions are sensitive to dirt, fats and oils. They may turn brown, especially if the previous cargo was ammonia (NH3).|
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 cause 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 favourable.
RF Toxicity / Hazards to health
If ventilation has been inadequate (frost) or has failed owing to a defect, life-threatening CO2concentrations 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 re-established 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.
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 vapour release during the voyage.
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. http://www.tisgdv.de/tis_e/ware/gemuese/zwiebeln/zwiebeln.htm#anfang
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. http://www.cargohandbook.com/index.php/Onions
|Color||Light and dark red color|
|Size||30mm and 70mm|
|Packing||Packed in 5 Kg, 10 Kg, 20 kg, or 25 Kg mesh bag as per customer requirement|
How are onions stored?
Onions in the field, ready for storage. The processes required to store onions properly. Once they are stored, onions undergo the following processes:
- The drying process: The processes of raising or lowering their temperature
- Conservation or storage proper: These processes are controlled by the Multiserverfrom AgroVentsystems BV.
The onion drying process
The aims of the onion drying process are:
- To remove all of the surface water or moisture from the onion
- To dry three or four layers of the onion skin, therefore sealing the onion
- To dry the stem and therefore also seal this part of the onion (remember that once the stem is cut, it is left open and must be kept as dry as possible.)
It is best to dry onions at between 25 and 30° Celsius.
The Multiserver programme for drying onions will ensure that the turbines work the maximum number of hours to extract moisture from the onions as quickly as possible, whilst maintaining the temperature between 25 and 30°C, and the relative humidity between 60% and 65%. In doing so, the aim is not to dehydrate the insides of the bulbs, but rather the 3 or 4 outer layers which separate the bulb from the outside. This process is completely controlled by the Multiserver, which constantly searches for the mixture of air with the highest drying capacity, by managing the doors, turbines, heaters and condensers.
The processes of raising or lowering the temperature of the onions
- Any changes to the temperature of the onions must be gradual and controlled, for two reasons:
- To prevent the cells in the bulb from being destroyed.
- To ensure that the centre of the bulb is at the same temperature as the outer layers.
This is why the Multiserver has two special programmes to control the heating or cooling of the produce.
Onion storage specialists advise cooling or heating the onions by 2 degrees per day at the very most, and recommend only 0.5 degrees per day. Such a slow change in the temperature of the onions has important consequences when planning to move the onions from the storage facility.
Let’s assume that the onions are stored at 2° Celsius.
The destination of the onions is 25° with 85% relative humidity, making the dew point 22.3°. This means that the onions must be heated by at least 20.3°, to 22.3°. At the maximum allowed increase of 2° per day, this process will take at least 10 days. These days must be kept in mind when planning the onion delivery. Especially in hot and humid climates, if the onions are not heated above the dew point, many of them will rot in the humidity.
The onion storage process
- A well-dried onionhas:
Crispy skin and a juicy middle.
- Before they can be stored, onions must tick the following boxes:
- The onion must be the right variety for storage.
- It must be harvested when it is completely physically ripe.
- The bulb must be firm and compact.
- The skin must be strong.
- The bulb must have a low water content.
- It must be grown to Good Agricultural Practices.
- It must not be fertilised with nitrogen during the last 6 weeks of its growth.
- It must be applied sufficient amounts of aglime.
- Onions can be stored at two ranges of temperatures.
- Between 0 and 4° Celsius
- Between 25 and 31° Celsius
Why these two ranges?
The onion, postharvest, is still alive. This means that it continues to respire. All plants give off water when they respire. The more they respire, the more water they lose, and losing water means losing weight, which is the last thing we want during the storage process.
Below is a graph indicating the relationship between the storage temperature and the intensity of the onion respiration, reflected in the weight (water) loss percentage over 5 months of storage.
Onion respiration with relation to storage temperature
The intensity of onion respiration is reflected in the percentage of weight (water) loss over 5 months of storage, with relation to the storage temperature. There are two ranges in which the onion respires relatively little, and consequently loses less weight:
- Cold storage between 0 and 4° Celsius
- Heat storage between 25 and 31° Celsius
The temperature ranges between 20 and 25° Celsius is very dangerous for the onion. It is inside this range that bacteria and fungi, enemies of the onion, are most active. Below and above this range, they remain inactive.
The graph demonstrates that within the range from 0 to 4° Celsius, weight loss is considerably less than within the range from 25 to 31° Celsius. On top of this, between 0 and 4° Celsius, onions can be kept healthy and suitable for storage for up to 12 months. Between 25 and 31° Celsius, they can be kept healthy and suitable for storage for up to 8 or 9 months.
However, when deciding at which range of temperatures to store your onions, you must bear in mind the local climate during storage time.
Why is this?
1) Temperate climates.
In a climate where the temperature varies between -10° Celsius and 15° Celsius it is much cheaper to cool your storage facility to between 0 and 4° Celsius. Less energy is wasted on keeping it cool.
2) Hot or tropical climates.
In a climate where the temperature fluctuates around 30° Celsius it is much cheaper to heat your storage facility to 25 to 31° Celsius. Less energy is wasted on keeping it at the right temperature. Another consideration is storing onions between 25 and 31° Celsius in the tropics.
As soon as an onion at 2° Celsius is exposed to humid air at 30° Celsius, it becomes moist.
This is what happens if you store onions at between 0 and 4° Celsius in the tropics:
When removing the onions from storage, they must be heated up to the outside temperature. This requires great energy and a lot of time. Meanwhile, onions stored at 30° Celsius can be removed immediately without the risk of moisture.
In both cases:
Relative humidity must be constantly kept between 55% and 65%. The temperature must remain constant; temperature fluctuations must be avoided during storage.
Bulbs must be kept in darkness. Whenever a light is switched on inside the storage facility, the onions in the light (on top of a pile) will react as if they were in direct sunlight, and start to sprout.
Please note: The onion bulb will lose significant weight through respiration (the bulb is still alive) if it is stored between 5 and 25° Celsius. The range between 20 and 25° is particularly dangerous because bacteria and mould thrive at this temperature.
During storage proper, ventilation must be kept to a minimum, because the onions are already dry (dry means dry!) and we do not want them to lose more weight through respiration, no matter how little that may be at this temperature. The little ventilation required is to avoid an increase in CO2 levels, and to eliminate the heat which the onions generate as they continue to respire slowly. https://www.onions-potatoes.com/storage/onions.php
Onions Exports by Country
Global sales from onions exports by country amounted to US$3.1 billion in 2016.
Overall, the value of onions exports were up by an average 20.8% for all exporting countries since 2012 when onions shipments were valued at $2.6 billion. Year over year, the value of global onions exports retreated by -3.1% from 2015 to 2016.
Among continents, Asian countries accounted for the highest dollar worth of exported onions during 2016 with shipments valued at $961.1 million or almost a third (30.9%) of the global total.
In second place were European Union exporters at 30.3% while 22.4% of worldwide onions shipments originated from North America. Smaller percentages came from Africa (8.4%), Latin America excluding Mexico (3.7%) and Oceania (mostly New Zealand and Australia) at 3.3%.
The 6-digit Harmonized Tariff System code is 070310 for onions and shallots. A shallot is a small bulb that resembles an onion. Shallots are used to make pickles or as an onion substitute.
Onions Exports by Country
According to global trading platform Alibaba, the following companies are examples of onions-trading exporters.
- Aalia Gmbh (Germany)
- Agricola Dece SL (Spain)
- BHK Krakow Joint Venture Sp ZOO (Poland)
- Camill Alimentos S/A (United States)
- JDGC (France)
- Jinxiang Luteng Trading Co, Ltd (China)
- Naqshbandi Enterprise (Pakistan)
- Silas Export (Netherlands)
- SMA Trading (Egypt)
- Sri Parvathi Exports (India)