| The only post-harvest treatment required for the long | | | | well ventilated conditions and using air at about 30°C, |
| storage of bulb onions is a thorough curing of the bulbs. | | | | 60-75% rh and 150 m³.h-1.m-3 is generally |
| Curing is a drying process intended to dry off the | | | | recommended for mechanical drying of onions. |
| necks and outer scale leaves of the bulbs to prevent | | | | 2.4 Cleaning |
| the loss of moisture and the attack by decay during | | | | Freedom from any impurity, which may materially alter |
| storage. The essentials for curing are heat and good | | | | the appearance or eating quality, is essential. Soil and |
| ventilation, preferably with low humidity. This dries out | | | | other foreign materials must be removed and badly |
| the neck and the two or three outer layers of the bulb. | | | | affected produce must be discarded. Cleaning may be |
| The outermost layer, which may be contaminated with | | | | carried out using air or by manually removing |
| soil, usually falls away easily when the bulbs are cured, | | | | unwanted materials on the bulb surface. |
| exposing the dry under-layer, which should have an | | | | Care should be taken to avoid physical injury on the |
| attractive appearance. Onions are considered cured | | | | bulb during these operations. |
| when neck is tight and the outerscales are dried until | | | | 2.5 Packaging |
| they rustle. This condition is reached when onions have | | | | General Information |
| lost 3 to 5% of their weight. | | | | Good packaging for onions must meet the following |
| If onions cannot be dried in the field, they can be | | | | criteria: (a) strong enough to retain the required weight |
| collected in trays, which are then stacked in a warm, | | | | of onions under the conditions of transport and |
| covered area with good ventilation. | | | | storage, (b) allow sufficient ventilation for the air around |
| In cool, damp climates, onions in bulk ventilated stores | | | | the bulbs to maintain relative humidity in the required |
| are dried with artificial heat blown through the bulk at a | | | | range, and (c) in many circumstances, provide a |
| duct temperature of 30 degrees Celsius. | | | | means of displaying legally required and commercially |
| Onions can also be cured by tying the tops of the | | | | necessary information (Brice et al., 1999). |
| bulbs in bunches and hanging them on a horizontal pole | | | | There are many traditional methods of holding onions |
| in a well-ventilated shades. Curing in shade improves | | | | for transportation and/or storage that do not fit into |
| bulb colour and reduces losses significantly during | | | | conventional packaging classifications. These include |
| storage | | | | 'string of onions', shelves and loose bulk In 'string of |
| Grading | | | | onions' packing, the bulbs are tied together by means |
| Onions after curing are graded manually before they | | | | of their tops to produce a bunch of bulbs is also a |
| go in to storage or for marketing. The thick neck, | | | | form of packaging. This is suitable for transporting |
| bolted, doubles, injured and decayed bulbs are picked | | | | small quantity of crop, and during storage, the bunches |
| out so also misshapen small bulbs. Sorting and grading | | | | are hung from the roof or from special racks. Shelves |
| is done after storage also to fetch better price. The | | | | for onion handling and storage are made from either |
| outer dry scales usually rub off during the grading | | | | wooden slats or metal mesh on a wooden or metal |
| process, giving the onions a better appearance for | | | | frame, and are usually fixed in position with the bulbs |
| market. It has been experienced that if storage is | | | | loaded and unloaded in the store. |
| arranged after proper sorting and grading losses in | | | | Ventilation (natural or forced) is usually achieved by |
| storage are reduced. | | | | passing air over the shelves. To achieve adequate |
| For local market the onions are graded based on their | | | | aeration of the bulbs, the depth of bulbs on the shelves |
| size. | | | | should be limited to 10 cm. |
| Extra large onion (>6 cm dia.) | | | | Onions are also stored loose bulk (instead of |
| Medium (4-6 cm dia.) | | | | containers) by heaping the bulbs directly on the floor or |
| Small (2-4 cm dia.) | | | | elevated platform. Because they are not restrained, |
| The extra large onions have great demand and | | | | the bulbs roll during store loading to completely fill the |
| fetches very good price. | | | | storage space. Bulk storage permits maximum |
| General Characteristics | | | | utilisation of store space, and uniform aeration is easier |
| The bulbs shall: | | | | to achieve than in stacks of bags or other rigid |
| • be reasonably uniform in shape, size colour | | | | packaging. |
| and pungency of the variety /type | | | | However, where bulk storage is to implemented, the |
| • be mature, solid in feel, reasonably firm with | | | | retaining walls must be strengthened when storing |
| tough clinging skins. | | | | larger quantities of bulbs, and arrangements need to be |
| • be throughout cured and dried. | | | | made for rebagging before subsequent marketing. It is |
| • be free from dust and other foreign material. | | | | also difficult to inspect bulbs regularly under these |
| • be free from defective, diseased, decayed | | | | storage conditions. Loose bulk handling of onion is most |
| and damaged bulbs caused by seed stems, tops | | | | suitable for large-scale operations where forced |
| • oots, moisture, dry sun scald burn, sprouting, | | | | ventilation can be provided during long-term storage. |
| mechanical or other injuries and staining. | | | | Soft cultivars (which are also generally sweet) 'Vidalia |
| • be free from moulds, soft rot and insect | | | | Sweets' should not be stored in loose bulk because of |
| attack. | | | | their high susceptibility to compression and impact |
| • % of seed stem or bolted bulbs shall not | | | | damage. |
| exceed 20% in Nasik kharif onions. | | | | Onions can be packaged and stored in a variety of |
| Bangalore and Krishnapuram onions will be free from | | | | containers such as boxes, cartons, bags, bulk bins, |
| bottle necks or doubles. | | | | pre-packs, plastic film bags, and stretch-wrapped trays. |
| Grade designations and definitions of quality for export | | | | Packages typically contain 25 kg and above, especially |
| of onions: | | | | for transporting crop from field to store and/or during |
| Different size but not below 15 | | | | storage. The same 25 kg bags or smaller bags may |
| 1. Tolerance for size in big onions: For accidental errors | | | | be used from store to market place. Decision on which |
| in sizing, not more than 5 % by weight of the bulbs in | | | | type of packaging to use depends on crop size, length |
| any lot may be of next lower grade than the minimum | | | | of storage and marketing requirements. A problem |
| diameter prescribed in Nasik, Saurashtra, Bellary or | | | | with packaging onions in boxes, net bags and bulk bins |
| Poona onions. In case of Podisu, this error in sizing not | | | | is that if they are too large, and airflow pattern tends |
| more than 10 % by weight. In this case, smallest onion | | | | to be around rather than through them. Under this |
| in bunch would be taken for measuring the diameter. | | | | condition, the respiration heat of the bulb results in a |
| 2. Defective, diseased and damaged shall mean | | | | warm, humid environment in the centre of the |
| malformed bulbs and the bulbs internally or externally | | | | package, which can result in decay or sprouting. To |
| damaged, diseased or discoloured material affecting | | | | avoid these problems in large stores, the capital |
| the quality. The decayed onions shall not exceed 2% in | | | | investment in packaging may be quite substantial. |
| any lot. | | | | Onion Bags |
| General: The grade shall be allowed to be packed only | | | | Sacks and nets used for onion packaging fall into three |
| against irrevocable letter of credit. | | | | groups: (i) general-purpose jute sacks, as used for |
| # NS grade: This is not a grade in its strict sense but | | | | many agricultural commodities, (ii) open-weave sacks |
| has been provided for the onions not covered under | | | | of sisal-like fibre, (iii) open-mesh nets, normally of plastic |
| regular grade. Onions under this grade shall be | | | | materials and (iv) big bags, used alternatively to crates, |
| exported only against a specific order from foreign | | | | containing up to 1000 kg . Jute sacks are readily |
| buyer inducting the quality. | | | | available in most developing countries, but their |
| Packaging | | | | disadvantages include: (i) generally too large - may |
| Packing should be small for easy handling during transit | | | | contain 100 kg onions, hence difficult to handle and an |
| and may vary according to market demand. Onions | | | | increased risk of mechanical damage; (ii) bulbs are not |
| are packed in jute (hessian) bags for transporting to | | | | visible through the fabric, and it is difficult to monitor |
| yard or brought as loose. For safe handling, 40 kg | | | | condition during storage; (iii) there is some resistance to |
| open mesh jute bags having 200-300 g weight should | | | | airflow if they are used in an aerated store; (iv) difficult |
| be used in domestic market. For export, common big | | | | to label effectively; and (v) recycled sacks may |
| onions are packed in 5-25 kg size open mesh jute | | | | encourage spread of post harvest diseases. |
| bags. | | | | Sisal sacks are made from sisal-like hard fibres and |
| Bangalore Rose and multiplier onions are packed for | | | | have an open weave, with thick threads spaced |
| export in 14-15 kg wooden baskets. Nylon net bags, | | | | between about 10 and 15 cm apart. The rough nature |
| when used for packing have resulted in less storage | | | | of the fibre provides a sufficiently stable weave. |
| loss because of good ventilation. | | | | These sacks are similar to jute sacks, but will allow |
| Handling | | | | limited visibility of the onions and impedance to airflow |
| Bulbs intended for storage must be free from cuts and | | | | is less. |
| handled with extreme care. Onions should not be | | | | Open-mesh nets are the most widely used package |
| dropped on to non-resilient surface from more than 6 | | | | for onions, and they are normally red or orange in |
| feet height. If onions are to be stacked after packing in | | | | colour. The slippery nature of plastics can result in the |
| store or trucks, the better height is 2-2.5 metres. | | | | movement of the threads allowing large holes to open |
| Losses due to rot is reported to be more if onions are | | | | up. To overcome this problem, alternative nets are |
| stored in gunny bags than in loose or wooden crates. | | | | industrially produced to give fully stable mesh and |
| Storage | | | | stronger bag. The principal techniques include: (i) using |
| Proper storage of bulbs is necessary both for | | | | extruded net from high-density PVC, (ii) knitted |
| consumption and also for seed production. Onions | | | | (warp-knitted) and asymmetric construction, and (iii) |
| should not be stored unless adequately dried either in | | | | special weave in which weft threads are double, and |
| the field or by artificial means. It is necessary to dry the | | | | twisted. They are also slowly degraded by sunlight, |
| neck tissue and outer scales until they rustle when | | | | and should not be left outdoors for long period before |
| handled otherwise the bulbs will rot in storage. | | | | use. In comparison with the other types of bags, they |
| Sprouting in onion is controlled by temperature. The | | | | offer several advantages, including: (i) light weight, small |
| temperature between 10-25°C increases sprouting. | | | | bulk when empty, (ii) usually available in 12.5 and 25 kg |
| Rooting is influenced by relative humidity (RH). | | | | sizes, (iii) fairly good visibility of bulbs, (iv) excellent |
| More the relative humidity, more is rooting. Weight loss | | | | ventilation, (v) hygienic, (vi) easy closing (draw-string |
| is more when temperature is above 35°C. Under | | | | types only), (vii) and crop brand and marketing |
| ambient conditions the onions are stored at a | | | | information may be printed around the middle of the |
| temperature of 30-35° C with RH of 65-70%. In cold | | | | bag for easy identification. |
| storage, temperature is maintained at 0-2°C while the | | | | Rigid Packages |
| RH is kept at 60-75%. | | | | A range of rigid containers is used to package onions |
| Sprouting is checked effectively if Maleic Hydrazide at | | | | for transportation, marketing, and/or storage (Opara |
| 2500 ppm is sprayed at 75-90 days after transplanting. | | | | and Geyer, 1999). The principal rigid containers are |
| Effect is, however, more pronounced in kharif season | | | | trays (10-15 kg of onions each), boxes (up to 25 kg), |
| than in rabi season. The storage rots could be | | | | and bulk bins (up to 1000 kg). These types of |
| checked if proper cleanliness is maintained in store and | | | | packaging enable segregation of onions into different |
| crop is sprayed with 0.1% Carbendazim after 90 days | | | | cultivars or sources. Choice of packaging material is |
| of transplanting and just before harvest. In India, the | | | | important as wooden bins, for example, are liable to |
| farmers practice different storage methods. The | | | | termite attack, and weathering during off-season. Rigid |
| onions are bulk stored in special houses with thatched | | | | containers are also expensive, need regular |
| roof and side walls are made up with bamboo sticks | | | | maintenance and a forklift is required for handling larger |
| or wire mesh for good air circulation. In North India, the | | | | containers. Where rigid containers are used for onion |
| sides are also covered with gunny cloth. Onions are | | | | storage, building design is simpler than that for |
| stored in these sheds by spreading them on dry and | | | | large-scale loose bulk storage as reinforcement of |
| damp proof floor or racks. Periodical turning of bulbs or | | | | retaining walls are not required to support the bulbs. |
| removal of rotten, damaged and sprouted bulbs should | | | | Handling damage of bulbs during filling and emptying |
| be done. Well-ventilated improved storage structures | | | | can be high, but damage is reduced during store |
| with racks or tiers having two or three layers of bulbs | | | | loading and unloading operations in comparison with |
| would be desirable for proper storage. | | | | loose bulk handling and storage. |
| The salient features of improved storage structures | | | | Stacking of containers must be carried out with care |
| are as below | | | | and to ensure that the ventilation air is forced through |
| • Construction of storage godown on raised | | | | the containers of bulbs and not around them. One of |
| platform helps in reduction of moisture and dampness | | | | the main advantages of rigid containers is that they |
| • Use of Mangalore tiles roof or other suitable | | | | facilitate regular inspection of produce, and when |
| material prevents built up of high temperature inside. | | | | problems occur with the stack, the area affected is |
| • Increased centre height and more slope is | | | | often limited to a few trays, boxes or bins which may |
| better for air circulation and preventing humid | | | | be more easily isolated and removed than in loose bulk |
| microclimate inside godown. | | | | handling system. |
| • Bottom ventilation provides free and faster | | | | Onion Pre-packs |
| air circulation to avoid formation of hot and humid | | | | Onions are commonly sold in retail outlets in pre-packs |
| pockets between the onion layers. | | | | with a capacity of 0.5-1.5 kg. Pre-packing offers the |
| • Avoid direct sunlight on onion bulbs to reduce | | | | following advantages over single bulbs in heaps or |
| sunscald, fading of colour and quality deterioration. | | | | bags: (i) price can be attached to produce, (ii) the |
| • Restriction on width of each stack to 60-70 | | | | collation of a number of pieces into one unit of sale |
| cm for cool humid weather, 75-90 cm for mild and | | | | may promote sale of a larger quantity than would be |
| humid weather and 90-120 cm for mild and dry | | | | purchased otherwise, (iii) provides a clean odourless |
| weather conditions | | | | unit for the customer to handle, and (iv) reduces time |
| • Restriction of stacking height to 100 cm for | | | | spent at the check-out. The use of weight/price |
| small and multiplier onion and hot weather and 120 cm | | | | labelling machines and bar-coding has reduced the |
| for mild weather and for big onion to avoid pressure | | | | need to pack to fixed nominal weights. During |
| bruising. | | | | preparation for retail, the quantity of produce is |
| • Cubicles should be made instead of | | | | measured by hand or machine and filled into the pack. |
| continuous stack leaving sufficient space for ventilation | | | | Then the actual weight and price and/or bar-code are |
| from all the sides. | | | | automatically calculated and printed on a label, which is |
| One cubic metre area of store accommodates about | | | | attached to the package. This mechanised weighing |
| 750 kg onions. | | | | and labelling system assists the packer in accurate |
| Transport | | | | record keeping and avoids losses due to inaccurate |
| Onion stocks are transported in bullock carts, tractor | | | | pack weights. The three main types of onion |
| trolleys and trucks as also railway wagons are used | | | | pre-packs are nets, plastic film bags, and |
| for longer distance movement within the country. | | | | stretch-wrapped trays |
| Onions are transported in ventilated ships as well as | | | | 2.6 Bulk Storage |
| sailing vessels / motorboats for export to Gulf and | | | | General Requirements |
| South-East Asian countries. It is also shipped in 3.5m | | | | The objectives of onion storage are to extend the |
| containers or 7m containers by loading on ships. | | | | period of availability of crop, maintain optimum bulb |
| ............................................................................................................ | | | | quality and minimise losses from physical, physiological, |
| 2.1 Pre-harvest Operations | | | | and pathological agents. Bulbs selected for storage |
| The condition of onion leaves is a good indicator of the | | | | should be firm and the neck dry and thin. Discard |
| maturity and general state of the bulb. Bulb onions | | | | thick-necked bulbs because they are most likely to |
| which are to be stored should be allowed to mature | | | | have high moisture content than optimum for storage, |
| fully before harvest and this occurs when the leaves | | | | and therefore would have short storage life. Skin |
| bend just above the top of the bulb and fall over. As a | | | | colour should be typical of the cultivar. Microbial |
| practical guide, farmers should conduct sample counts | | | | infections such as Aspergillus niger occur during |
| on the number of bulbs, which have fallen over in a | | | | production of onions but these will only develop on the |
| field; and when the percentage of bulbs, which have | | | | bulbs during storage where the storage environment is |
| fallen over, reaches about 70-80% then the entire crop | | | | conducive for their growth. Prior to storage, crop must |
| should be harvested. Harvesting could commence | | | | be cleaned and graded, and all damaged or diseased |
| earlier when 50-80% of the tops have gone over, | | | | bulbs removed. |
| before it is possible to see split skins exposing onion | | | | Careful harvest and pre-storage treatments with |
| flesh Storage losses at optimum maturity are normally | | | | minimal mechanical loads are important to achieve a |
| lower than those harvested before the tops collapse. | | | | long storage period. Both store room temperature, |
| Bulbs generally mature within 100-140 days from | | | | relative humidity, and atmospheric composition affect |
| sowing, depending on the cultivar and the weather. | | | | the length of storage that can be achieved. Several |
| Spring onions mature for harvesting after 35-45 days | | | | technology options are available for bulk storage of |
| from sowing. Harvested crop should be allowed to dry | | | | onions, including low-temperate storage, |
| or cure and ripen in the sun for several days after | | | | high-temperature storage, 'direct harvest' storage and |
| lifting. Onions can yield up to 5 t.ha-1 under good | | | | the use of controlled atmosphere (CA) stores. The |
| growing and management conditions. | | | | recommended storage conditions under these |
| 2.2 Harvesting & Transport | | | | systems are summarised below. |
| Manual harvesting is the most common practice in | | | | Storage at Low Temperature |
| most developing countries. This is normally carried out | | | | For successful low temperature storage, good |
| by levering the bulbs with a fork to loosen them and | | | | ventilation and a low level humidity in the range of |
| pulling the tops by hand. In developed countries, | | | | 70-75% is essential. To maintain good quality crop, the |
| especially in large scale farms, mechanical harvesting is | | | | period of storage varies but may be up to 200 days. |
| commonly used. The harvesting techniques adopted | | | | For maximum storage period and minimum losses |
| are influenced by weather condition at harvest time. In | | | | bulbs should be fully mature at harvest, and dried until |
| areas where warm, dry weather occurs reliably, the | | | | the 'neck' of the bulb is tight. For large-scale |
| curing and bagging of the crop can be done in the field | | | | commercial storage, onions are usually stored under |
| (two phase harvesting). In wetter, temperate regions, | | | | refrigeration and the most commonly recommended |
| mechanical harvesting and artificial heating and | | | | conditions are 0°C with 70-75% rh. Regular ventilation |
| ventilation for drying are essential for reliable production | | | | and monitoring of both temperature and relative |
| of high quality bulbs on a large scale. | | | | humidity in the store are necessary to avoid significant |
| The following steps are followed during two-phase | | | | fluctuations in environmental conditions. During the first |
| harvesting of onions: (a) mowing the leaves (if | | | | few days of storage the fans should provide an |
| necessary); (b) stubbing, undercutting and sieving the | | | | adequate airflow, to remove water in the outer skins |
| onions to remove stones and clods; (c) roll the soil in | | | | and to dry bruises. High air speed is needed for a |
| the row to get a plane surface; (d) drying the bulbs | | | | period of up to 1 week, until the skin of the upper onion |
| (windrowing) 8 to 10 days in the field; (e) turning the | | | | layers in the bulk rustles. Excessive humidity in-store will |
| bulbs 1 to 2 times; (f) harvesting, sieving and | | | | lead to the development of roots and promote rotting |
| hand-grading, overloading into a trailer or in crates; and | | | | while higher temperatures will result in sprouting and |
| (g) transport. For one phase harvesting usually | | | | promote development of pathological disorders such |
| commercial potato harvesters have been adapted. | | | | as Botrytis rots (Thompson, 1982) Bulbs freeze below |
| After mowing the leaves the crop is immediately | | | | -3°C and a range of storage temperatures and |
| harvested, sieved, hand graded and loaded onto the | | | | relative humidities have been recommended for safe |
| trailer. Because of the additional operations involved, | | | | storage of onions (Table 5). Spring (green) onions |
| labour costs for two-phase harvesting are about 30 to | | | | store best at about 0°C and very high humidity |
| 100 % higher than for one phase harvesting. The main | | | | (95%) (Table 6). The maximum length of storage |
| disadvantage of one-phase harvesting is the high | | | | under these conditions varies from just a few days to |
| energy consumption required for mechanical drying. | | | | about 3 weeks. |
| Using combine harvesting, the standardised working | | | | Ventilation must be carefully applied inside the store to |
| hours has been calculated to be 2.7 to 2.9 hr.ha-1 for | | | | achieve the required temperature and humidity levels |
| stubbing, 2.4 to 2.6 hr.ha-1 for turning and 8.9 to 11 | | | | without inducing condensation of water on the surface. |
| hr.ha-1 (KTBL, 1993). | | | | Onion Storage at High-temperature |
| Harvested bulbs are placed in containers (basket, bins) | | | | Onions can be stored at high temperatures of over |
| or tied into bunches and placed directly on the floor of | | | | 25°C at a range of relative humidities (75-85%) |
| a trailer for transport. These trailers can be pulled by | | | | which is necessary for minimising water loss. |
| an animals (such as donkey) or mechanical transport | | | | Storage at temperatures of 25-30°C has been |
| such as a tractor. Both packaging and transport | | | | shown to reduce sprouting and root growth compared |
| systems must be selected to ensure minimum handling | | | | to low-temperature storage (10-20°C). However, |
| damage to produce. Hard surfaces should be | | | | weight loss, desiccation of bulbs, and rots occurred at |
| cushioned with leaves, foam or other appropriate | | | | high temperatures, making the system uneconomic for |
| force decelerators. | | | | long periods of storage that is required for successful |
| 2.3 Curing & Drying | | | | onion marketing (Thompson et al., 1972; Stow, 1975). In |
| Both curing and drying remove excess moisture from | | | | tropical climates, high-temperature storage of onions |
| the outer layers of the bulb prior to storage. The dried | | | | can be achieved under both ambient and heated |
| skin provides a surface barrier to water loss and | | | | storage conditions. Under these conditions, ventilation |
| microbial infection, thereby preserving the main edible | | | | must be carefully applied inside the store to achieve |
| tissue in a fresh state. Drying also reduces shrinkage | | | | the required temperature and humidity levels. |
| during subsequent handling, reduces the occurrence of | | | | 'Direct Harvest' Storage |
| sprouting, and allows the crop to ripen before fresh | | | | The need to cure onions can pose considerable |
| consumption or long-term storage (Opara and Geyer, | | | | challenges in situations where the climatic condition is |
| 1999). This process of dehydration is sometimes called | | | | unpredictable during the harvest period. To overcome |
| 'curing', but the use of the word 'curing' for onion drying | | | | these problems, the 'direct harvest system' has been |
| is rather inaccurate since no cell regeneration or | | | | developed and used extensively, particularly by |
| wound healing occurs as in other root crops such as | | | | growers in the UK, since the early 1980s. The bulbs are |
| yam and cassava. Drying reduces bulb weight and | | | | harvested while green, topped, loaded into store, dried |
| since they are sold mostly on a weight basis, achieving | | | | and cured using well controlled ventilation system, and |
| the desired level of dehydration is critical. Weight | | | | thereafter held in long-term low-temperature storage |
| losses of 3-5% are normal under ambient drying | | | | as required (Table 7). During stage I, removal of |
| conditions and up to 10 % with artificial drying. | | | | excessive surface moisture is achieved at high airflow |
| In traditional small-scale operations, onion drying is | | | | rates, ignoring the rh of the air. Stage II is completed |
| carried out in the field in a process commonly called | | | | when the skins have been cured on the bulb. |
| 'windrowing'. It involves harvesting the mature bulbs and | | | | Adequate control of the storage condition at the |
| laying them on their sides (in windrows) on the surface | | | | various stages is critical to the success of this storage |
| of the soil to dry for 1 or 2 weeks. In hot tropical | | | | system in maintaining required bulb quality. |
| climates, the bulbs should be windrowed in such a way | | | | A is used in combination with coldstorage to extend |
| to reduce the exposed surface to minimise damage | | | | the storage life of onions. Recommended air |
| due to direct exposure to the sun. In wet weather, the | | | | composition and temperature regimes are summarised |
| bulbs can take longer time to dry and may develop | | | | in Table 8. Spring onions generally tolerate higher CO2 |
| higher levels of rots during storage. The side of the | | | | and O2 levels than bulb onions, and the levels of CO2 |
| bulb in contact with wet soil or moisture may also | | | | and O2 combination required varies depending on the |
| develop brown strains or pixels, which reduce the | | | | storage temperature (Table 9). Commercial CA |
| appearance quality and value. Obviously, successful | | | | storage of onion bulbs is limited partly because of |
| windrowing is weather dependent and therefore | | | | variable success and inconsistent effects on bulb |
| cannot be relied upon for large scale commercial onion | | | | quality. However, high carbon dioxide (0-5%) and low |
| production business. Bulbs harvested for storage | | | | oxygen (1-3%) levels in combination with low |
| require in total 14-20 days of ripening or drying before | | | | temperature storage has been shown to reduce |
| being stored. Harvested onions may also be placed in | | | | sprouting and root growth (SeaLand, 1991; Hardenburg |
| trays, which are then stacked at the side of the field to | | | | et al., 1990). The combination of CA storage (5% CO2, |
| dry. In some tropical regions, the bulbs are tied together | | | | 3% O2) and refrigerated storage (1°C) also resulted |
| in groups by plaiting the tops, which are then hung over | | | | in 99% of the onion bulbs considered marketable after |
| poles in sheds to dry naturally. | | | | 7 months storage; however, 9% weight loss occurred |
| Harvested bulbs can also be taken straight from the | | | | (Smittle, 1989). |
| field and dried artificially either in a store, shed, barns, or | | | | Onion response to CA storage varies among cultivars. |
| in a purpose-built drier. This method is commonly used | | | | Therefore, experiments should therefore be conducted |
| when crops are stored in bulk but it can also be | | | | under local conditions to determine the appropriate |
| applied to bags, boxed or bins. Under this method, bulbs | | | | level of gas composition suitable for safe storage of |
| are laid on racks and heated air is rapidly passed | | | | local cultivars. CA storage generally increases the |
| across the surface of the bulbs night and day | | | | pungency of characteristic cultivars. For the 'Viladia |
| [O'Connor, 1979; Brice et al., 1997]. Drying may take 7-10 | | | | Sweets' which are known for their sweetness and |
| days and is considered complete when the necks of | | | | low pungency, the recommended storage conditions |
| the bulbs have dried out and are tight and the skins | | | | are (Smittle, 1989): 1 ºC, 70-80% rh, 3% O2, 5% CO2, |
| shriek when held in the hand. The control of humidity | | | | 92% N2, and ventilation rate of 5.m3.h-1.m3 of onions. |
| level in the store is critical. Under very high humidity, | | | | 2.7 Processing |
| drying is delayed and fungal infection can increase. | | | | Onion bulbs are generally chopped into desired sizes |
| However, if relative humidity is too low (below 60%), | | | | and shapes using a knife. Many commercial devices |
| excessive water loss and splitting of the bulb outer | | | | are also available for chopping onions. In some food |
| skins can occur, resulting in storage losses and | | | | preparations, the onions are blended with other |
| reduction of bulb value. Placing onions on wire mesh in | | | | ingredients to produce the desired flavour. |