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Adequate information was not available on post-harvest technology of onion particularly under Indian conditions till NHRDF initiated the work on this aspect at their own and in collaboration with other agencies as well as under All India Coordinated Research Project on Post-Harvest Technology of ICAR. The trials on various aspects were carried out to assess the performance and the recommendations are now available for adoption by onion growers.

The impact of post-harvest technology is found better when it is combined with pre-harvest factors determining shelf-life and thus integrated approach is absolutely essential as no single factor can show required impact as also the response of various factors depend on prevailing weather conditions.

Among pre-harvest factors , use of good storer varieties such as Agrifound Light Red and Arka Niketan has proved much beneficial. Restriction on use of excessive and delayed nitrogen application (not exceeding 100 kg/ha under normal fertility condition and application of N in 2-3 splits within 45  days of transplanting), use of ammonium sulphate instead of urea for topdressing, more use of organic manures to fulfil nutritional requirement, use of vermicompost, neem cake etc. have also showed encouraging results. Many new formulations of organic manures are now available and these can be tried by onion growers.

Over-watering of onion crop is always harmful. Irrigation at 1.25 ID/CPE with reduced N and irrigation frequency depending on weather conditions as well as more gap between 2 irrigations till 60 days after transplanting is beneficial. Similarly last irrigation given 10-15 days before harvesting reduces microbial losses in stored onions compared to withholding of irrigation before 5 days.

Major response was, however, recorded with pre-harvest pesticides sprays on decay in storage giving better results for 0.02% streptocyclin and 0.1% carbendazim at 10 days before harvesting. Application of thiophanate methyl @ 0.1% + streptocyclin or klorocin @ 0.2% + ekalux @ 0.25% immediately after neck cut and combined with field and shade curing is promising under North Indian conditions while use of maleic hydrazide @ 2000-3000 ppm 75 days after planting for kharif onions and @ 2500 ppm combined with 0.1% carbendazim spray at 10 days before harvesting in rabi onions give promising results.

Stage of harvesting plays a major role in determining the shelf-life of onions as it is linked with physiological maturity of bulbs. Harvesting one week after 50% neckfall and field curing by windrow method for 3-5 days till foliage turn yellow is recommended. Curing of kharif onions in sun or solar dryer with foliage and storage with dried foliage is better while curing in perforated crates with forced air circulation is recommended for rabi onions.

Stage of curing is another important step of post-harvest handling to reduce losses and maintaing quality of bulbs in storage. Shade curing after field curing and neck cut to remove excess moisture from surface of bulbs and to remove the field heat before packing, transportation or storage is essential operation. Shade curing for 10 days is optimum as extended shade curing have no any detrimental effects. It also helps the detachment of soil attached to roots, shedding of dried roots and thereby eliminating the chances of microbial infection in storage.Removal of foliage leaving 2.5 -3cm neck is beneficial.

Sorting and grading should be done at field level to minimize post-harvest losses at subsequent stages. Use of consumer packing for graded bulbs, avoid drop of bulbs from more than 30 cm height, avoid sunscald by eliminating over-drying of outer scales directly in sun etc. improve shelf-life of onions.

Perforated hessian bags and plastic-woven bags are used for onion packing to permit proper ventilation. Tier system of transportation on poor roads, restriction of loading height in trucks and wagons, providing ventilation in railway wagons and quick movement of onion wagons or truck loads are other factors which can help in minimizing the post-harvest losses of onions.

Use of irradiation improves shelf-life of onions.

Exposure of onion bulbs after harvesting when bulbs are in dormant stage with 60-90 Gy inhibit their sprouting regardless of crop season, environmental condition and type of storage. However, to reduce the microbial and other losses, combined use of irradiation with improved storage and providing the irradiation facilities at production level are to be considered.

Onions when spoiled if not disposed off immediately cause nuisance and environmental pollution. Such spoiled bulbs, scales, peels and rejected portion of onion bulbs from processing units form a large quantity and thus conversion of this into compost or manure or vermicompost is suitable alternative. Such manures after complete decomposition, since has manurial value comparable with farmyard manure, can be used for various crops.

Storage of onions

In Maharashtra, Gujarat, Haryana and Western Uttar Pradesh large-scale storage of onions is taken in conventionally-designed structures. In other states, the storage is taken only on small scale but now showing increasing trend after the post-harvest technology and improved storage structures have been popularized by NHRDF.

Traditional storage practices result in substantial losses in stored onions, hence use of improved storage structures as well as use of good storer varieties, judicious use of fertilizers, timely irrigation and post-harvest technology are essential to reduce the losses in stored onions.

Storage temperature and humidity affect loss in weight, respiration rate, sprouting, rotting and quality of bulbs in storage. The dormancy in onion bulbs is the main factor to determine as how long the bulbs can be stored. Inherent characters of dormancy based on equilibrium of inhibitors in onion bulbs also gets affected by temperature where lower (00C) and higher (300C) temperature increase the dormant state of onion bulbs and moderate (10-150C) temperature enhance the sprouting losses by breaking dormancy. Higher temperature, however, increases the rottage if accompanied with high humidity and desiccation/water loss is reduced at higher humidity but it increase the rooting and rotting. Hence equilibrium of temperature and relative humidity by providing sufficient aeration/ventilation is needed in improved onion store houses. The improved storage godown designed by NHRDF is based on the above factors.

Salient features of improved storage structures

Construction of structure on raised platform to prevent moisture contact and dampness.

Use of Mangalore tiled roof or other suitable material to prevent build-up, high temperature inside.

Providing bottom ventilation for free and faster air circulation to avoid formation of hot and humid pockets between the onion layers.

Avoid direct sunlight on onion bulbs to reduce sunscald, fading of colour and quality deterioration.

Restriction on width of each stack to 60-75 cm for hot and humid weather, 75 - 90 cm for mild and humid weather and 90-120 cm for mild and dry weather conditions.

Maintenance of stacking height to 100 cm for small and multiplier onions and hot weather and 120 cm for mild weather and for big onions to avoid pressure bruising.

Providing cubicles instead of continuous stack and sufficient space for ventilation from all sides.

One cubic meter area of store accommodates about 750 kg onions. Accordingly construction of godown for required capacity and construction of more units instead of single big structure and in zigzag manner when constructed in more rows to have better aeration.

Providing 2-tier if space available is insufficient.

Periodical disinfection of structures and premises to check rottage. The cost-efficient  structures is based on locally - available materials and labour.