The following was adapted by Roger Bullard for EDN from the book reviewed in this issue, Onion Storage In The Tropics – a practical guide to methods of storage and their selection. The wild species giving rise to the bulb onion (Allium cepa) came from southwestern Central Asia, and from it a variety of landraces and cultivars have been developed which flourish under a wide range of climatic conditions.
Onions are normally harvested at the end of the bulbing process when “the leaf blades are no longer able to support themselves, the neck softens and the leaves collapse. At this point the bulb has reached maturity. It then enters a period of dormancy during which little change appears to occur…"
“Onions grow best under dry conditions and at moderate temperatures. Under these conditions onions mature fully and, if they are cured before storing, may last for a long time without loss of quality.” Onions grown under very hot or wet conditions face difficulties from fungal and bacterial diseases. In addition to lower yields, the storage losses will probably be higher because bulbs may have been infected before they went into storage. Harvesting after the leaves have completely dried but before the bulbs have begun to resprout, and topping (cutting off the leaves) after most of59-5 Permanent shed for outdoor drying of onions (Ministry of Agriculture, Jamaica). the leaf has dried, will decrease storage losses to these plant pathogens. Poor quality “culls” should be removed before the onions are dried and stored.
It is best not to promote excessive bulb growth with too much nitrogen fertilizer and irrigation.
Cultivars vary considerably in how well they store. Cultivars suitable for storage should produce a number of outer dry scales or skins which form a vapor barrier around the bulb, thereby minimizing moisture loss and the entry of fungi or bacteria. Locally adapted onion varieties, selected over many years within the tropics, will probably store better than the imported types, especially the ‘short-day’ varieties from temperate climates.
Before storing, bulbs should be properly dried and cured. Use some technique that will both remove surface moisture and allow high temperature formation of strong, intact outer protective skins and neck closure of the onion. High temperatures (e.g. 27° C or 80°F), low humidity (< 60% RH) and good ventilation are important in drying. Laying the onions on the soil in windrows is the simplest form of field drying. Alternatively, they may be removed from the field and placed in a heap, or spread in a shallow layer, preferably on a cement slab to avoid contamination of soil-borne pathogens. Problems associated with outdoor drying can include poor drying rates, sun scald, and rain.
A simple outdoor shaded structure can avoid these problems. A good design for natural ventilation is to make shallow trays with screen or woven bottoms. Arrange trays vertically, spaced sufficiently apart to catch the prevailing winds. If clear corrugated plastic sheeting is available, heat required for the curing process can be achieved by using it in the roof structure and thereby capturing passive solar energy.
Maintaining the bulbs at suitable temperature and humidity is vital to the successful storage of onions. The optimum relative humidity range is from 65-75%. There are two favorable temperature regimes. Minimal storage losses occur at 0-5°C (32-41° F), but for the farmer or villager not having refrigeration, 25-30°C (77-86° F) would be the best choice. Temperatures that are either too high or too low will increase rotting or sprouting. These are the greatest enemies to successful storage of onions. A high technology solution to the sprouting problem is to add chemical "sprout suppressants” prior to storage.
Stored onions should be arranged so that some air reaches each onion, if possible. The storage structure should be designed to maximize natural ventilation. In the tropics there are a variety of ways to do this in home storage. Strings of onions may hang from the ceiling or roof beams or horizontal bamboo rods or racks. In Belize they are hung over the fireplace where wood smoke may have an inhibiting effect on sprouting. They may be stored in bags in the home or in the shade. Ventilation is facilitated by arranging the bags in a particular way. In Pakistan bulbs are sometimes laid on beds of coarse sand. An experiment in the Philippines compared storage of onion layers separated with 50mm of sand, sawdust, rice straw, rice husk or wood shavings. All materials improved storage performance, but sand and sawdust were best.
Many traditional on-farm structures for storing onion are found worldwide. They are constructed from mud, bamboo, straw, wood and other materials with thatching or some fabricated material to protect the interior from rain. A typical good design would be a simple ventilated structure constructed from bamboo. The floor would be at least 15 cm (6 in.) from the ground with framework constructed from poles about 8 cm (3 in.) in diameter placed 0.6 m (2 ft.) apart. The roof is grass thatch and the four sides and base are constructed from split bamboo poles with gaps of 2.5 cm (1 in.) between them to allow good air flow. Onions would be layered within this structure in one of the manners discussed above which will allow air flow to each stored onion.
[Editor: Missionary Tom Post told me of a dramatic example of the importance of onion storage. An innovative farmer in Belize found a way to produce nice bulbs in a region where few farmers could achieve this. If I recall correctly, the farmer received about as much income that year from a rather small plot of onions as from the rest of his farm. But the next year he could hardly sell the onions. It turns out that the storekeepers who bought his onions the year before had suffered huge losses during storage.]
“Successful onion storage is the outcome of a process which begins at the crop planning stage and which continues through cultivation, harvesting and post-harvest handling right through to sale of onions to the consumer.”
The physical method in which the onions are held (i.e. bulk, tray, box, bin, or bag) and designing the storage area for maximum natural ventilation are important considerations. If power is available, one can make further improvements in the storage conditions by strategically placing fans as a means of forced aeration.