The purpose of a curing period is to allow natural dormancy to develop and to dry the onion sufficiently. A properly cured onion will have a dry shrunken neck and dry outer scales. The respiration rate of a cured bulb is lower than that of an uncured bulb. Fully mature bulbs are harvested and cured by exposure to temperatures up to 35° in low (less than 50 percent) relative humidity. Air movement must be provided at the rate of i cubic foot/ minute/cubic foot of onions (60 cubic meters/hour/cubic meters of onions). Immature onions require twice the rate of air exchange.
Following curing, the temperature of stored onions is lowered gradually to 0° C, or slightly higher, with the relative humidity at 60 to 70 percent. Air exchange in the storage facility is important to prevent any condensation on the bulbs. Also, when the bulbs are removed from the storage, they should be conditioned for several days at 20 and 50 percent relative humidity.The tops and roots are removed during harvest. When this is not possible, they should be removed after curing, before storage or sale.
Freshly harvested onions are dormant and will not sprout for a variable period of time (depending on cultivar). Storage will prolong this dormancy. Sprouting will increase in storage temperatures above 4.4°C, decreasing again as temperatures exceed 25°C. To reduce the frequency of sprouting after the rest period, onions may be field treated with maleic hydrazide (MH-30) at 2.2 to 3.4 kg/ha when the tops are still green but beginning to senesce.
Cultivars intended for long-term storage should be firm with a thick dry neck; free from greening, root growth, sunburn, or freeze damage; and well covered with dry scales. Bulbs with fleshy, soft necks are susceptible to persistent rot, especially if storage humidity exceeds 70 percent. Flavor in onion is associated with pungency (propyl disulfides and other disulfides) and with sugars (glucose, fructose, and sucrose). Both sugar content and pungency are related to percentage dry matter. Short-day and long-day types differ in their flavors. Pungency and dry matter content are important quality attributes in onions for processing.
Onions normally are shipped in 22.y-kg mesh bags. The bulbs are graded by size, with jumbo and pearl sizes frequently used by processors. Those intended for international trade are packed in 25-kg bags.Green onions are pulled before bulbing, when the basal diameter exceeds 6 mm, and the roots are trimmed near the base. They should be washed free of soil. Discolored stalks are discarded. The green onions are bunched and packed in ice to preserve crisp texture and quality.
Vacuum cooling is possible but requires prewrapping in ventilated polyethylene bags to retard wilting. Storage life of green onions is limited to approximately one week at o°C and 90 to 95 percent relative humidity. Shallots are harvested by hand when the bases are at least 6 mm in diameter. The outer leaf is stripped off and the roots are trimmed before washing and bunching. If grown for dry bulbs, they are handled in a similar manner as onion bulbs.
source: Marid Digest, World Vegetable Center, Taiwan