- Malheur Experiment Station
Properly drying onions prior to storage is key to their preservation and prevents the development of bacteria, mold, and freezing of the onions. Drying is especially important in wet climates or if the onions have been exposed to extended periods of moisture through the harvesting season. The perishability of onions is directly related to their respiration rate; less moisture present at the time of storage means a longer shelf life (Sargent, Talbot, & Brecht, 1988).
It is extremely important that the onions are dry before they are lifted from the field and that all excess moisture is gone before they are stored. A dry outer layer of skin protects and maintains onion freshness and quality during storage. Bulbs harvested for storage require a total of 14 to 20 days of drying and curing prior to being stored (Opara, 2003). An onion is dried correctly if the neck is tight and outer scales are of uniform color and dry to the touch (Adamicki, n.d.). Proper technique prevents shrinkage and sloughing off of the onion caused by excessive drying (Matson, 1985).
Curing, like drying, requires heat application (natural or artificial) before the onions are placed in a storage facility. Proper curing optimizes maximum storage life and quality of onions, protects them from damage and disease, and promotes natural dormancy. Curing processes vary from grower to grower in each geographical growing region. Cost effectiveness and sustainability are dependent on weather conditions and available resources.
Artificial curing can reduce the incidence of neck-rot and spoilage of the onions and may be necessary if onions are exposed to significant amounts of moisture, humidity, or low temperatures during the harvesting season. Artificial curing methods involve blowing hot air on onions placed on large pallets. The hot air—near 115° F (approximately 66°C) – must blow at approximately 3-4 feet per second for a period of 16-24 hours. It is important to monitor the onions to avoid cooking or overheating, as temperatures exceeding 125°F (52° C) for 24 hours or 115°F (46°C) for 48 hours can severely damage onions in storage (Vaughan, Cropsey, & Hoffman, 1964).
Natural curing may be beneficial if onions are grown in a climate hot and dry enough for them to be cured outside. In natural curing, onions are typically windrowed, topped, and left to dry in the field in bags or crates for a period of at least 5 days. During this time, precipitation could disrupt the curing process. If weather does not permit windrowing, other drying methods may be necessary. It has been noted that for optimal curing, the use of bulk pallets or wooden crates should be considered (Vaughan, Cropsey, & Hoffman, 1964). It should also be noted that if curing in crates or pallets is not possible, curing onions in burlap bags is suggested (Vaughan, Cropsey, & Hoffman, 1964). An onion has been correctly cured when the neck is dry and shrunken (Matson, 1985).
Other natural curing methods minimize handling by allowing crops to cure in place. In the cure-in-place method, water supply to onions is cut 1 to 2 weeks before lifting, working best in areas with dry, warm harvesting seasons. Once fields have been dried, onions can be undercut (removal of roots) by a machine, then lifted, topped, and loaded in a one step process for immediate bagging or storing. This minimal handling process maximizes efficiency by reducing costs, energy usage, and the amount of harvesting processes.