Onions should be adequately cured in the field, in open sheds, or by artificial means before or in storage. Adequate curing in the field or in open sheds may require 2 to 4 weeks, depending on the weather.
The best skin color develops at 75 to 90° F. The most common method of curing in northern areas is by forced ventilation in the storage by blowing heated air at 75 to 85° F, through the bottom of the onion pile to the top, at two to three cubic feet of air per minute for each cubic foot of onions (or 30-50cfm/ton).
Use the higher airflow rate initially to remove surface moisture and seal necks. If the weather is cool and wet, forced air at 75 to 85° F and 70% relative humidity is recommended.
If the onions are also wet, forced air at 85° F and a relative humidity of 25-35%. Relative humidity should be used as soon as storage loading is completed. This should be continued until the outer skins and neck are dry.
Onions are considered cured when the neck is tight and the outer scales are dry and make a rustling sound when handled.
This condition is reached when onions have lost 3 to 5 % of their weight. If not adequately cured, onions are likely to decay in storage.
The common form of decay is gray mold rot (Botrytis), which occurs at the top of the bulb hence its name “neck rot”.
High temperatures and high humidity (80%) during curing with good air circulation favor development of desirable skin color.