Storage and Processing
When the onion crop is mature (stops growing), it is harvested. The harvest typically takes several weeks because of drying and curing time.
Field curing normally takes from 6 to 20 days. A properly cured onion has a dried, shrunken neck and dry outer scales without punctures or splits. The grower's goal is to have completed this process along with full maturation. But if harvest must occur during rain or dew periods, when onion bulbs are immature, or when maturity must be hastened, the grower must speed up the curing process mechanically.
After the onion bulb is lifted, its roots quickly dry, no new leaves are produced, and its respiration rate dramatically decreases. The bulb will continue to size up. The neck must dry and shrink, and a maximum number of outer scales must be retained.
The most important factor for the grower to consider at this point is the condition of the wrapper layers. The bulb should have several layers of wrapper scales. This is important because a mature onion with several wrapper scale layers will cure more rapidly than an immature onion with fewer wrapper scales and a higher moisture content. When the wrapper scales are dry, they are not only an important protection against storage rots but also contain a natural sprout inhibitor. In addition, the better the quality of the wrapper scales on an onion bulb, the better it will store. Excessive drying, however, will cause sloughing off of the outer scales (baldness) and excessive shrinkage of the onion.
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Before the onions are lifted for storage, dew should be completely evaporated from the onions and from the ground. If the bulbs are wet, rot organisms in the soil may separate the dry wrapper scales from the root plate. Many times surface drying will be used to speed maturation and curing of immature onions. They are dried in bulk storage by forcing air through the pile. Drying is also used when onions must be harvested while wet, and to ensure that the storage environment will not promote early sprouting, disease and rot development. The relative humidity of the drying air must be kept below 75%. Surface drying can extend up to 3 days when the humidity content of the ventilating air is low. If surface drying is used, quickness is the key; weight loss during curing should not exceed 5%.
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After drying and curing, the temperature of the stored onions is slowly lowered following the normal seasonal temperature monthly drop for the geographic area. It is important that condensation does not occur as it encourages rot.
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If refrigeration is used, the storage temperatures are slowly lowered to 33° F. Too rapid cooling will cause stress and damage. Good ventilation and circulation is important. The best holding temperature for onions is 33° F. with a relative humidity between 60 and 70%. Higher humidity promotes root growth and bulb decay.
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If onions are to be stored outside for a brief time prior to shipping, they must be in a shaded area. Direct sunlight can warm the onions too much and result in translucent scales. Translucent scales can cause serious losses, especially if shipment is delayed and the onions sit in storage.
Onion storage can be improved by spraying maleic hydrazide on the crop when 50 to 60% of the tops have fallen over. This chemical will prevent sprouting.
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The onion's temperature must be brought up to the packing room temperature very slowly over a two to three day period. If water condenses on the onions, it can prevent the removal of loose outer bulb scales (shucking) and results in cleaning problems. Wet onions must be dried prior to shucking, a loss of time and income.
If the onions are artificially heated to bring their temperature up to the packing room temperature, too much moisture can be removed. This can result in bald onions.
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Packaging
Dry onions are sorted, cleaned, sized and graded just prior to packaging. They are commonly packaged in 50 lb. sacks. Some are now also packaged in consumer packs of 2, 5 and 25 lb. mesh sacks.
Dry bulb onions from the northwest are shipped across the U.S. at ambient temperatures but protected from freezing. Processed frozen onions are shipped in 12, 20, 45 and 1150 lb. totes.
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