Onion Harvest and Storage
|Agdex #: 258/50|
|Author: Susan MacKinnon|
|History: October 2000|
The successful harvest and subsequent storage of onions requires that: onions are free from disease, have reached maturity, that harvest equipment is working properly and that onions are quickly cured and dried.
As the onion reaches maturity, a number of events occur: the neck tissue softens, the tops begin to fall over and the roots start to die. During this period, food material stored in the leaves is translocated to the storage bulb, resulting in a rapid increase in bulb size. Trying to rush maturity, by breaking or rolling the tops, undercutting the roots, or applying a foliage desiccant, similar to those used in potato production, appears to have little effect in hastening the maturation of the crop. In fact, these measures can some times be responsible for reduced yield and quality.
If onions are intended for storage, the use of a sprout inhibitor is recommended. The sprout inhibitor is translocated to the shoot apex where it prevents cell division. In stored onions, it suppresses sprout and root growth. There should be 5 - 8 green leaves on the onion to insure absorption and translocation of the product to the bulb. Plants with less than 3 green leaves or all the tops down, will not absorb sufficient sprout inhibitor for good control. Application too early will result in the development of bulbs with soft, spongy centres.
A sprout inhibitor will lengthen the storage life of an onion, but will not be of benefit on varieties that naturally have a short storage life.
Ideally, the onion crop should be lifted when 90% of the leaves have fallen over, minimizing the chance of skin damage. Practically, the crop can be lifted when 50 - 80% of the plants have soft necks and the foliage has started to collapse.
If field drying is an option, it is important that lifting and harvesting be done when little or no rain is expected for several days. Onions that have been undercut or windrowed and allowed to remain in the field during warm, humid or rainy weather are very susceptible to infection by decay organisms. Continuous moisture on the roots will promote rooting, and can result in accelerated bulb sprouting. If the onions are topped, wait a minimum of one or two hours before lifting, to allow the neck sap time to dry. This will help prevent the entry of disease organisms and reduce staining. Let all the dew evaporate off the onions before harvesting.
Onions may appear to be very durable but they bruise easily. A bruised onion may appear to have no visible damage but after a week or two in transit or exposure to adverse storage conditions, the extent of rough handling will become apparent.
|Bruising can occur for a number of reasons but the most common are:|
|There are three phases to storage management - curing, drying,
|Curing removes external moisture from the onions. The
excess moisture must be removed quickly to minimize the opportunity for
disease spread. Moisture is removed by ventilating with air heated to 300C.
Warm air not only dries the onions, but has a greater capacity to absorb
moisture. A minimum of 4 cfm per cubic foot of onions is required for fast
curing. Heated air should be continually exhausted to keep the humidity
below 75%. The heat can be turned off, once the air leaving the top of the
pile or bins has reached the same temperature as the air being blown in.
Leave the fans running until the onions feel dry on the outside. Do not
overdry the onions because excessive moisture loss can result in the loosening
of the outer skin.
Curing should be completed within one to three days. The main cause of neck rot problems in storage is improper curing of the necks.
|Drying reduces the respiration rate of the bulbs and
allows for the development of natural dormancy. Drying should be completed
within 10-15 days. The onions should be exposed to a continuous slow drying
at a temperature between 24-260C and 70-75 % relative humidity. Maintaining
the correct humidity level during this period is important. A humidity above
75% produces an onion with a dark skin colour but can also cause staining,
a humidity level below 60% removes too much moisture and causes skin cracking.
As the onions cure, the skin will develop a light brown colour. Curing is
complete when the relative humidity stabilizes and the necks are completely
|Cooling process should start once the onion necks are paper dry and skin is the desired colour. The temperature of the storage should be cooled 10C every two days until a temperature of 50 C is reached. Try not to cool the onions too quickly as this will cause skinning. On days when the ambient air temperature is higher than the temperature inside the storage, keep the storage closed. Bringing warm air into the storage will produce condensation on the onions.|
|Accurate thermometers and hygrometers to measure temperature and humidity
are a necessity.
Careful handling and temperature management is essential when onions are removed from cold storage. Onions should be warmed to ambient air temperature slowly, otherwise water will condense on the cold onions from exposure to the warm air, causing them to attract dirt and dust, and encourage mold growth and root development. Also, exposure to low humidity will result in the loss of skins during subsequent grading and packing.
More information may be obtained from Farm Extension Services of the PEI Department of Agriculture and Forestry, phone (902) 368-5663 or 1-800-959-8929