Onions for Dehydration
Last revised February 12, 2010
Bulb onion varieties are classified according to day length. Those grown in the Pacific Northwest are classified as long-day varieties requiring 14 or more hours of day length before bulbing occurs. Technically, all onions are "long-day" plants, in that bulbing begins as day length increases. Short-day varieties are those requiring only 12 to 13 hours of day length for bulbing. These are grown in southern states generally below the 35th parallel. They are not suitable for dry bulb production in the northwest, except as pearl or boiler varieties, or for overwinter production. Some short-day material or short-day x long-day hybrids may possibly be used to extend the dehydrated onion harvest season if planted early enough in spring.
Key economic factors are the soluble solids content and pungency of the variety. White types are most commonly used to facilitate processing. Yellow types are used only for special purposes. Varieties developed specifically for dehydration may have soluble solids of 23% or more, and high pungency. Many are hybrids, developed and provided by the dehydrator company to its growers only, and are proprietary.
Varieties available through commercial seed companies are usually strains of Southport White Globe that are selected for dehydration, such as Dehyso and Dehydrator No. 14. Others may also be available from certain seed companies. Still others may be varieties that are also used in bunching onion production.
Other strains of Southport White Globe and White Creole, and hybrids of these types which are used for dehydration in more southerly latitudes (such as California's San Joaquin and Imperial valleys) could produce very early, small bulbs (and lower yields) in production areas in Oregon. Such varieties might be useful in extending the harvest season by allowing harvest to begin several weeks early. Such short day varieties must be tested carefully for bulb size potential since some may produce pearl or small boiler size bulbs.
The Hermiston-Boardman area has successfully grown high quality onions for dehydration. The area's water and soil resources are particularly suited for producing onions with very low bacterial counts giving the area a special advantage in production and marketing.
SOIL TEMPERATURE AND PLANTING DATE
The optimum temperature range for germination is 48 to 90 F. March and April plantings in central Oregon, and late February to April plantings in eastern Oregon and the Columbia Basin are considered ideal.
Onions for dehydration must have friable soils with good internal drainage. Soils must be friable to allow rapid mechanical separation of onions from the soil at harvest. Because onion bulbs for dehydration tend to be small, clods are difficult to separate from the bulbs, therefore loamy sands or similar sandy soils are best. Clay soils that may adhere to the onion bulbs or form hard clods are not desirable.
Onion seed numbers approximately 7,000 per ounce. Adjust seeding rates to compensate for seed count and germination test percentage. Onions for dehydration are all direct seeded. Three to 4 pounds of seed may be used per acre depending on seed quality, seeds per pound and expected seedling mortality. Populations of about 300,000 plants per acre, uniformly spaced, are considered ideal, so seeding rates often exceed 350,000 seeds per acre. Planters of all types are modified by growers for use, with Planet Jr., Graymor, Beck and Stanhay units being common.
Seeding with a precision seeder is recommended. Vacuum seeders such as Stanhay or Gaspardo are becoming popular. The Stanhay belt planter has a special shoe that plants 3 lines l.5 in. apart with each planter unit. The Stanhay vacuum planter also has the capability of planting 3 rows per planter unit. Each vacuum plate meters seed to three separate lines.
Planters should be set to drop seeds 3/4 to 1 inch deep. Depth of seeding has an effect on bulb shape since the onion stem plate (the base of the onion bulb) forms at the point where the seed germinates. Shallow planting results in flatter bulbs, while deeper seed placement results in taller, and sometimes top-shaped bulbs. To reduce greening, depth of planting is very important with the deepest practical planting preferred. When possible, onions for dehydration should be planted in 2-inch deep furrows and the bulb portion of the plants covered with soil at about mid-season (see also section on greening prevention below).
Onions for dehydration are planted at very high populations because of their generally small bulb size. In order to accommodate this high population, the seed should be distributed in split rows, or single rows planted in multiple row beds or sets, usually 4 to 6 rows per set. An average spacing between rows of 6 inches will result in 87,122 lineal feet per acre and produce 348,000 plants per acre at 4 plants per lineal foot. For ease of digging onions may be planted on raised beds.
Schematics of planting configurations described:
two beds on 40-inch centers
with six rows per bed.
four, four-line sets spaced to
provide soil for covering bulbs.
Where overhead irrigation and 40-42 inch beds are used, and where modified potato diggers are used for harvest, it may be better to place the line sets closer together and farther from the edge of the bed. This would reduce exposure of onions in the outside lines to sunlight as the edges of the beds erode, and reduce the necessity to cultivate between the sets to rebuild the beds. The distance between the sets would be limited only by the distance needed between planters.
In California and areas of the Northwest where furrow irrigation requires the use of raised beds, plant populations of about 18-24 plants per bed-foot, with 4-6 rows per bed and beds at 40 inch centers is considered ideal. In the Hermiston area where tractors and cultivation equipment are set for potato spacings of 34 inches between rows, reduce plant populations to 16-20 plants per bed-foot to maintain the same plant population per acre. The following table shows what populations would be at various multiple-row bed spacings and plant populations per bed-foot:
PLANTS PER ACRE AT VARIOUS MULTIPLE ROW SPACINGS
|Distance between row sets or beds (inches)||Plants per acre at 18 and 24 plants/bed-foot|
Where overhead sprinkler irrigation is used the use of raised beds is not recommended. Although raised beds may facilitate mechanical digging they may also result in greater difficulty in keeping the onions covered with soil, resulting in excessive exposure of onion bulbs to sunlight and increased greening.
Onions are shallow-rooted, and unless moisture supply is constant, they bulb early and the resulting sizes may be small. Maintaining moisture near the surface, at the onion stem plate, is important in root generation. Onion roots generate at the stem plate only when moisture is present. Proper moisture management is important in alleviating pink root problems, general root health, and therefore bulb growth vigor. Also, maintaining an even soil moisture is important in reducing incidence of double-center bulbs.
Each irrigation should thoroughly wet the soil to the 24-inch depth. In the Treasure Valley area, 20-25 inches may be needed depending on seasonal variation, variety, planting date and location. Watering should be terminated after the bulbs have reached full size. Approximate summer irrigation needs for the Hermiston area have been found to be: 3.5 inches in May, 5.0 in June, 7.5 in July, and 7.0 in August.
Bacterial and fungal contamination in the finished product in onions grown for dehydration is of special concern. The water and soil resources in the Hermiston-Boardman area have resulted in dehydrated product of high quality and very low contamination. To maintain this advantage, care should be exercised in the source of irrigation water and in irrigation practices. Avoid production in fields that rely on irrigation from stagnant surface water sources or water sources that might be otherwise contaminated. Use well or Columbia River water where possible for overhead irrigation.
Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
Irrigation is usually cut off about 2-3 weeks before onions are lifted or undercut to facilitate curing.
GREENING AND SUNBURN PREVENTION
White onions must be protected from greening and sunburn prior to, and during harvest and curing. Sunburn or sunscald may occur if freshly topped, partially cured onions are exposed to sunlight and temperatures over 90 F for several days .
Mechanically undercut onions when the tops are down and dry. This is usually done after the tops are flailed off, but may be done while the tops are still on, but at least after 65 to 100% of the tops are down. Onions are usually windrowed in the undercutting operation either after the tops are flailed off, or with the tops still on, to combine 4 or more rows (2 or 4 beds) together just prior to the onions being loaded for transport to the dehydration or storage facility. If windrowed with the tops on, they should be handled in such a way as to cover the onions with their tops.
Greening is a risk when white onions are exposed to excessive sunlight any time after bulbs begin to mature. Depth of seeding, row spacing and stand density and nitrogen rates are also important factors (see appropriate previous sections). To avoid greening, the lower portion of the onion plants may be covered with soil after bulbs begin to mature, or approximately 4-6 weeks before harvest. Research from Texas indicates that covering bulbs 4-6 weeks before harvest did not have a detrimental effect on yield or any effect on the incidence of rots.
Special equipment is used to cover onions with soil during the growing season or at harvest. Some of this equipment consists of special cultivators that are used to replace soil that may have washed off the edge of the plant beds. Modified rototillers (corrugators) from which all but certain blades have been removed are also used. The blades remaining lift soil from between onion beds and specially installed deflectors built into the rototiller hood deflect the soil over the onions. Care must be exercised to avoid burying growing plants.
Although covering white onions whose tops have fallen will help reduce greening, topping such onions becomes more of a problem and needs to be considered. Special toppers such as the Top-Air and Veggie-Vac machines are available. These machines top the onions after they are lifted and before windrowing or loading. Attempting to top onions "in place" (before lifting) may cause serious problems with dust.
Topped onions may also develop greening if they are left in the field exposed to sunlight for more than one week. To avoid this, they should also be covered with soil immediately after topping if the tops are flailed off some days prior to harvest.
HARVESTING, HANDLING, CURING, AND STORAGE
Yields of "dehy" onions can vary greatly due to differences in the variety's day length requirements that affect bulb size. Yields in the Pacific Northwest have ranged from 15 to 25 tons/acre, commonly around 18 tons/acre.
The various steps in harvesting "dehy" onions in Oregon may be performed by the grower, the processor or a combination of the two. Growers use modified onion and potato harvest equipment to undercut, top, windrow, and remove onions from the field. Timing of these operations is important in maintaining high quality and yield.
Dehydrated flakes produced from onions that had been stored at 85 F for 4 months discolored less in storage than flakes made from onions stored at cold temperatures.
After curing, the relative humidity in the storage should be maintained between 60 and 70%. If the storage is too dry, the outer bulb scales will crack excessively, resulting in bald onions during handling. Although this is undesirable in fresh-marketed onions, it may be desirable in onions intended for dehydration.