Proper drying is essential to the successful marketing of the sweet onions grown in North Carolina. Most serious postharvest losses result from improper drying. Only by following proper postharvest handling procedures can growers ensure buyer satisfaction and marketing success.
Drying conditions: Temperature: ....................... 100 F Relative humidity: ................... 65% Storage conditions: Temperature: ........................ 32 F Relative humidity: ................... 70% Freezing temperature: ................. 31 F Storage life: Refrigerated .............. 2 to 3 months Controlled atmosphere ...... 6 to 8 months
The production of onions in commercial volumes is relatively new to many North Carolina growers. This publication has been prepared to acquaint onion growers, packers, and shippers with proper postharvest handling procedures.
To maximize yield and quality, onions should be harvested only when mature. The bulbing phase is often very rapid and occurs near the end of the onion's active growth period. During this time, the onion tops will begin to fall over and die. Onions should be ready to harvest when approximately 10 to 20 percent of the tops have fallen over. Harvesting begins with a shallow undercutting 1 to 2 inches below the bulbs. Undercutting initiates and hastens the onion's change from growth to dormancy. The practice of clipping the tops with a rotary mower just before harvest is not recommended in North Carolina because it increases the risk of disease.
Although most western-grown and some northern-grown onions are field dried, the typical harvesttime climatic conditions in North Carolina prevent complete field drying. Onions that have been undercut and allowed to remain in the field during warm, humid, or rainy weather are very susceptible to infection by decay organisms. Therefore, North Carolina growers who rely solely on field drying are taking a substantial risk.
Mechanical harvesters that gather the onions into bulk containers or bags are common in some onion-growing regions. These machines may be specially designed for onion harvesting or may be modified potato diggers. However, because the type of onion grown in North Carolina is very susceptible to mechanical damage, hand harvesting is generally recommended. Hand harvesting into bags or bulk containers may be the least-cost alternative for most growers and offers the advantages of gentler handling and field grading. Before they dry completely, onions are particularly susceptible to mechanical damage and to the diseases that result from that damage.
During harvesting, field crews normally trim the tops and roots with a pair of clippers. Tops should not be cut less than 1 inch from the bulb, and roots should not be trimmed shorter than 1/4 inch. When hand harvesting, field crews should be well supervised and instructed not to handle the onions roughly. Care exercised during harvest will result in fewer problems at the packing shed and in the market. There are many similarities between the harvesting and handling of onions and sweetpotatoes. Many North Carolina onion growers are familiar with the harvesting system now used for sweetpotatoes. The 20-bushel wooden pallet bins used for sweetpotatoes are excellent for onions.
Some growers may prefer to gather onions into mesh bags. The bagged onions are taken from the field, thoroughly dried, removed from the bags for grading, and returned to the same bags for sale. Handling onions in mesh bags is an alternative for growers who lack bulk handling equipment. Figure 1 shows North Carolina onions ready for harvest.
Figure 1. North Carolina onions ready for harvest.
Several obvious physiological conditions indicate thorough drying. These include the complete drying of roots, foliage, and several layers of skin on the bulb. The dry skins should have a uniform color and texture. The best indicator of complete drying is the condition of the neck of the onion. The neck should be dry nearly to the surface of the onion and should not slide back and forth when squeezed between the thumb and forefinger. Onions that are packed before they are thoroughly dry will quickly decay. The most common postharvest disease is "neck rot," which results when Botrytis and similar pathogens enter an incompletely dried neck wound. Once an onion is infected, there is nothing that can be done to stop the decay.
Because of the poor natural drying conditions typical during the harvest season in North Carolina, growers should make some provisions for artificially drying their onions. Artificial drying can sometimes be accomplished by using ambient (unheated) air only. However, using heated air results in more thorough drying and a substantially shortened drying time.
Air heated with gas or other types of heaters (much the same as with grain or peanut drying and tobacco curing) will dry onions satisfactorily in 48 to 72 hours, depending upon the condition of the onions and the atmospheric conditions. Air heated to 100 F is sufficient for rapid drying. The air temperature should be measured just as it strikes the onions with an accurate thermometer. Don't guess! Air temperatures above 110 F will damage the onions.
In most drying situations, the air is partially recirculated. The relative humidity can be regulated by altering the proportion of outside air admitted to the heated air stream. This adjustment can be made both to control the drying rate and to conserve energy. A wet-bulb temperature of 85 to 90 F yields rapid yet energy-efficient drying.
Fans used to pull the air through the drying onions should be carefully sized to give adequate airflow without wasting energy on excess capacity. Fans should be sized to provide from 3 to 5 cubic feet of air per minute per bushel of onions. A reduction of 5 to 8 percent in the onion's weight is usual during drying. Onions should be graded immediately after drying. The tops and roots are dry and brittle and can be removed easily in the grading and packing line.
Such an arrangement may also be assembled with a heat source inside a closed building, as shown in Figure 2. Adding a suitable properly vented gas or oil heater will reduce drying time considerably. The building should be relatively tight and have a concrete floor. A building with a dirt floor is not suitable for dry-ing onions. A moderate amount of building insulation (for example, 1/2 inch of foil-backed polyisocyranuate foam) would improve energy efficiency. The relative humidity inside the building can be regulated with a combination of louvered vents and exhaust fans.
Figure 2. Fan and pallet bin arrangement used to dry onions.
A more sophisticated system that uses permanently mounted fans and a heat source will dry as many as 10,000 bushels (500 20-bushel pallet bins) at one time. Such a structure is known as a horizontal air ventilation system. This system could, with slight modifications, be used for several crops. For example, such a building might be used to cure and store sweetpotatoes. With the addition of refrigeration, the same building can be used for forced-air cooling of fruits and vegetables. Sample plans may be obtained through your county Cooperative Extension Center or from the Department of Biological and Agricultural Engineering at North Carolina State University.
Box barns are particularly well suited for drying onions. Wooden or metal boxes of the same dimensions as the tobacco boxes may be built for less than $100 each. Depending on the make and model of the barn, these boxes may have a capacity of up to 75 bushels each and can be trucked to the field for filling. Figure 3 illustrates onion drying in a tobacco-curing barn.
Figure 3. Onions drying in a box-type tobacco barn.
The most common form of damage is the bruising and cutting that occurs when onions strike unprotected surfaces. Losses can be reduced up to 25 percent by padding the various surfaces along the grading and packing line. The addition of 1 to 2 inches of foam to key areas will almost eliminate impact damage in those areas. Reducing the drop heights below the grader and at other points along the grading and packing line may also reduce losses.
Onions are commonly packed in mesh bags containing 25, 30, or 50 pounds. They may also be packed into various consumer-size bags weighing from 1 to 10 pounds and shipped in master containers. Bags, however, have been shown to offer little protection and can subject the onions to severe damage if mishandled. The recent industry trend has been toward the use of 48- or 50-pound fiberboard cartons. USDA grade standards for Granex-Grano-Bermuda-type onions are U.S. Number 1, Combination, and U.S. Number 2. Size often determines price but is not related to quality.
Onions should be transported and stored separately from other kinds of produce. Many types of fruits and vegetables will readily absorb the odor of onions. Well-dried onions also draw moisture readily from fresh vegetables.
Onions will freeze at about 31 F. The effects of freezing, even for short periods, is cumulative. That is, several short periods below 31 F are just as damaging as a single longer period. Onions that have frozen become soft and decay quickly. On the other hand, onions that have been dried and are otherwise in good condition but allowed to remain above 50 F are subject to sprouting. Sprouting onions cannot be marketed and are very susceptible to decay and severe weight loss. Table 1 shows the differences in storage losses at different temperatures.
Table 1. Effect of Storage Time and Temperature on Onion Losses
Percentage of loss at various temperatures _______________________ Storage period 70 F 40 F 34 F
Two weeks 8.9 5.2 5.3 One month 10.2 7.0 6.1 Three months 25.2 15.9 10.6 Six months 61.8 32.7 14.0
Source: Hurst, W. C., et al. (1985). Shelf life and quality changes in summer storage onions. Journal of Food Science 50(3):213-220.
Size alone usually does not determine marketing quality or grade. Rather, buyers and consumers often select onions on the basis of intended use. For example, larger onions are easier to slice and dice and are therefore preferred by food-service operators. Onion growers and shippers must implement strict quality-maintenance practices to produce a clean, uniform, defect-free pack. For ultimate marketing success, the product supplied must closely match the buyer's requirements. Research suggests that the market for high-quality North Carolina onions will continue for the foreseeable future.
M. D. Boyette, Extension Agricultural Engineering Specialist
D. C. Sanders, Extension Horticulture Specialist
E. A. Estes, Extension Economist
2,000 copies of this public document were printed at a cost of $721, or $.36 each.
Sponsored by the Energy Division, North Carolina Department of Economic and Community Development, with State Energy Conservation Program funds, in cooperation with North Carolina State University. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Energy Division, North Carolina Department of Economic and Community Development.
North Carolina Cooperative Extension Service