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Postharvest Handling of Asparagus

The international market for fresh asparagus (Asparagus officinalis var. altilis) is growing. Yet because cut asparagus has one of the highest respiration rates of all fruits and vegetables and undergoes rapid senescence (aging) after harvest, supplying the market requires careful postharvest handling. Harvesting procedures, cooling times, and storage temperatures can all dramatically affect asparagus quality and shelf life.

Varieties The most common hybrid of asparagus produced for international trade is UC 157 (F1). It is derived from the Mary Washington variety and has high yields and export percentages. F1, which has compact tips, is also dark green, thick, cylindrical, and tolerant of Fusarium. UC 157 (F2), also derived from Mary Washington, yields less than the F1 and has tips that are less compact and tend to open faster. Other varieties include Jersey Giant, Jersey Centennial, Greenwich, Bearing Giant, Palmetto, Backlim, Boolim, Gijnlim, Lucullus, Limburgua, Limbras, and Brock Special.

European hybrids include Franklin F1 (Dutch), which is straight with closed tips and is suitable for both white and green asparagus. Others include D¹Argenteuil, Connovers Colossal, Blanco Aleman, and Giant Mammoth. In France, selections developed for white asparagus include Alexandra Marionet, Darbonne 4, Lorella, and Valliere Rocault. For green asparagus, selections include Darbonne Vert, Jacq-Ma Verte, and Verte de California.

These different hybrids have been developed for specific growing conditions and are not suitable for all regions.

Quality Requirements Markets may require green or white asparagus, although most now tend to favor green. Buyers judge quality on the apparent freshness, straightness, stem diameter, color, and condition of the bracts on the stem tips. High-quality asparagus is turgid, of uniform straightness and color, with tightly closed bracts. The stem should not bend easily, but should snap or break if bent more than 90 degrees. Round stems are required. Flat or angled stems with ridges are indicative of old asparagus. Most consumers prefer asparagus that is long (up to 22 centimeters) and thick (up to 2 centimeters base diameter).

Poor-quality asparagus is characterized by a high fiber content and a hard and woody stem. Fiber content can be kept to a minimum when:

Harvest Maturity There are two main harvesting systems for asparagus. In the Taiwanese system, the plant is pruned down to the ground, and shoots that subsequently develop are immediately harvested. Harvesting is carried out over a period of three months, followed by a three- month pause, after which harvesting can begin again.

In the second system, Modified Taiwanese, when the initial pruning is carried out, the three most developed fronds are left intact and only the new shoots are harvested. Alternatively, after complete pruning, three stems are allowed to develop fully, and the remaining ones are harvested. Harvesting in this system can be carried out for four months.

Harvest frequency depends on growing conditions and ambient temperatures, and can vary from twice a day to once every two days. Normally, harvesting can commence in the second year, but for no more than two to three weeks. In the third year, harvesting is carried out during a 30- to 45-day period; this period increases with age to a maximum of 90 days. Harvesting for more than 90 days reduces the resistance of the plant to pests and diseases.

Green asparagus is harvested when the stems are at least 23 centimeters above the soil surface and before the scale-like leaves on the tip separate (a closed tip is considered a sign of good quality). The shoots should not be longer than 27 centimeters. Harvesting is usually carried out manually, although there are machines available for mechanical harvesting.

In manual harvesting, a sharp knife or a spade-like harvesting tool is used. This tool is a metal bar 30 to 45 centimeters long, 6 to 10 millimeters wide, with a handle at one end and a flat V-shaped blade at the other end. The stem is cut just below the surface of the soil. Quality requirements normally specify the percentage of white‹that is, the portion of stem cut from beneath the soil surface‹permitted.

White asparagus stays white because it is kept covered with mounds of soil about 35 centimeters high. It is cut 20 to 26 centimeters below the surface of the soil bed, when the tip of the stem just breaks through the soil surface. Care must be taken to prevent damage to the cut stem and to neighboring stems.

Harvested stems should be placed upright in shallow plastic field crates, which should be kept in the shade and transported to the packing facility as soon as possible. Stem tips should be below the top of the field crate to prevent damage when stacking. Field transport should be in cool trucks or in covered trailers to prevent dehydration or other quality losses through contact with sun, wind, or rain. As soon as possible after harvesting, the stems must be placed in cold water; even brief exposure to the sun can dry the stem, rapidly reducing its quality. In the field, wet bags or cloths may be used to help prevent drying.

Grading and Packing Packhouse operations can be manual or mechanized. The main processes in both operations are pre-classification, trimming of stems to the required length, grading by stem diameter, grouping into bunches, packing, and precooling.

On arrival at the packing facilities, the stems are carefully removed from the field crate. Pre-classification is carried out to remove stems that are short, thin, bent, soft, damaged, or have open bracts. In manual operations, acceptable stems are held in groups of 5 to 10 with the points of the tips in alignment; they are then trimmed with a knife or guillotine to the required length (23 centimeters).

After they are trimmed, the stems are placed in a cold water tank for cleaning and precooling. Iced water is normally acceptable, although the ice should not come into direct contact with the asparagus. The water should contain 100 to 150 parts per million chlorine solution. The stems should be left in water that is 3oC to 5oC for a maximum of 20 minutes. Rapid cooling of white asparagus prevents color changes. Hydrocooling is the preferred method of precooling for asparagus because it is fast and does not result in weight loss.

On removal from the water, asparagus is graded based on the diameter of the stem. Under mechanized operations, the stems that pass through the preclassification are placed onto a conveyor belt. The stems are placed perpendicular to the belt direction, with the tips all to one side. The distance between the tip of the stem and the edge of the belt should be exactly 23 centimeters (or the stem length required), so that the base of longer stems will protrude over the edge of the belt. The stems move with the belt and pass a radial saw positioned adjacent to the belt. Stem bases are trimmed to ensure uniform length. Care must be taken to ensure that no pieces of the asparagus skin are torn away.

Stems continue along the conveyor and under a cold water drench for precooling. Water temperature should again be 3oC to 5oC, with volumes of 10 to 15 gallons per minute per square foot, and contain 200 parts per million chlorine. Treatment should last approximately five minutes. Afterward, the stems are removed from the belt and size graded manually by diameter.

At this point, whether manual or mechanized operations have been used, the asparagus must be graded, packed, and placed under cold storage as soon as possible. Care is required because susceptibility to bruising is higher when the stems are cooled. Allowing the stems to heat up again reduces both the storage and shelf lives.

Grading is always in accordance with valid market standards and may be based on color and stem length and diameter. The stem diameter grades depend on market requirements; the following are U.S. diameter grades as measured one inch from the cut base.

European Union (EU) regulations (nos. 183/64 and 1677/88) state that asparagus shoots should be classified according to color:

Shoots should also be whole, fresh, sound, free from damage, practically unbruised, clean, and free from external moisture and foreign taste and smells. Shoots should not be hollow, split, peeled, or broken. Under EU standards, sizing by length is based on the following:

The diameter is measured at the midpoint of the stem. Minimum diameter and tolerances of Class I are shown in the table on page 5.

Packers should be separated according to those who pack small and medium stems and those who pack large and extra-large stems. As the stems pass by on the conveyor, packers remove stems that appear to fit into their category. All packers should have a size frame to identify the exact size category. Size frames can be made of plastic or wood.

After size separation, stems are grouped into batches and tied with elastic or plastic tags, or placed in plastic sleeves. Batches should be specific weights depending on market needs, but normally 0.5, 1, or 2 kilograms. Shoots on the outside of the bundle must correspond to those on the inside. Bunching can be done in plastic cups or other recipients; as stems are graded, they are placed into the recipients. When the containers are full, the stems are ready for banding. Plastic or paper tags are normally preferred because they cause less damage than elastic bands and can be used for labeling as well. One or two ties may be used. If plastic sleeves are used, they should be ventilated to allow for air and heat transfer.

During all operations, care has to be taken to prevent mechanical damage to the stems and to ensure uniformity in product color, length, and diameter.

Characteristic	White Asparagus (and Purple)	Green Asparagus (and Green-Purple)
Minimum Diameter	10 mm	6 mm
Diameter Range	10-16 mm is standard; stems with diameters greater than 16 mm are permitted, but must be bundled with stems of similar diameter (see "tolerance" category below).	6-12 mm is standard; stems with diameters greater than 12 mm are permitted, but must be bundled with stems of similar diameter (see "tolerance" category below).
Tolerance within Bundle	+10 A bundle that contains one stem with a 17-mm diameter may not contain any stems with diameters greater than 27 mm. For that bundle, the range in stem diameters would be 10-27 mm.	+8 A bundle that contains one stem with a 13-mm diameter may not contain any stems with diameters greater than 21 mm. For that bundle, the range in stem diameters would be 6-21 mm.

Packaging The type of packaging used for asparagus depends on market requirements, but box designs made from wood, corrugated paper, and plastic are available. All designs are in the ³pyramid² form‹with two or all four walls sloping inward from the base to the top. In packaging that is water resistant, including waxed corrugated cartons, a sponge is dampened and placed in the base of the carton before the stem bunches are laid inside. In all cases, the stems have to be packed vertically‹that is, upright.

The carton dimensions and the respective net weights vary by market; the net weight required may be between 6.8 and 14.5 kilograms (15 to 32 pounds). Standard asparagus carton designs are available in most importing countries; this is the only product packed in pyramid cartons. Adequate side wall ventilation is essential to ensure maintenance of storage temperatures and to prevent carbon dioxide and ethylene build-up. Water-resistant packaging with a minimum of 12 millimeters of clearance above the shoots is preferred, to allow for growth.

Labeling should include the packer; country of origin; number of bundles; unit weight of each bundle; and stem color, quality, and size.

Precooling Under the handling systems previously described, the asparagus undergoes hydrocooling to remove the majority of the field heat. After packing, the cartons are placed in a cold room at the required storage temperature. Where hydrocooling is not carried out, cold room cooling can be used, although forced air cooling is not recommended.

Storage Conditions Optimum storage conditions for asparagus are 1oC to 2oC, with 90-95 percent relative humidity. With uniform temperatures and adequate air circulation, asparagus can be stored for two to three weeks. Rapid cooling is essential for quality maintenance. The quality loss in one hour at 27oC would take 14 hours at 2oC. Storage life is highly correlated to temperature, and quality declines rapidly at temperatures above 5oC. This loss is manifested by hardening of the stem, loss of green color, water loss, and development of off flavors. Quality loss occurs faster in stems harvested with open or opening bracts. Exposure to ethylene or storage with ethylene-producing products should be avoided because asparagus is highly sensitive to the gas; low concentrations of ethylene cause loss of color and opening of the bracts.

Rapid precooling, cold chain maintenance, and optimum storage are the essential factors in maintaining asparagus quality.

Transport Because asparagus is highly perishable, all local transport should be refrigerated. If air shipments are made, and the stems remain on a runway for extended periods of time without refrigeration, stems will undergo irreversible damage. Heat, dryness, light, and ethylene all are capable of reducing quality in a few hours.

Potential Postharvest Losses‹Physical Problems Mechanical Damage: Broken tips and crushed stems are the most common types of damage inflicted by machines. Damage can occur at any stage of the handling chain, but is most common during selection, grouping, and packing. Damage can be minimized by careful handling and by reducing the number of handling operations. Frost Damage: When subjected to temperatures of -0.8oC or lower, asparagus stems suffer frost damage, which can be detected in changes in stem texture and color. Stems become soft, watery, and gray at the tips.

Chilling Damage: Symptoms are the same as for frost damage, except the stems do not become watery. Chilling damage occurs during storage for extended periods (three to four weeks) at 0oC to 3oC; it is more evident at 0oC. If stems are stored for two weeks or less, chilling damage is rarely a problem. For longer storage, 2oC is an optimal storage temperature.

Irregular Growth: Elongation is the opening or separation of the bracts, which reduces the visual quality. Doubling is shown by growth in a non-vertical direction. This growth can occur after harvesting if the stems are laid on their sides for a day or more at storage temperatures higher than the optimum. Doubling can also occur if the stem tips reach the top of the carton and are then directed to one side. Both elongation and doubling can be minimized by storage at 0oC to 2oC.

Aging: When the stems bend over as a result of stem growth, feathering occurs. This is a sign of senescence (aging) and indicates that the stem was harvested too late or that storage conditions after harvest were inappropriate. Feathering can be avoided by careful selection at harvest, rapid precooling, cold chain maintenance, and optimal storage conditions.

Potential Postharvest Problems‹Pathological Factors Bacterial Soft Rot (Erwinia carotovora): This is the most common disease in asparagus. It causes stem tips to be gray, pale, and watery. The disease is initially characterized by bacterial growth in the cut base, which exudes drops. Subsequent bacterial growth causes the base to become soft and watery. As the base collapses, fibers appear as ridges. The bacteria typically enters through the cut surface of the base or through injured areas; it is more frequently seen in feathered stems. The incidence can be reduced by using a clean, sharp cutting instrument, 200 parts per million chlorine in water baths, and careful handling to avoid mechanical damage.

Fusarium Rot: This disease is characterized by white mold growing mainly on the bracts and stem tips and results in lesions on soft tissue and watery areas, which turn yellow or dark green with age. The disease occurs during long-term storage: three days at 21oC or four weeks at 2oC.

Additional Information Falloon, P.G., Nikoloff, A.S. & Conner, A.J. ³Asparagus breeding in New Zealand.² Asparagus Research Newsletter. 6: 12-23, 1989.

Fundaci=n Chile. Espárragos. 11 pp. 1991.

Germany. Export Manual for Tropical Fruits and Vegetables: Asparagus. GTZ - Protrade. 27 pp. 1991.

Guatemala. Gremial de Exportadores de Productos No-tradicionales. Espárrago: gufa de producci=n, manejo postcosecha y mercadeo. 44 pp. 1988.

King, G.A., Henderson, K.G. & Lill, R.E. ³Shelf-life of Stored Asparagus Is Strongly Related to Postharvest Accumulated Heat Units.² Annals of Applied Biology. 112: 329-335. 1988.

King, G.A., Hurst, P.L., Irving, D.E. & Lill, R.E. ³Recent Advances in the Postharvest Physiology, Storage and Handling of Green Asparagus.² Postharvest News and Information. 4(3): 85-89. 1993.

Klieber, A. & Wills, R.B.H. ³Optimization of Storage Conditions for UC 157 Asparagus.² Australian Journal of Experimental Agriculture. 32: 529-534. 1992.

Lipton, W.J. ³Postharvest Biology of Fresh Asparagus.² Horticultural Reviews. 12: 69-155. 1990.

Robb, A.R. "Physiology of Asparagus (Asparagus officinales) as Related to the Production of the Crop." New Zealand Journal of Experimental Agriculture. 12: 251-260. 1984.

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