Postharvest Handling and Storage of Asparagus

Harvesting

Asparagus quality and market value depends on the level of care during harvesting and handling. Postharvest quality begins in the field. Careful supervision and instruction of the harvesting crew are essential, and good sanitary personal habits should be required of workers at all times. Asparagus spears are cut by hand, as they emerge through the soil from the underground crowns. Spears are usually harvested at a height of 8-10 inches, with the tips slightly closed. Harvested spears are prepared for market by grading, sizing, and bunching, based on freshness, stalk diameter and length, spear color, tip tightness, and extent of bruising. After trimming the butt end, bunches are packed upright in boxes containing moisture pads or water.

Figure 1. Bunches of trimed asparagus are packed upright in boxes

Maturity and Quality Indices

High quality fresh asparagus spears are dark green and firm, with tightly closed and compact tips. Stems are straight, tender and glossy in appearance. Stalk diameter is not a good indicator of maturity and associated tenderness. However, many consumers do have a preference for either thick or thin spears. Diameter grading, to ensure that each bundle is made up of similar sized spears, offers consumers a more attractive, uniform selection at the point of purchase. Spears with green butts are preferred over those with white butts, because the latter are associated with increased toughness. However, it has been shown that a small amount of white tissue at the butt will delay decay development under typical commercial handling practices.

Ethylene Production and Respiration

Asparagus produces little ethylene, <0.1 µL·kg-1·hr-1 at 20°C. Exposure to ethylene will accelerate lignification (toughening) of the spears, while low temperatures will minimize ethylene-induced toughening. Freshly harvested asparagus has one of the highest respiration rates among produce. However, rates decline after harvest and the apical tips have higher rates than basal portions of the stalks.

Pre-cooling and Storage Conditions

Asparagus has a high metabolic rate after harvest and thus is one of the most perishable crops. Rapid cooling to 0-2°C soon after harvest is essential for optimal postharvest keeping quality. It has been shown that a 4-hour delay in cooling can cause a 40% increase in shear force due to tissue toughening. Hydro-cooling is the most effective method for quickly cooling asparagus. Hydro-coolers use either an immersion or a shower system to bring products in contact with cold water.

Recommended commercial storage conditions for asparagus are 0-2°C with 95-99% relative humidity. Low temperature is essential to delay senescence (ageing), tissue toughening, and flavor loss, while high humidity is critical to prevent desiccation and loss of glossiness. With rapid cooling and these storage conditions, asparagus can be stored for up to 3 weeks. However, extended storage (>10 days) at 0°C may result in some chilling injury (see description below). Storage in controlled atmosphere with 5-10% CO2 is beneficial in preventing decay and reducing the rate of spear lignification. These effects are more pronounced if the temperatures cannot be maintained below 5°C.

Physiological Disorders

• Elongation and Tip Bending - Asparagus will continue to grow and elongate after harvest if not cooled promptly and stored in low temperatures. Spear growth and elongation are also promoted if the butt is in contact with water. Tip bending occurs as a response to gravity, such upward growth when the spears are horizontal or when the tips are deflected upon reaching the package top.
  
• Spear Toughening - This disorder results form tissue lignification and fiber development, and progresses from butt to tip. Spear toughening occurs rapidly at temperatures >10°C and is accelerated by the presence of ethylene. Bruising and tip breakage are often the results of rough handling, and can also promote toughening due to wound ethylene.
  
• Feathering - Expansion and opening of the tips is a sign of senescence. This is often observed when asparagus is harvested over-mature or after an extended period of storage, especially at higher than optimal temperatures.
  
• Chilling Injury - Asparagus can develop chilling injury after extended storage (>10 days) at 0°C. Symptoms include loss of glossiness and graying of tips, with possibly a limp, wilted appearance. Severe chilling injury may result in darkened spots or streaks near the spear tips.
  
• Freezing Injury - Damage due to freezing will be initiated at 0.5°C in asparagus. Symptoms include water-soaked appearance and extreme tissue softening.

Postharvest Disease

Decay is an important source of postharvest loss in asparagus. The most prominent disease is bacterial soft rot, caused by Erwinia carotovora or Pseudomonas ssp. Decay, characterized by soft rot pits, may initiate anywhere on the spears, but is most frequently found on the tips or butts. Spears that are re-cut above the white portion of the butt have been reported to be more susceptible to this type of rot. Some fungi (e.g. Fusarium, Penicillium, and Phytophthora) are also associated with postharvest decay and spoilage of asparagus.

References

• Cantwell and Kasmire, 2002. Postharvest handling systems: flower, leafy, and stem vegetables. In: Kader (ed.), Postharvest Technology of Horticultural Crops, 3rd ed. University of California, Agriculture and Natural Resources, Publication 3311.
  
• Lidster et al., 1988. Commercial storage of fruits and vegetables. Agriculture Canada Publication 1532/E
  
• Lipton, 1990. Postharvest biology of fresh asparagus. Hort. Rev. 12:69-155.
  
• Luo et al., 2004. Asparagus. In: Gross et al. (eds.), The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks, 2nd ed. U.S.D.A. Agriculture Handbook Number 66, (in press)
  
• Suslow, 2005. Asparagus: Recommendations for maintaining postharvest quality; Produce Facts. Postharvest Technology Research and Information Center, University of California, Davis.