Postharvest Handling and Storage of Asparagus
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.
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.
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