Harvesting and Handling
The edible portion of asparagus is a rapidly growing stem (shoot) with scale leaves that arise at nodes. There are two forms of asparagus in the marketplace, namely white (blanched) and green. White asparagus is widely used in Europe and Asia; green asparagus is popular in the U.S. and is produced predominantly in California and Washington. Asparagus has a high metabolic rate after harvest and is among the most perishable crops.
High quality asparagus spears are dark green and firm with tightly closed and compact tips. Stems are straight, tender and glossy in appearance. Spears with green butts are preferred over the spears with white butts as the latter are associated with increased toughness. However, a small amount of white tissue at the butt will delay decay development under typical commercial distribution conditions.
Asparagus spears are harvested as they emerge through the soil from the underground crowns. Typically, spears are cut when they reach 20 – 25 cm., with spear tips still tightly closed. Tender, immature asparagus may be harvested for special market. Stalk diameter is not a good indicator of proper maturity and associated tenderness.
Harvested spears are prepared for market by grading, sizing and bunching. Grades are based on freshness, length and diameter of the stalks, colour of spears, tightness of the spear tips, and the extent of bruising. Spears of larger diameter are considered to be superior in quality with less fibre. After trimming the butt-end, the bunches are packed upright in such a way as to minimize geotropic bending (curving away from gravity) in transit. The packaging should include water-saturated pads in contact with the butt-end to maintain turgidity. However, excessive free water at elevated storage or shipping temperatures may lead to increased decay. Headspace is provided in the packaging to allow for spear elongation without tip curvature or breakage.
Cooling and Storage
Asparagus is highly perishable and must be cooled immediately to 0°/2°C. A 4 hour delay in cooling resulted in an average 40% increase in shear force due to tissue toughening. Asparagus is typically partially cooled during the washing selection and packing operation, and then hydro-cooled to near 0°C after packing. Maintaining a low storage temperature is critical to delay senescence, tissue toughening and flavour loss. High RH is essential to prevent desiccation and to maintain freshness.
Asparagus continues to grow and elongate after harvest if not cooled immediately and stored at low temperatures (<5°C). Contacting water at the butt will also promote spear growth and elongation. Tip bending occurs as the result of upward growth of the tips when the spears are horizontal. Held in an upright position, tip bending may still occur if the tips reach the top of the package and are physically deflected. Freezing injury occurs at temperatures below -0,5°C, and results in water-soaked appearance and tissue softening.
Controlled atmosphere considerations
Elevated CO2 at 5-10% (typically 7%) in air is beneficial in preventing decay and reducing the rate of toughening of the spears. The beneficial effect is most pronounced if temperatures cannot be maintained below 5°C. Short (CA) exposure to higher CO2 concentrations (12-20%) is safe and beneficial only if temperatures can be maintained at 0° – 1°C. Signs of CO2 injury are small to elongated pits, generally first observed just below the tips. Severe injury results in ribbiness. The combination of intermediate O2 (2% to 10%) may or may not provide benefit compared to air enriched with CO2 alone. At O2 levels below 2%, off-odours and discolouration may develop.
Bacterial soft rot, Blue mould, Carbon dioxide injury, Chilling injury, Fusarium, Phytophthora, Pitting, Purpler spot, Tip rot, Watery soft rot.