Asparagus

Infobox on Asparagus
Example of Asparagus
Freshness facts
Optimum carrying temperature	0°C / +2°C
Highest freezing point	-0,6°C
Acceptable product temp. at loading into containers	Max. 2°C above carrying temperature
Optimum humidity	95%
Ventilation setting for containers	25 m³/hr
Storage life	2-3 weeks
Climacteric / non-climacteric	Non-climacteric
Ethylene production	Very low
Ethylene sensitivity	Medium
Modified / controlled atmosphere	5%-9% CO2 at 3°/6°C or 10%-14% at
Potential benefits	O2 none; CO2 - high
Availability
Australia/New Zealand South Africa S. America (Argentine/Chile) C. America N. America N. Europe Netherlands	September - December September - December August - December June - February February - June March - June April-July

Asparagus

Harvesting and Handling

During the growth of asparagus, the stalk shoots (asparagus spears) sprout upright out of the rootstock (rhizome). The tip (head) of the spear, which is approx. 20 - 30 cm long, is covered with small scale-like leaves. Depending on the variety, the plants are cultivated in raised mounds or level beds.

Asparagus is a bush-type plant. After harvest, the shoots develop into shrub-like plants, ensuring the absorption of nutrients. In the autumn, the above-ground parts die back. The rhizomes regenerate after the end of harvest, so allowing the re-growth of strong asparagus plants. Asparagus is considered a particularly healthy vegetable, due to the wide range of nutrients and minerals and high vitamin content. Differing cultivation and harvesting methods result in the following three colour variants:

Blanched or white asparagus
Blanched asparagus is grown in raised mounds. As soon as the tip peeks through the soil of the mound, the asparagus is harvested (cut). Immediate harvesting prevents the asparagus from changing colour.

Purple asparagus
If asparagus is not cut immediately after its tip emerges from the soil, the part of the plant above ground turns purple to blue, as the pigment anthocyanin is formed by exposure to sunlight.

Green asparagus
Green asparagus is grown in level beds where it is exposed to sunlight. Firstly, the pigment anthocyanin forms, resulting in purple colouring and then the asparagus turns green due to chlorophyll formation.

The quality of asparagus cannot be determined from its colour, as this mainly depends on the cultivation and harvesting methods adopted. Signs of perfect quality are an undamaged, tight head (it must not be open) and straight growth. In addition, asparagus must not be woody or exhibit any shrivelled or discoloured cut ends. Postharvest, asparagus is washed briefly under running water and then cooled down immediately. To ensure that the product does not become too hot, harvesting should where possible be performed in the morning and evening.

Asparagus is packaged loose or in bundles in cartons, fruit crates, crates, trays and baskets made of wood or plastic. The content varies, depending on the type of packaging, from 500 g (trays, bundles) to 10 kg (boxes, fruit crates). During transport, the product should be covered with perforated film, to prevent drying-out and quality impairment. The possible deposition of moisture on the inside of the film does not impair quality, but rather assists in keeping the product fresh.

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.

Storage disorders

Bacterial soft rot, Blue mould, Carbon dioxide injury, Chilling injury, Fusarium, Phytophthora, Pitting, Purpler spot, Tip rot, Watery soft rot.