Postharvest Handling Systems: Flower, leafy and stem vegetables

The leafy, stem, and floral vegetables are represented by the following commodities:

Leafy vegetables: lettuce, cabbage, Chinese cabbage, Brussels sprouts, rhubarb, celery, spinach, chard, kale, endive, escarole, other leafy greens, green onion, Witloof chicory, radicchio, sprouts

Stem vegetables: asparagus, kohlrabi, fennel

Floral vegetables: artichoke, broccoli, cauliflower

Mushrooms

Most of these vegetables are marketed throughout the year since they are harvested from various California production areas. For this reason, no long-term storage is required. In general, these commodities are characterized as very perishable, with high respiration and water loss rates.

Harvesting

Virtually all leafy vegetables are cut by hand; but harvesting aids may be used with some (Brussels sprouts, celery, and parsley). Mechanical harvesting systems have been developed for crisphead lettuce celery, cabbage, Brussels sprouts, and cauliflower, but they are not presently used in California. The determination of horticultural maturity varies with commodity, but in general, size is the principal criterion. For others, the solidity of the head determines harvest maturity.

Stem vegetables are also hand harvested. A limited amount of asparagus has been experimentally machine harvested. Asparagus is generally hand cut when spears are at least 23 cm (9 inches) above the soil surface. All floral vegetables are hand harvested, but harvest aids (conveyors) are sometimes used for broccoli. Maturity of floral vegetables is determined by head size and development.

Field Packing

Field packing is used for all leafy vegetables, except Brussels sprouts. The products are selected for maturity and quality, and then cut, trimmed, packed in cartons or crates, transported to cooling facilities, cooled, put into temporary cold storage prior to loading or loaded directly, and transported to market. Field packing generally provides greater marketable yields because of reduced mechanical damage. Wrapped and unwrapped lettuce, celery, cauliflower, broccoli, and spinach are mostly field packed, though the latter three are still packed in packinghouses by a few shippers.

Small celery stalks may be trimmed and packed as hearts in the field or field packed in bulk containers after harvesting and transported to packinghouses for trimming, sorting, prepackaging, and packing as celery hearts. Wrapped lettuce and cauliflower are hand selected, cut and trimmed, and then placed on mobile field units where they are wrapped and packed into cartons. They are then palletized and transported to the cooling facilities for cooling and subsequent handling. Rough handling in field packing is a mayor cause of lettuce and cauliflower marketing losses. Keeping the commodity clean is a problem in field packing operations, particularly when fields are muddy.

Packinghouse Operations

The floral and stem vegetables not packed in the field are selected, cut, placed in bulk containers, then transported to packinghouses for all subsequent handling operations. Compared with field packing, packinghouse handling requires more energy and results in more physical damage to the product, reducing marketable yields.

Packinghouse operations needed to prepare these products for market include:

• Trimming and cleaning with chlorinated water (desirable concentration is about 200 ppm chlorine).

• Sorting and grading to eliminate defective products.

• Sizing, in some cases (all sizing is subjective and done by hand).

• Wrapping or tying individual units (cauliflower, broccoli), or in some cases, prepackaging (Brussels sprouts, broccoli, cauliflower florets).

• Packing in shipping containers (often wax-impregnated) or wood crates.

Cooling

Delays between harvest and cooling should be avoided, especially during warm weather. Different cooling methods may be applied to the same commodity. The most common cooling methods in commercial use are:

• Vacuum cooling for crisphead lettuce, leaf lettuce, spinach, cauliflower, Chinese cabbage, bok choy, cabbage and other leafy vegetables, and mushrooms.

• HydroVac cooling (vacuum cooling with injection of water prior to vacuum cycle) for celery and many other leafy vegetables.

• Hydrocooling for artichoke, leaf lettuce, celery, spinach, some green onions, leek, and many other leafy vegetables.

• Package-icing and liquid-icing for broccoli, spinach, parsley, green onions, and Brussels sprouts.

• Room cooling, primarily for artichoke and cabbage, and for the other leafy vegetables in some operations (not generally recommended for this group of vegetables because it is too slow).

• Forced-air cooling (sometimes with initial spraying of water), primarily for cauliflower and to a limited extent for other leafy and stem vegetables, including sprouts and mushrooms.

Recommended Storage Conditions

In general, these products respond best to storage temperatures of 0° to 1°C (32° to 34°F). Freezing must be avoided. These products are frequently loaded into refrigerated trailers and containers immediately after cooling. For temporary storage, a temperature of 0° to 2°C (32° to 36°F) and a relative humidity of 90 to 95 percent is recommended.

Longterm storage is not recommended, except for cabbage, Chinese cabbage, and celery. In storage, air circulation should be minimized to that required for proper temperature control, excess carbon dioxide should be removed, and adequate oxygen levels should be maintained. Exposure to ethylene should be avoided throughout the handling system. Ethylene induces "russet spotting" disorder in lettuce and decreases the shelf life of all green, leafy vegetables. Exposure to light causes undesirable greening in Belgian endive; this can be retarded by maintaining the product at low temperature.

All the leafy, stem and floral commodities respond favorably to modified atmospheres, although this technique is used on a limited scale commercially. Low oxygen atmospheres (2 to 3 percent oxygen) favor longer shelf life in all products except asparagus and mushrooms. The recommendations for carbon dioxide modification are more variable. Spinach, which is highly perishable, does not tolerate low oxygen atmospheres and is routinely washed and packed in perforated polybags.

Robert E. Kasmire, UC Extension Vegetable Specialist and
Marita Cantwell, UC Extension Vegetable Specialist