Postharvest Handling Systems: Underground vegetables (roots, tubers and bulbs)

The edible portions of this group of vegetables develop mostly underground and include several botanical structures.

Roots: beet, carrot, celeriac, radish, horseradish, parsnip, turnip, sweet potato, cassava, Jicama
Tubers: potato, Jerusalem artichoke, yam
Bulbs: onion, garlic, shallot
Others: ginger rhizomes, taro (dasheen) corms

Vegetables within this group can also be divided into two subgroups based on their postharvest temperature requirements:

Temperate-zone underground vegetables: beet, carrot, celeriac, radish, horseradish, parsnip, turnip, potato, onion, garlic, shallot, daikon, salsify, water chestnut

Subtropical and tropical underground vegetables: sweet potato, yam, cassava, ginger, taro, jicama, malanga

The commodities in this grouping have several common characteristics. They are all storage organs, principally of carbohydrates; they generally have low respiration rates (depends on the stage of development); they are considered relatively nonperishable, especially if the tops are removed; they continue growth after harvest (rooting and sprouting); and they all can be stored for relatively long periods.

Harvesting

Maturity indices vary with commodity. Many of these products may be harvested and marketed at various stages of development (e.g., "new" or immature potatoes versus mature potatoes, "baby" or immature carrots versus mature carrots). Criteria commonly used to harvest these commodities are as follows:

Carrot: size, length of root

Radish: days from planting, size

Potato: drying of foliage, setting of skins

Cassava and taro: drying of foliage begins

Garlic and onion: drying and bending over of tops

Sweet potato: drying of foliage

Both mechanical and manual harvest are used for this group of vegetables. Most roots and tubers are harvested mechanically and transported in bulk to packinghouses or processing facilities. Garlic and onion for fresh market are mechanically undercut, hand harvested and trimmed, cured in the field, and field-packed or transported to packinghouses. Sweet potato harvesting is still done mostly by hand, although vine cutting and lifting are done mechanically. Vine-killing chemicals may be used on potatoes before mechanical harvesting. Carrots are undercut, lifted by their tops, and detopped during mechanical harvest. Physical damage during the harvesting operations can be extensive and is a major cause of postharvest losses.

Postharvest Procedures

Curing

One of the simplest and most effective ways to reduce water loss and decay during postharvest storage of root, tuber, and bulb crops is curing after harvest. In root and tuber crops, curing refers to the process of wound healing with the development and suberization of new epidermal tissue called wound periderm. The type of wound also affects periderm formation: abrasions result in the formation of deep, irregular periderm, cuts result in a thin periderm, and compressions and impacts may entirely prevent periderm formation.

In bulb crops, curing refers to the process of drying of the neck tissues and of the outer leaves to form dry scales. Some water loss takes place during curing. Removing decayed bulbs before curing and storage ensures a greater percentage of usable product after storage.

When onions and garlic are cured in the field they are undercut, then hand pulled. Sometimes the roots and tops are trimmed and the bulbs then are allowed to dry in field racks or bins from 2 to 7 days or longer (depending on ambient conditions). Sometimes they are pulled and cured before trimming. Curing may be done in windrows with the tops covering the bulbs to prevent sunburn. Where ambient conditions are unfavorable, curing may be done in rooms with warm forced air. Onions develop the best scale color if cured at temperatures of 25° to 32°C (77° to 90°F).

Storage

Onion, garlic, potato, and sweet potato are often stored after curing and before preparation for market (cleaning, grading, sizing, and packing). These products, and other root crops such as carrot and turnip, may be stored from 3 to 10 months in mechanically refrigerated or ventilated storages.

Preparation for market

The following operations are commonly used to prepare root, tuber, and bulb crops for market:

Cleaning. Dry brush or wash and partially dry, removing excess moisture.

Sorting. Eliminate defective products and plant debris.

Decay control. Postharvest fungicides are used on some of these commodities such as sweet potato; chlorination of wash and flume waters provides sanitation for carrot and potato.

Sizing. Size mechanically or by hand. Mechanical sizers are generally diverging rollers or weight sizers. Modified volumetric sizers are used for potatoes. Carrots present special sizing problems, since they must be sized by diameter (diverging rollers) and by length (manually or with a gravity-length sizer).

Grading. Separate into quality grades.

Packing. Pack into consumer units (bags, trays) and then pack into master shipping containers; or bulk pack into shipping containers (bags, boxes, and bins).

Loading into transit vehicles. Bulk transport to processing plants is sometimes used for onion, potato, and radish. For fresh market, most products are loaded in packed shipping containers (palletized if boxes, manually stacked if bags).

All temperate-zone root crops except potato, onion, and garlic can be hydrocooled. In areas where potatoes are harvested during hot weather, they may also be hydrocooled. Tropical root crops, potato, onion, and garlic are occasionally room-cooled before shipment to market. Potatoes may be cooled to 13° to 16°C, (55° to 60°F) before shipment when they are harvested, packed, and shipped to market during hot weather.

Potatoes and onions destined for storage are cooled during the early phase of the storage period with cool air forced through storage piles or bins. Cooling may be done with cold ambient air or with air cooled by mechanical refrigeration.

Special treatments. For storage, onions and potatoes are generally sprayed with maleic hydrazide (MH) a few weeks before harvest to inhibit sprouting during storage. Aerosol applications of CIPC (3-chloro-isopropyl-N-phenyl carbamate) are often circulated around stored potatoes to further inhibit sprouting. Rodent control is also necessary storages.

Non-refrigerated storage methods. Some growers occasionally store mature potatoes in the ground several weeks before harvest. Ground storage is also used for several of the tropical and subtropical roots, including cassava and jicama. Pits, trenches, and clamps are used for storage of harvested tropical roots and tubers. Pits are occasionally used for short-term, small-scale storage of potatoes in some areas.

Ventilated storage in cellars and warehouses is used for potatoes, sweet potatoes, garlic and onions. Newer facilities with temperature and relative humidity controls provide forced-air circulation through bulk piles of potatoes or onions, or through and around stacks of bulk bins

Recommended Storage Conditions

Temperate-zone root vegetables

In California, temperate-zone root vegetables are not usually stored. When they are stored, the following conditions should be maintained: 0°C (32°F), 95 to 98 percent RH, and adequate air circulation to remove vital heat from the product and prevent carbon dioxide accumulation.

Potatoes can be stored up to 10 months under proper conditions. Most long-term potato storage facilities are in the northern U.S. For fresh market, potatoes should be stored under the following conditions: 4° to 7°C (39° to 45°F), 95 to 98 percent RH, enough air circulation to prevent oxygen depletion and carbon dioxide accumulation (about 0.8 cubic feet per minute per 100 pounds of potatoes), and exclusion of light to avoid greening. Greening is due to chlorophyll synthesis and is associated with accumulation of a toxic alkaloid, solanine. For processing (e.g., chipping), the proper conditions are 8° to 12°C (46° to 54°F), 95 to 98 percent RH, adequate ventilation, and exclusion of light. This higher temperature storage retards undesirable sweetening of the potatoes and consequent dark color of the processed products. Seed potatoes are best kept at 0° to 2°C (32° to 36°F), 95 to 98 percent RH, with adequate ventilation.

Garlic should be kept at 0°C (32°F) for long-term storage (6 to 7 months); 28° to 30°C (82° to 86°F) can be used for storage up to 1 month. Ventilation of about 1 cubic meter of air per minute per cubic meter of garlic with 70 percent RH is adequate.

Onions vary in their storage capability. The more pungent types with high soluble solids contents store longer, whereas mild onions with low soluble solids contents are rarely stored for more than 1 month. Storage temperatures should be either 0° to 5°C (32° to 41°F) or 28° to 30°C (82° to 86°F), as intermediate temperatures favor sprouting. Relative humidity should be maintained at 65 to 70 percent, and ventilation rate should be from 0.5 to 1 cubic meter of air per minute for each cubic meter of onions. Avoid light exposure to prevent greening. The storage potential of onions depends on the cultivar.

The use of controlled or modified atmospheres for this group of commodities is negligible. Limited commercial CA storage (3 percent oxygen, 5 percent carbon dioxide) of mild types of onions has been tested recently. Low concentrations of ethylene in the storage environment of carrots and parsnips induce bitterness.

Tropical-zone root vegetables

The general storage recommendations for tropical-zone root vegetables, show that most of these products are chilling sensitive. Some of these root crops (e.g., cassava) are successfully field-stored but deteriorate rapidly if harvested and held under ambient conditions.