Postharvest Cooling and Handling of
Cabbage and Leafy Greens

[ Cabbage GIF ]

Freshness Facts


Preferred cooling method: 
     Room cooling, hydrocooling, and icing.
     (Icing is suitable for leafy greens only.)
Optimum temperature: ..................... 32 F
Freezing temperature: .................... 30 F
Optimum humidity: ......................... 95%
Storage life:
        Cabbage ................. 2 to 3 months
        Leafy greens ............. 1 to 2 weeks
Cabbage and leafy greens (collards, kale, mustard greens, and turnip greens) are important commercial vegetable crops in North Carolina. Although growers in the state supply only about 4 percent of U.S. cabbage production, during the month of June they produce one-fourth of the do-mestic supply. Leafy greens are shipped from the state year-round, but the bulk of the crop is shipped in late spring and early summer. Mishandling can easily damage these crops, but most serious postharvest losses can be prevented. Careful attention to proper postharvest handling can ensure buyer satisfaction and marketing success. This publication has been prepared to acquaint growers, packers, and shippers of cabbage and leafy greens with proper postharvest handling procedures.

Harvesting

Cabbage

Although there have been occasional attempts to harvest cabbage mechanically, almost all North Carolina fresh-market cabbage is harvested by hand. Machine harvesting is much faster than hand harvesting, but unless extreme care is taken, machine harvesting can result in significant damage to the cabbage. For this reason, mechanically harvested cabbage may be better suited for processing than for the fresh market. Machine-harvested cabbage intended for the fresh market must be unusually uniform in size and maturity.

A mature head of cabbage generally weighs from 3 to 5 pounds, depending upon variety. Cabbage should be harvested promptly when the heads are firm and mature. Delaying harvest even a few days beyond maturity can result in split heads and increased incidence of field disease. Unharvested cabbage may develop significant infestations of alternaria leaf spot and downy mildew, particularly during wet weather. These diseases can be spread through normal harvesting and handling. Harvesting immature heads, however, reduces yield, and the heads are too soft to resist handling damage. Immature heads also have a shorter shelf life than mature heads.

Field workers have a major influence on quality. They should be made aware of the importance of good sanitation practices, be properly instructed in selecting for maturity, and be cautioned against handling cabbages roughly. An experienced picker should be able to determine the level of maturity quickly and consistently by feel and by the size of the head. The head is harvested by bending it to one side and cutting it with either a Russel knife or a common butcher knife. Harvesting knives should be sharpened frequently to reduce effort and lessen picker fatigue. The head should not be removed by snapping or twisting it since this practice damages the head and results in inconsistent stalk length and trim. Broken stalks are also more susceptible to decay.

The stalk should be cut flat and as close to the head as possible, yet long enough to retain two to four wrapper leaves. Extra leaves act as cushions during handling and may be desired in certain markets. Yellowed, damaged, or diseased wrapper leaves should be removed, however. Heads with insect damage and other defects should be discarded. It is essential that heads not harvested be left undamaged because fields may be harvested as many as three times for maximum yield. Harvested cabbage can be placed in bags, boxes, wagons, or pallet bins, depending on the harvesting method employed.

Harvesting aids can significantly reduce harvest labor costs, improve harvest efficiency and cabbage quality, and speed the harvest operation dramatically. Aids may be as simple as a modified farm trailer for transporting cabbage and boxes or as sophisticated as a self- propelled unit costing thousands of dollars. The more complex machines conveniently integrate and automate most of the harvesting and packing functions into a single unit. An effective but simple harvesting aid employs a simple belt conveyor attached to a tractor that slowly passes through the field alongside the pickers. Workers place harvested heads on the conveyer belt, which carries the heads to a bulk bin, wagon, or even a mobile packing station. The conveyer can be a homemade conversion of a grain conveyer or a factory-built model especially designed for cabbage harvesting. When equipped with a canopy and high-flotation tires, a harvesting aid can be operated during rainy weather.

Collards and Other Greens

Collards can be harvested as leaf-collards, in which case only leaves of the proper size and maturity are harvested, or as head-collards, in which case the entire plant is taken. Leaf-collards can be packed loose or gathered into bunches of 8 to 12 leaves and secured with a rubber band. Head-collards are seldom bunched. Head-collards are usually harvested when plants have 16 to 20 mature leaves. Leaves that show cold injury, yellowing, mechanical injury, or insect damage should be discarded. It is essential that all leaves be of high quality and uniform in color. Collards intended for machine harvesting are normally planted very close together (much closer than when head-collards are desired) and harvested at the four- to eight-leaf stage.

Turnip greens, mustard greens, and kale may also be harvested as single leaves or as whole plants. Leafy greens are still predominantly harvested by hand in North Carolina. However, machines are available that cut and gather greens into bulk containers. Fields are usually harvested several times, but care must be exercised to prevent damage to the plants. At the packing shed, the greens are removed from the bulk containers onto a grading belt for cleaning, sorting, and packing.

Postharvest Handling

Harvested produce should always be removed from direct sunlight and transported to the packing shed as soon as possible. Cabbage and leafy greens are particularly susceptible to wilting and other damage from high temperatures. When there is a delay of more than an hour or two between harvest and packing, a water drench or spray arrangement can help prevent dehydration and overheating.

Cabbage

Cabbages are generally packed in 50-pound fiberboard cartons, 50- to 60-pound wire-bound crates, or mesh bags. The industry has been slowly abandoning the mesh bag in favor of cartons or crates because bags offer only minimal protection from rough handling. Cartons and crates are also easier to palletize. A recent market innovation is the shipping of cabbage in heavy fiberboard bulk pallet bins holding 500, 750, or 1,000 pounds, as shown in Figure 1. Some specialty cabbage (such as red, savoy, and Chinese types) are packed in 25-, 30-, or 40-pound cartons, depending upon market preference. Uniformity and the proper count per carton are important; 18 to 22 heads per 50-pound carton is customary.

[ Bulk pallet bin of cabbage ]

Figure 1. Bulk pallet bin of cabbage in the packing house.

Proper packing and cooling are essential to maintaining the freshness of cabbage. Freshness can be tested by rubbing two heads together; if they are fresh, they will make a squeaking sound. Cabbage should be cooled immediately after packing. A refrigerated room controlled to 32 F and 95 percent relative humidity is ideal. In this environment, the center of a medium-sized cabbage should take about 18 hours to cool from 80 F to 36 F. It is usually not necessary to cool cabbage by more rapid means, although some packers use forced-air cooling fans to greatly decrease cooling time. For more information on forced-air cooling, refer to Cooperative Extension publication AG-414-3, Maintaining the Quality of North Carolina Fresh Produce: Forced-Air Cooling.

Greens

Greens that have been harvested during rainy weather or harvested by machine often are contaminated with soil. They are normally washed in fresh water or in water chlorinated at 75 ppm before they are graded and packed. (For more information on chlorination, refer to the current North Carolina Agricultural Chemicals Manual and Cooperative Extension Service publication AG-414-4, Maintaining the Quality of North Carolina Fresh Produce: Hydrocooling.

Mustard greens, turnip greens, and kale are banded into 1-pound bunches or packed loose into 21- or 24- pound cartons or wire-bound crates. Leaf-collards can be treated similarly. Head-collards are usually packed with 8 to 16 bunches per container. Leaves that have yellowed, show signs of disease, or have other obvious defects should be discarded.

Rapid cooling either by hydrocooling alone or in combination with package icing is essential to maintaining the quality of leafy greens. For more information on icing, refer to Cooperative Extension Service publication AG-414-5, Maintaining the Quality of North Carolina Fresh Produce: Crushed and Liquid Icing. Greens may be held in refrigerated storage under ideal conditions for as long as two weeks. Southern-grown cabbage has been held in common cold storage for two to three months. The use of controlled atmosphere storage has been shown to extend this period somewhat. Cabbage and all types of leafy greens freeze at about 30 F and are sensitive to ethylene gas. These items should not be stored or shipped in proximity to fruits and vegetables that are known to produce ethylene, such as apples, pears, peaches, or tomatoes.

Marketing

To market a crop successfully, producers must always be aware of the supply and demand, which vary with season and locality. Although per capita consumption of vegetables in the United States increased nearly 30 percent between 1980 and 1990, cabbage consumption apparently declined somewhat. In 1990, cabbage farm cash receipts were 18 percent lower than 1980 levels, and interstate shipments of cabbage to selected major markets were about 20 percent below 1980 levels. It is estimated that North Carolina cabbage production decreased 10 percent during that decade. California, Florida, and Texas remain the dominant cabbage supply areas, providing nearly 60 percent of the nation's supply.

U.S. production and consumption of leafy greens have increased somewhat in recent years, and import competition has intensified. North Carolina collard and leafy greens production increased 40 percent between 1980 and 1990, with most production occurring in the eastern part of the state. Peak supply availability for greens usually occurs during the winter season, with California, Georgia, and Texas leading in production. To gain and retain a market for their crops, cabbage and greens producers need to reduce cost per unit and offer buyers improved quality by using proper postharvest practices, such as cooling and strict quality control. A market window may exist for summer marketing of cabbage grown in the mountains. Supply is smallest and grower prices are highest for cabbage and leafy greens during the summer months. A prudent grower always makes marketing and postharvest handling arrangements before planting.

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Prepared by
M. D. Boyette, Extension Agricultural Engineering Specialist
D. C. Sanders, Extension Horticulture Specialist
E. A. Estes, Extension Economist

2,000 copies of this public document were printed at a cost of $695, or $.34 each.

Sponsored by the Energy Division, North Carolina Department of Economic and Community Development, with State Energy Conservation Program funds, in cooperation with North Carolina State University. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Energy Division, North Carolina Department of Economic and Community Development.

Published by

THE NORTH CAROLINA AGRICULTURAL EXTENSION SERVICE

North Carolina State University at Raleigh, North Carolina Agricultural and Technical State University at Greensboro, and the U.S. Department of Agriculture, Cooperating. State University Station, Raleigh, N.C., Chester D. Black, Director. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. The North Carolina Agricultural Extension Service offers its programs to all eligible persons regardless of race, color, or national origin, and is an equal opportunity employer. 
8/92-2M-TWK-220533                                    AG-413-5