Postharvest Cooling and Handling of
Green Beans and Field Peas

[ Peas IMAGE ]

Freshness Facts


Preferred cooling method: ........... Hydrocooling
Alternative cooling method: ... Forced-air cooling
Optimum temperature: .................. 37 to 45 F
Freezing temperature: ....................... 31 F
Optimum humidity: ............................ 95%
Storage life: ....................... 5 to 10 days
Harvested green beans and southern peas (genera Phaseolus and Vigna, respectively) consist primarily of fast-growing, immature plant tissue that is easily damaged by rough handling, dehydration, and attack by decay organisms. In addition, immature plant tissue produces relatively large amounts of respiration heat. Careful handling and quick, thorough postharvest cooling of these crops aids greatly in maintaining quality and significantly lengthens shelf life.

Harvesting

Manual Harvesting

Because of their growth habit, some varieties of beans and peas cannot be harvested mechanically. The quality and hence the market value of in-shell beans and peas depends on the timeliness of harvest, the level of maturity of the pods, and the care in handling.

Postharvest quality control begins in the field. Clean hands and sanitary personal habits are required of workers at all times when handling food products like beans and peas. Therefore, careful supervision and proper instruction of the harvesting crew are essential to the success of any hand harvesting operation. Careful supervision includes random checks of harvesting bags or pails for trash and poor-quality beans. Packing house problems and buyer complaints often result from a poorly instructed and supervised harvesting crew.

Beans and peas should be removed from the plants cleanly without tearing them or causing undue damage to the pods or plants. Overhandling or rough handling of the pods will result in both visible and latent damage. In addition, harvested pods should never be packed tightly into harvesting containers or allowed to remain in the sun for extended periods.

Rules for Manual Harvesting

  1. Keep your hands clean. Remember that you are handling a food product. The law requires you to wash your hands after each visit to the rest station.
  2. Pick all mature pods on the bush before moving on to the next bush.
  3. Harvest only those pods that are ready. Leave immature pods for the next harvest.
  4. Avoid overfilling your hands; do not squeeze or roll the pods.
  5. Do not put trash or cull pods into the container.
  6. Never allow harvested pods to remain in the sun.

Mechanical Harvesting (In-Shell)

Most large commercial green bean and southern pea plantings are harvested mechanically. The major advantage of mechanical harvesting is the significant reduction in management and labor costs. One mechanical harvester can replace as many as 100 manual pickers plus their supervisors, minimizing harvesting costs, but there is some reduction in quality. Many varieties of green beans and southern peas have been specifically developed for once-over mechanical harvesting. These varieties are better able to withstand the forces on the pods moving through a mechanical harvester. Some of these varieties are also considered to be more durable for shipping.

Mechanical harvesters have become highly developed, with one-row or multirow capacity. These machines generally have opposing brushes that strip the pods from the vines, leaving only the plant stems. The harvested material is transported through various separators to remove dirt, leaves, and other foreign materials. The pods are then placed in bags, pallet bins, or dump hoppers for transport to the packing shed. For successful mechanical harvesting, fields must be free of weeds, and the green beans and peas must be uniform in size and maturity.

Mechanical harvesters must be carefully adjusted and operated to reduce the amount of trash and unacceptable pods. Most of these machines have no means of discriminating between quality levels and will harvest immature, overmature, diseased, or damaged pods that would ordinarily be discarded by a human picker. Eliminating large quantities of unacceptable pods and foreign matter in the packing house is difficult, expensive, and requires the harvested crop to be handled excessively. Under the best circumstances, a mechanical harvester and the required sorting machines subject the pods to some damage. Any additional handling can so adversely affect the shelf life and appearance of the product that it will be discounted or refused by buyers.

Mechanical Harvesting (Shelled)

Machines for harvesting shelled green beans often combine the functions of harvesting and shelling in one machine. The pods are initially harvested by either a rotary brush (as discussed in the previous section) or with a conventional reel and sickle-bar arrangement that cuts the plants off several inches above ground level. The pods are shelled and the trash and leaves are separated and discarded while the cleaned peas or beans are conveyed to a storage bin. The rotary brush-type harvesters operate at speeds of 0.5 mile per hour or less.

Determining Quality at Harvest

Snap beans, including bush, pole, and half-runner types, are grown for the fresh market and processing. Pod diameter, not length, is the best indicator of quality. Buyers prefer pods with no bulge or only a slight bulge, indicating tender, young seeds. (See the latest revision of the USDA standards for snap beans.) Overmature snap beans with bulging pods are tough and fibrous, while immature pods (pin beans, sieve sizes one and two) are more susceptible to wilting. Uniform sizing of beans in the crate or carton is critical to acceptance by fresh market buyers. The shape must be fairly straight, the color bright and typical for the cultivar (green or yellow), and the appearance fresh, turgid, and without blemishes. Freshness is evidenced by a distinct, audible snap when the bean is broken.

Lima beans, commonly called butter beans in the southern United States, are grown for fresh markets, processing, and pick-your-own operations. The degree of bean development in the pod indicates processing and marketing quality. Buyers and processors require a well-filled pod. Avoid harvesting flat and immature green pods. Processors require 30 to 40 percent well-matured pods (yellow with whitish purple beans) before beginning harvest for maximum shelling efficiency. Shelled beans should be bright and moist with the skin capable of being easily pricked by fingernail. Dark brown pods indicate dry, overmature beans that are unacceptable in the fresh market. Pods displaying rusty brown spots or other blemishes indicate disease, injury, or the possibility of deterioration and should be discarded.

Southern peas include all black-eyed, crowder, cream, and purple-hull types. Pods should be harvested when mature (half green and half pale) but not overmature (that is, deep purple for the purple-hull type). Overmature peas are too dry and starchy for the fresh market, but excessively green or immature pods contain small seeds and are also not acceptable for fresh or processed markets. Processors prefer harvest to begin when pea fields contain 30 to 40 percent mature pods (that is, when the color is reddish green for purple-hull peas). At this maturity stage, pea color is mostly green and the moisture content is high.

Field Handling

If possible, do not begin the day s harvesting until moisture on the plants has evaporated. Harvesting after the pods have dried will help prevent the spread of postharvest diseases and will result in less contamination by dirt and foreign matter. Once harvested, in-shell beans and peas should be protected from direct sunlight. Heat increases their rate of respiration, which is already relatively high after harvest. Delayed postharvest cooling and exposure to the sun leads to quality deterioration, resulting in shrivelling and weight loss (Table 1). Therefore field-packed beans and peas should be shaded while they are held in the field. Limiting the time between harvesting and cooling to no more than 1 or 2 hours will help maximize shelf life.

Manually harvested green beans and southern peas are often field packed. Machine-harvested beans and peas are loaded into field wagons or truck trailers for transport to the packing shed or processing plant. With one-row harvesting and careful handling, green beans can be harvested directly into market hampers. Without careful handling, snap beans in particular may be bruised and broken during loading operations. To prevent damage and a decrease in pack- out yields, workers should be cautioned not to stand on the beans during loading and unloading. Installing running boards or perches on the wagon or trailer allows workers to unload beans without getting into the trailer bed. Common pitch forks and rakes are not recommended to unload the beans and peas from combine elevator belts or onto the packing line because the sharp metal tines may break and puncture many beans. Special teflon-coated tools are commercially available for this purpose.

Field-Shelled Beans and Peas

Extreme care must be exercised when the harvesting process includes in-field shelling. Shelled green beans and southern peas are extremely perishable and very susceptible to damage. The quality of mechanically harvested and shelled southern peas may decrease greatly during field operations. Temperatures may exceed 140 F in the center of the load. The postharvest changes that occur in these products include color loss, slime development, and eye-bleed. Some processors have made efforts to prevent heat buildup during loading, holding, and hauling by ventilating the trailer beds with an air duct system. The system consists of an air scoop located at the front of the trailer with an opening that forces air through a duct running the length of the trailer when the truck is in motion. Ventilation holes in the duct force air up through the loaded piles of beans or peas to dissipate heat. Fans may also be connected to the duct when the trailer is stationary.

Research has shown that the quality of shelled peas may be stabilized until the trailer reaches the processing plant by ventilating the load with a large fan attached to the rear of the trailer. Cool outside air is distributed through the trailer while the peas are being loaded and held in the field. Hot spots and the attendant color loss can generally be avoided if cool, moist air is evenly forced throughout the load at a rate of 20 cubic feet per minute per square foot of trailer floor surface.

Table 1. Effect of Delayed Cooling on Snap Bean Quality 

Delay Time (hours)                  Weight Loss (%)
        1                                  2.2
        3                                  2.8
        5                                 10.0
Duplicate boxes held, cold stored, and reweighed.
Source: W. Hurst, University of Georgia, 1982.

Cleaning and Grading

Careful supervision of labor is the key to ensuring uniform cleaning, sizing, and packing of hand- harvested green beans and southern peas. Smaller-acreage growers may use a grading table or belted conveyor located at the packing shed to remove trash and culls. Spreading in-shell beans and peas out on a belt or flat surface helps to dissipate field heat before packing and shipping.

For marketing purposes, beans and peas may be segregated by U.S. Grade Standards. Most buyers will accept only produce that meets U.S. No. 1 quality or higher. Single copies of the grade standards for snap beans, lima beans, southern peas, and most other vegetables and fruits can be obtained from the following address:

Standardization Section
AMS, F&VD, Fresh Products Branch
U.S. Department of Agriculture
P.O. Box 96456, Room 2056-South
Washington, DC 20090-6456

A Typical Grading Line

Although methods and machines vary widely, a typical cleaning and grading procedure may include the following components:

Offloading belt - Freshly harvested beans or peas are unloaded from the truck or trailer onto a belt conveyor.

Gravity separator - Adhering soil, rocks, and heavy field trash drop out.

Trash eliminator - An air blast removes leaves, stems, and other light field trash.

Pin-bean eliminator - A rotating drum tumbler removes immature pods through slots.

Broken-bean eliminator - A rotating tumbler with shallow, cuplike depressions catches and removes broken pods.

Vibrating tables - Good pods are further segregated from field trash.

Vibrating washers - Pods are rinsed with clean water to remove adhering soil particles and to remove some of the field heat.

Grading tables - Pods are manually inspected to remove overmature, blemished, decayed, or other defective pods.

Carousel-type automatic box filler - Beans are moved by vibration into wirebound crates or waxed cartons, which are weighed and unloaded onto an automatic box-closing machine.

Cooling - Filled containers are hydrocooled or forced-air cooled and placed into cold storage or immediately shipped.

Postharvest Cooling

Because harvested green beans and southern peas produce large amounts of respiration heat, immediate and thorough postharvest cooling aids greatly in maintaining quality and substantially lengthens shelf life. In addition, prompt and thorough cooling can reduce the effects of dehydration and lessen damage caused by decay-producing organisms. Remember, however, that while postharvest cooling is essential for maintaining quality, it will not improve the quality of a poor product.

When refrigeration is not immediately available, alternatives such as shade, harvesting during the coolest part of the day, and drenching the produce with cold well water after harvesting should be employed. Field-packed containers, if properly cleaned and destined for immediate local sale, may be cooled with well water. Wetting may also initiate evaporative cooling if sufficient air circulation is present. Once the beans or peas have been packed in cartons and wetted, air circulation must continue until the products are properly refrigerated.

Green beans and southern peas intended for distant fresh markets should be immediately cooled after harvest. The placement of field-warm beans and peas in a refrigerated space, known as room cooling, is recommended only as a last resort. Room cooling may be of some benefit but is slow because it relies only on natural conduction and convection to transfer heat. Palletized and bulk containers of snap beans and in-shell peas may require more than 16 hours to cool sufficiently in cooling rooms. To promote cooling and prevent the buildup of respiration heat, the containers should be loosely stacked, leaving space between the pallets for air circulation.

Forced-Air Cooling

In forced-air cooling systems, circulating fans are used in a cooling room to pull refrigerated air through produce containers, greatly improving the cooling rate. Experiments have shown that forced-air cooling is five to eight times faster than standard still-air room cooling. For more information, ask your county Extension agent for a copy of publication AG-414-3, Forced-Air Cooling, in this series of fact sheets.

Hydrocooling

Hydrocooling, the process of bringing large quantities of chilled water into contact with the produce, is the preferred method for cooling green beans and southern peas. It is particularly useful where large volumes of produce must be cooled quickly for shipment to distant markets. Because water is a much better heat transfer medium than air, hydrocooling is very rapid. In this process, the produce is wetted either with a commercial hydrocooler that rains the water onto the produce containers as they pass on a conveyor or by immersion into a flume or tank of chilled water.

Flume hydrocooling is a relatively new method for cooling fresh market snap beans rapidly. Cleaned and graded beans are deposited directly into a long flume system containing chlorinated water chilled to 34 to 38 F. Tests have demonstrated that flume hydrocooling is an effective method for fast, uniform cooling, lowering the temperature of the produce from 85 F to about 45 F in about 6 minutes. Rapid cooling helps prevent brown-end discoloration of the beans.

The disadvantage of hydrocooling is that the beans and peas are wetted. Significant postharvest disease problems will inevitably occur if the produce is allowed to rewarm after hydrocooling or if the water is not properly chlorinated. Warm, wet beans and peas are particularly liable to develop any of a host of postharvest diseases. These include nesting (caused by Pythium species or Rhizopus species), gray mold (caused by Botrytis cinerea), and watery soft rot (caused by Sclerotinia species). Although hydrocooling is the preferred cooling method, it should not be used unless adequate refrigeration facilities are available for continuous cooling and storage.

Although the skin of beans and peas offers considerable protection against infection, pathogens (disease-causing organisms) can enter the produce through a variety of openings. Wounds such as punctures, cuts, and abrasions as well as stems and stem scars provide potential points of entry. The probability of pathogens entering the produce increases with the size of the opening, the depth of submergence, the length of time in the water, and the water temperature.

Always use chlorinated water when washing and hydrocooling beans and peas. Chlorine is a germicidal agent that can control decay-causing organisms found on produce. A free chlorine concentration of about 55 to 70 ppm at pH 7.0 (neutral) is recommended for sanitizing most fruits and vegetables. It may be necessary to add chlorine to the solution more often if the pH is higher and if the temperature of the solution is more than 80 F. In practice, free chlorine concentrations of 150 ppm and more have been used. For more information, ask your county Extension agent for copies of publications AG-414-4, Hydrocooling, and AG-414-6, Chlorination and Postharvest Disease Control, in this series of fact sheets.

Practical Rules for Successful Chlorination

  1. Closely evaluate the need to wet the produce. Wetting the produce greatly increases the likelihood of infection and spread of postharvest diseases.
  2. Monitor the condition of the water. Check the chlorine concentration and pH frequently using test papers or electronic equipment.
  3. Avoid overexposure. Do not allow the produce to remain in contact with the solution longer than necessary.
  4. Change the water frequently. Chlorination efficiency is poor in very dirty water. If the produce is very dirty, wash it with clean water before it comes into contact with the chlorinated water.
  5. Dispose of wastewater property. Before you install chlorination equipment, plan how you are going to dispose of the wastewater safely.
  6. Practice good sanitation. Hose off the packing equipment and floors daily and remove the dirt and trash that may have settled in the chlorination tank. Sanitize equipment with a spray of 4 pints of 5.25 percent sodium hypochlorite solution in 10 gallons of water.

Packaging

Green beans and in-shell southern peas are marketed in a variety of containers, depending on the demands of the market. Some of the more common containers are 1-bushel wirebound crates, hampers holding 26 to 31 pounds each, and telescoping or semitelescoping fiberboard cartons holding 25 to 30 pounds each.

Packers occasionally cut snap beans for the fresh market trade, while others shell lima beans and southern peas. Cutting and shelling greatly increases bean and pea respiration rates and the susceptibility to disease organisms. Prepackaging in low-density polyethylene bags having an oxygen transmission rate of 10,000 to 15,000 cubic centimeters per square meter per day (cc/m^2/day) at 73 F will achieve a modified atmosphere of approximately 3 to 5 percent oxygen and 20 to 30 percent carbon dioxide. These conditions reduce brown discoloration in freshly cut snap beans, stickiness and brown spotting in shelled limas, and sprouting and sliminess in shelled peas.

Storage and Shipping

Green beans and in-shell southern peas should be stored at 37 F to 45 F and 95 percent relative humidity. Under these conditions, snap beans will maintain quality for 7 to 10 days, in-shell lima beans for 5 to 7 days, and in-shell peas for 6 to 8 days. However, temperatures of 38 F and lower may cause significant chilling injury to beans and peas, depending on the cultivar. Chilling injury will appear later during distribution as obvious surface pitting and russeting, making the products unmarketable. Snap beans and in-shell beans and peas should never be placed in contact with ice, as this will cause water spots on the pods. After shelling, limas and peas may be stored at 38 F.

When beans and peas are to be stored or transported in mixed loads with other commodities, it is important to consider the compatibility of the produce in regard to temperature, humidity, and the presence of ethylene gas. Ethylene is given off by some fruits (apples, cantaloupes, bananas, and tomatoes) and will hasten the maturity and decline in quality of green beans and peas. Storing beans and peas with ethylene-producing items is discouraged. Beans and peas also readily adsorb the odor of peppers, onions, and cantaloupes. Common storage and shipment with these items should also be avoided.

Economics

It is difficult to estimate the acreage of fresh market green beans and southern peas grown in North Carolina. Data are not collected by federal market news agencies, and state market news personnel report snap bean acreage only if the beans are a major source of income for local producers. Unofficial or indirect estimates of snap bean acreage changes include county Extension agent estimates, shipment and unloading data, and cash sale receipts. These measures suggest that production has increased modestly, somewhere between 2 and 4 percent per year since the mid-1980s. Leading snap-bean-producing states include Florida, Wisconsin, California, and Georgia. North Carolina snap bean harvesting usually peaks in June, with primary competition coming from growers in Georgia, Tennessee, and South Carolina. Based on shipment arrival data collected by the U.S. Department of Agriculture in the top 22 U.S. cities, the leading snap bean markets in decreasing order are New York City, Boston, Los Angeles, San Francisco, Chicago, and Atlanta. However, primary destinations for locally grown snap beans are Atlanta; Columbia, South Carolina; and New York City, with smaller quantities sent to buyers in the Baltimore-Washington area and Philadelphia.

A 1986 Virginia survey sought buyer opinions about the relative importance of various handling and product characteristics in marketing vegetables. Overall, buyers felt that proper postharvest handling and cooling procedures were the most important features in successfully marketing a perishable crop. Reliability, consistency and quality of pack, and adherence to packaging standards were also important. Buyers indicated that volume was the least critical consideration. However, it was noted in the study that, for some major markets, shipment volume was a very important consideration in gaining initial access to selected buyers. Snap bean growers likely face a fairly stable to modestly increasing national demand.

Because the primary competitors for North Carolina snap bean growers are growers in adjacent states (likely having similar production, transportation, and distribution costs), it is critical that local producers build a reputation for providing quality beans in the quantity demanded. As competition intensifies, it is important that our reputation be maintained and improved. If production growth slightly exceeds marketing growth, some local growers will find snap bean production highly unprofitable. Local production and postharvest handling skills likely must be improved before increased market share and expanded marketing opportunities will be realized. For Additional Information The following publications in this series on the postharvest cooling and handling of North Carolina Fresh produce are available from your county Cooperative Extension Center.

Prepared by

M. D. Boyette, Extension Agricultural Engineering Specialist
J. R. Schultheis, Extension Horticulture Specialist
E. A. Estes, Extension Economist
North Carolina Cooperative Extension Service

and

W. C. Hurst, Extension Food Science Specialist, University of Georgia
P. E. Sumner, Extension Agricultural Engineering Specialist, University of Georgia

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.

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2,000 copies of this public document were printed at a cost of $950, or $0.48 per copy.

Published by

North Carolina Cooperative Extension Service

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
3/94-2M-TWK-240129                                    AG-413-8