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.
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
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
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.
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.
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.
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
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:
AMS, F&VD, Fresh Products Branch
U.S. Department of Agriculture
P.O. Box 96456, Room 2056-South
Washington, DC 20090-6456
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.
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.
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
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.
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.
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.
M. D. Boyette, Extension Agricultural Engineering Specialist
J. R. Schultheis, Extension Horticulture Specialist
E. A. Estes, Extension Economist
North Carolina Cooperative Extension Service
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.
2,000 copies of this public document were printed at a cost of $950, or $0.48 per copy.
North Carolina Cooperative Extension Service