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Organisation: Centro Internacional de Agricultura Tropical (CIAT) (http://www.cgiar.org/ciat/)
Author: A.L. Jones
Edited by AGSI/FAO: Danilo Mejia (Technical), Beverly Lewis (Language&Style), Carolin Bothe (HTML transfer)

CHAPTER IV PHASEOLUS BEAN: Post-harvest Operations


2.1 Pre-harvest operations

2.2 Harvesting

2.3 Transport

2.4 Threshing

2.5 Drying

2.6 Cleaning

2.7 Packaging

2.8 Storage


2. Post Production Operations

Traditional high-yielding (> 1000 kg ha-1) bean environments are in subtropical regions like those in Chile, Argentina, the Pacific coast of Mexico, the United States, and in some Asian and European countries. In these areas the crop is often irrigated and sole cropped, so diseases and insect problems are few. Bean farmers in these regions are more affluent than those in the tropics and grow crops to sell to large cities within the country or to export to other nations. However, an estimated 90 percentage of world dry bean production occurs under stress conditions where average yields are low (< 600 kg ha-1). Such regions are found in tropical and subtropical Latin America and Africa, where small-scale farmers often intercrop beans for subsistence and apply few or no inputs because they have limited resources. Diseases, insects, adverse climatic and edaphic factors, and other problems cause severe yield loss (28).

Wherever beans are grown on a small-scale, the whole family becomes involved in the harvesting and cleaning of the crop in most of Africa. Women are primarily responsible for bean production in Kenya, Rwanda, Burundi and Uganda. The bean is one of the few crops a woman can grow and be allowed to market for cash. Thus Ugandan women groups are targeting beans as one of the crops to be produced on a large scale (17 and 21). In fact, women make substantial contributions to the agricultural labour, household income generation, choice of crops planted and choice of seeds. Women also assume a large role in Peru, Ecuador, and Bolivia (Andean zone).

Latin America is the part of the world where women help the most by picking, threshing, and cleaning crops. In Africa, women help during all stages, or may be the only ones who do the work. This profile changes when machinery is used. Women are not usually involved in mechanical harvesting.

2.1 Pre-harvest operations

Pre-harvest operations are diverse. Developed countries use highly mechanised techniques. In Latin America, except Argentina where beans are produced on large holdings with high technical input, small holders usually produce beans. Mexico, Brazil, Chile, and Cuba have three types of bean producers, large-, medium-, and small-scale. Colombia, Venezuela, Dominican Republic, Peru, Guatemala, and Costa Rica have limited areas of large-scale, highly mechanised production (33). For mechanised harvesting, the plant needs to be uniform and upright with pods off the ground. Breeding for an improvement in plant architecture would help mechanised harvesting become more efficient and cut down on losses.

At harvest, the variety needs to be ready all at the same time. If plants are too mature pods open. Breeding for uniform ripening is being pursued. Some disease and insect resistant varieties are available but dissemination of new varieties is slow. Networks such as PROFRIZA (Proyecto Regional de Frijol para la Zona Andina) and PROFRIJOL (Proyecto Regional de Frijol para Centro América, México y el Caribe) in Latin America, and ECABREN (Eastern and Central Africa Bean Research Network) and SADC (Southern Africa Development Community) in Africa are helping in this aspect. They encourage small holders to produce seed and develop improved varieties (10).

2.2 Harvesting

Once harvested and separated from the plant, the bean seed continues to ripen, thus biochemical reactions occur, which deteriorate the quality. Therefore at harvest, humidity content, temperature, and climate affect or modify the deterioration agents (12).

There are two types of mechanised dry bean harvesting: conventional undercutting, rodding, or winnowing then combining; and the direct harvest system requiring only one pass of the combine. The latter system has some problems associated with it, such as high header losses and the difficulty of threshing immature plants and weeds. The reduced harvest cost, and lower risk from high winds and water staining, more than compensate. Moisture levels should be about 13 percentage-15 percentage (26). Depending on the size and type of machinery used, 1 hectare of beans may be harvested in 1-2 hours.

In developing countries, harvesting is mostly manual. Plants are pulled up and placed in rows (if a threshing machine is used), or more commonly in piles, very early in the morning to avoid pods opening. When harvesting is done totally by hand, 1 hectare requires 50-80 men hours. Climbing species have to be harvested pod by pod as they mature upwards. Humidity should be about 12 percentage-13 percentage (12). Unthreshed beans are not left long, usually removed for shelter by nightfall for fear of rain. Careful harvesting is important to the bean yield. Mistakes at this point could undo all the benefits of earlier proper practices.

Small-scale farmers need equipment they can use in their fields, thus small-scale technology needs to be developed.

2.3 Transport

Problems of transport do not usually affect small-scale farmers. On the farm, distances are short and covered often on foot. Most of the crop does not leave the farm; some is kept for seed and the rest eaten. Although farms are usually distant from markets, and roads may be in poor condition, the time taken to transport dry beans is not as consequential as it is for green foods. Intermediaries usually take the cost of transport and make the profit on it. There is price duplication on beans between the original seller and the buyer to cover the cost of storage and transport. In theory, small holders could accomplish this themselves in a co-operative. This has not worked in some Latin American countries because the intermediary is also usually the supplier of other goods to the farmer (e.g., chicken feed). The farmer, who does not sell the crop to the intermediary, loses the complementary services. Intermediaries often use the price differential for different grain types to quote farm-gate prices for new varieties.

Large volumes of beans are transported by truck or semi-trailer for domestic markets and by intermediary bulk containers for export. In both cases, handling is kept to a minimum and the more advanced procedures have moisture gradients and temperature controls. Sanitation of containers must be carefully inspected and controlled (31).

2.4 Threshing

Once harvested, the bean plants are either left in the field or taken elsewhere to dry. Bigger producers use drying silos or dryers designed for sacks. Plants must not be piled too high, as air must pass through them to avoid the risk of heating. Smaller amounts may be dried on patios, wooden platforms, under house pilings, in ceiling spaces, hung from wires under a roof and in fine weather, directly on fences or bars around the house.

Threshing methods vary widely. Big-scale producers use moving machinery, which require calibration to avoid losses. Standing machinery can be used for smaller production, carried to the field and run on diesel oil or gasoline. Usually the dried plants are piled on plastic sheets or jute bags then beaten with sticks or run over by animals, tractors, or even light trucks. Beans used for seed are best threshed by hand as this causes least damage but is only suitable for small amounts. In Guatemala and El Salvador, a thin walled box is used on a table made of strips of bamboo, wood, or 12-mesh wire. Separations are wide enough for beans to pass through; the chaff remains on the table. This method has the advantage of less beating of the seed and is useful in areas of small production (12).

Photo 3: Hanging beans to dry under patio roof HANGING BEANS TO DRY

Photo 4: Threshing by beating with sticks THRESHING

2.5 Drying

Drying can be done artificially or naturally (see Table 2). The artificial methods are used for larger production. Natural methods prevail in developing countries and women help in the operation.

The object in drying the seed is to achieve a final humidity of 11 percentage-12 percentage for better storage. Three practical rules should be observed: for each 1 percentage humidity reduced, double the storage potential; for every 5 _C lower temperature of seed, double the storage potential; if the sum of the temperature (in _C) and humidity (in wb) is less than 45, storing conditions are adequate (12).

Table 2. Some different systems for drying beans

Drying systems

Examples

Conditions required

Recommendations

Artificial:

     

Stationary

False-bottomed drier - air flow forced through perforated floor from open chamber below

Continuous air flow

Temperature not to exceed
40_ C, RH 40% at start, 70% at end

 

Tunnel system - forcing air over seeds packed in hemp sacks

 

Do not use more than 4-9 _C temp. differential above air
supply temperature

Do not dry below 13%

Intermittent

Silo modifications of brick or
wood (700-800 kg capacity)

Fast - air temp. < 70 _ C
Slow - < 60 _C
(0.25% per hour)

 
 

Drying wagons or trailers

   

Natural:

     

Sun and wind

On patios and roads in wave
form < 10 cm thick

Immediately after cleaning. Early hours before sun heats concrete/asphalt

Use plastic sheeting under and cover with jute or cloth to absorb humidity

 

Suspended trays with wire
netting base

Seeds piled < 10 cm high, periodically stirred

Trays suspended 50 cm from ground and parallel to it, or slightly angled (23_) in direction of wind and sun

 

Coffee dryers

< 60 _C, well ventilated

 

SOURCE: Compiled by author from (12) and (14).

2.6 Cleaning

Big-scale producers use air and sieving machinery with padded equipment to clean the crop. The distance and number of drops is kept minimal to avoid damaging the seed. The finished product is then bagged (14).

Most small-scale methods of cleaning use sieves and air, then a manual pick through for damaged or discoloured seed. Sieves usually have a metal mesh, with mesh size according to seed size, smaller particles falling through. In windy areas, natural air currents are used, the seed being allowed to fall onto the ground or onto sackcloth from the height of a person with arms raised. The wind takes away the lighter material. Alternatively, electric fans may be used or a motor pump.

The small-scale farmer might use the portable cleaner type Clipper 3W (bicycle), which works with an electric motor, petrol, or with foot pedals, like riding a bicycle. Manual sorting is traditionally done on a table. It is more efficient if the surface is painted pale blue for better contrast and the seed placed in a box with a slanted bottom and an exit at the end (12).

Women play a major role in cleaning seed. Their hands are smaller and defter for this kind of work.

Photo 5: Sieve method of cleaning SIEVE METHOD OF CLEANING

Photo 6: The wind blows away lighter material HANGING BEANS TO DRY

Photo 7: Manual sorting of beans MANUAL SORTING OF BEANS

2.7 Packaging

Commercially, beans are often transported direct in container trucks. Various types of bags are used by the bean industry-laminated paper, burlap, and polypropylene are most used in shipments (2).

Beans are usually sold loose in open sacks or in clear plastic bags so that the colour and quality can be easily seen.

Photo 7: Beans for sale in open sacks BEANS FOR SALE IN OPEN SACKS

2.8 Storage

Farmers have three main reasons for storing bean seed: to keep it safe for consumption; to keep it safe for the next harvest; and to speculate the selling price. When everyone harvests at the same time prices go down, so some farmers reserve seed for later sale at a higher price.

The condition of dry beans going into storage and the storage conditions they experience affect their final quality (26). Conventional storage types for quantities over 1 ton have temperature and humidity controls. The traditional storage methods follow the same principal to keep seed with fewer than 12 percentage humidity and in dry ventilated conditions. Small-scale farmers lower the moisture content of stored bean seed to less than 12 percentage without necessarily understanding the reasons behind it. They just know from experience that humidity will rot the seed.

Bean moisture control, storage temperature, and relative humidity strongly influence dry bean quality and the final product. High moisture and high temperature during storage results in "bin burn", a defect that gives the beans a brown discoloration and an off-flavour. Beans given bad storage conditions may result in defective cooked grain texture. Bean grains that do not soften enough because they fail to absorb water during soaking are called "hard shell". Those that absorb enough water but do not soften enough during a reasonable cooking time are called "hard-to-cook".

Hard grain is both genetically and environmentally controlled. The incidence of hard shell increases as seed moisture content decreases experienced under conditions of high temperature and low relative humidity. Hard shell has been found by most researchers to be highly heritable, with relatively few genes involved. Natural reversibility under high relative humidity occurs.

In contrast, hard-to-cook defect is irreversible and develops during storage under high temperatures (> 21 ºC) and high relative humidity. The mechanism by which the hard-to-cook defect develops is still not clearly understood. The inheritance and genetic variability of hard-to-cook has not been determined (27).

Small-scale farmers usually store bean seed for less than 6 months. They sell their surplus as soon as possible because they need cash and are afraid of losing seed to weevils. They leave the risks of storage for intermediaries. Beans being kept for seed can be treated with insecticides and stored safely for longer periods.

Types of storage range from sophisticated silos to pits in the ground, with all kinds of containers being used on a small scale. Ensuring that containers are clean before use is important.

Table 3. Examples of storage facilities for dry beans

Type

Examples

Storage period

Storage
purpose

Storage
conditions

Comments

Bulk

Silos - wood, steel,
or concrete

3-6 m

> 6 m

Marketing

Later sale

14%-15% humidity

12%-14% humidity

Heated air temp not to exceed
45 ºC, RH < 40%
Protect from contamination of other crops, chemicals, etc.

200-1000 kg

Containers - plastic, metal, wood, aluminium

3-6 m

Consumption, some marketing, some seed

11%-12% humidity

Hermetically sealed containers best

< 200 kg

Earthenware pots, straw baskets, sacks, gallon tins

< 3 m

Consumption, some seed

< 12% humidity

Containers mostly stored in farmer's house.

Source: Compiled by author from many of sources cited.

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