Thursday, July 18, 2024

Millet and Sorghum Harvesting and Threshing

Millet and sorghum harvesting and threshing can simply be described as those methods used in harvesting and threshing millet and sorghum. Some of the methods used in harvesting and threshing millet and sorghum include the following;

Manual Harvesting

In Africa, and especially in the Sudano-Sahelian area, these cereals constitute the staple food in the human diet. They are harvested almost exclusively by hand, with a knife after uprooting or bending the taller stems to reach the spikes.

Harvesting and removal from the field takes 10 to 20 days per hectare, according to yields. Harvested ears are stored in traditional granaries while the straw is used as feed for cattle or for other purposes (e.g. thatching).

Gradual Mechanization of Threshing

Women separate the grain from the ears with a mortar and pestle, as it is needed for consumption or for marketing purpose. The threshed grain is cleaned by tossing it in the air using gourds or shallow baskets.

This traditional method is arduous and slow at (10kg per woman-day). Consequently, research has been conducted for some years on how to mechanize it.

The mechanical threshing of sorghum ears does not raise any special problems: conventional grain threshers can be used with some modifications; such as adjustment of the cylinder speed, size of the slots in the cleaning screens, etc.

On the other hand, the dense arrangement of spikelets on the rachis and the shape of millet ears (especially pearl millet), make their mechanical threshing excessively difficult.

The first millet and sorghum threshers were developed in Senegal in the 1960-70s: the Siscoma BS 1000 and the Marot DAK II. Giving relatively high outputs (about 1000kg per hour) they have been intended for village farmers’ groups, cooperatives or private contractors going from village to village to work on big threshing layouts.

The multipurpose “Bamba” thresher, better suited to rural communities, has a capacity of about 300kg per hour. The Senegalese pool of millet and sorghum threshers currently amounts to 120-150 units.

As regards mechanized harvesting at family level, some hand-operated threshers (Champenois) were developed and tested experimentally but they did not prove very successful. CIRAD is currently working on the design of powered millet threshers of low capacities (50 to 100kg per hour).

Grain Cleaning

Threshing operations leave all kinds of trash mixed with the grain; they comprise both vegetable (e.g. foreign seeds or kernels, chaff, stalk, empty grains, etc.) and mineral materials (e.g. earth, stones, sand, metal particles, etc.), and can adversely affect subsequent storage and processing conditions.

The cleaning operation aims at removing as much trash as possible from the threshed grain.

1. Manual Cleaning of Grains

The simplest traditional cleaning method is winnowing, which uses the wind to remove light elements from the grain.

2. Mechanized Cleaning

The most rustic equipment is the winnower a fan-originated current of air passes through several superposed reciprocating sieves or screens. This type of machine was widely used in the past for on-farm cleaning of seed in Europe.

It can be either manually powered or motorized; capacities range from a few hundred kilogram’s to several tones per hour.

In Europe, with the use of combine harvesters and the development of centralized gathering, cereal winnowers have been progressively replaced by seed cleaners in the big storage centers. These machines, also equipped with a system of vibrating sieves, are generally capable of very high outputs (several tens of tones per hour).

In developing countries, mechanizing the cleaning operation at village level has seldom been felt as a necessity, because of the lack of quality standards in grain trading. However, because of the current trend towards privatization of marketing networks, the demand for cleaning machines will probably increase.

The local manufacture and popularization of simple and easily portable equipment, such as winnowers or screen graders suited to cereal crops, need to be encouraged. CIRAD/SAR has recently developed cleaning machines of the rotary type with outputs of a few hundred kilograms per hour.

Drying of Grains

Millet and Sorghum Harvesting

In the process of drying, heat is necessary to evaporate moisture from the grain and a flow of air is needed to carry away the evaporated moisture.

There are two basic mechanisms involved in the drying process; the migration of moisture from the interior of an individual grain to the surface, and the evaporation of moisture from the surface to the surrounding air.

The rate of drying is determined by the moisture content and the temperature of the grain and the temperature, (relative) humidity and velocity of the air in contact with the grain.

In general, the drying rate decreases with moisture content, increases with increase in air temperature or decreases with increase in air humidity.

At very low air flows, increasing the velocity causes faster drying but at greater velocities the effect is minimal indicating that moisture diffusion within the grain is the controlling mechanism.

Grains are hydroscopic and will lose or gain moisture until equilibrium is reached with the surrounding air. The equilibrium moisture content (EMC) is dependent on the relative humidity and the temperature of the air.

It is very important to appreciate the practical significance of the EMC. Under no circumstances is it possible to dry to moisture content lower than the EMC associated with the temperature and humidity of the drying air.

Read Also : Six (6) Factors Affecting Mechanized Harvesting Of Field Crops

1. Sun Drying

The traditional practice of grain drying is to spread crop on the ground, thus exposing it to the effects of sun, wind and rain.

The logic of this is inescapable; the sun supplies an appreciable and inexhaustible source of heat to evaporate moisture from the grain, and the velocity of the wind to remove the evaporated moisture is, in many locations, at least the equivalent of the airflow produced in a mechanical dryer.

Even today, sun drying of grain remains the most common drying method in tropical developing countries. It is first employed when the crop is standing in the field prior to harvest; maize cobs may be left on the standing plant for several weeks after attaining maturity.

Although not requiring labour or other inputs field drying may render the grain subject to insect infestation and mould growth, prevent the land being prepared for the next crop and is vulnerable to theft and damage from animals.

Drying in the field may also be carried out after harvest with the harvested plants laid in stacks with the grain, maize cobs or panicles raised above the ground and exposed directly to the sun.

Drying on flat exposed surfaces is the most common way of drying grain after harvesting and threshing. For drying, small amounts on the farm grain may be spread on any convenient area of land.

Contamination with dirt cannot be easily avoided with this method and cleaner dried grain can be obtained by drying the grain on plastic sheets, preferably black.

Purpose-constructed drying floors are commonly used where there is a need to dry large quantities of grain during the season, e.g. at most rice mills. The floors are usually made of concrete or brick, these materials presenting a relatively smooth and hardwearing surface.

Floors should be constructed to withstand the movement of vehicles and sloped or channeled to hasten the runoff of rainwater. The paddy is spread in a thin layer on the floors and raked at intervals, preferably 7-8 times daily, to facilitate even drying. At night the paddy is heaped into rows and covered with sheeting.

The grain can reach temperatures as high as 60°C under clear skies and the rate of drying can be extremely high. Under these circumstances kernel cracking and loss of head rice can be appreciable, particularly if paddy is dried to below 14% moisture.

Covering the paddy around midday may be beneficial under particularly hot and sunny conditions.

Experiments at IRRI have shown that cracking can be reduced by 25% if paddy is dried in the shade but the benefit from the improved quality is generally more than offset by the longer drying times and hence reduced throughput and increased costs.

In rainy weather, even though drying will be slow, every effort should be made to prevent wet freshly-harvested paddy from over-heating with deterioration in quality by spreading on floors rather than let it remain in heaps and sacks.

Under these conditions or when there is great demand for drying space paddy can be dried to 17-18% moisture and then temporarily stored for 15-30 days before final drying.

2. Crib Drying

Compared with paddy, cob maize can remain at relatively high moisture contents, in excess of 20% with natural ventilation for considerably longer periods, from one to three months. The maize crib in its many forms acts as both a dryer and a storage structure.

The rate and uniformity of drying are controlled by the relative humidity of the air and the ease with which air can pass through the bed of cobs. The degree of movement of air through the loaded crib is largely attributable to the width of the crib.

3. Solar Dryers

An improved technology in utilizing solar energy for drying grain is the use of solar dryers where the air is heated in a solar collector and then passed through beds of grain.

There are two basic types of solar dryer appropriate for use with grain: natural convection dryers where the air flow is induced by thermal gradients; and forced convection dryers wherein air is forced through a solar collector and the grain bed by a fan.

Natural convection dryers are generally of a size appropriate for on-farm use. The dryer consists of three components, a solar collector, the drying bin and a solar chimney.

For a 1 ton capacity dryer the collector is 4.5 m long and 7.0 m wide with the solar absorber base of burnt rice husks or black plastic sheet covered with clear plastic sheet.

The drying bin is 1.0 m long and 7.0 m wide with a base of perforated steel or bamboo matting.

The solar chimney provides a column of warm air that increases the thermal draught of air through the dryer.

The forced convection solar dryer can be considered as a conventional mechanical drying system in which air is forced through a bed of grain but the air is heated by a flat plate solar collector rather than by more conventional means.

Considerable work has been undertaken in developing low-cost and efficient solar collectors for crop drying applications.

The simplest type of collector is the bare plate which consists simply of an air ducts the uppermost surface of which acts as the absorber plate. The covered plate collector in its many forms utilizes a translucent cover above the absorber plate.

The optimum design suitable for use at farms and mills in developing countries is probably the bare plate collector which is capable of operating at a collection efficiency of 40-50% with airflow of 0.10 kg/s.m².

A major advantage of the bare plate collector is that it can be easily incorporated into the roof of a dryer or storage building. Corrugated iron is a popular and inexpensive roofing material in many areas and when painted black forms an excellent solar absorber.

A false ceiling can be fixed to the roof joists so forming a shallow duct running the length of the building and easily connected to a fan via ducting at one end of the building.

The heat available from the collector is weather dependent and consideration should therefore be given as to whether solar energy should be the sole source for heating the air or a supplement to more conventional heating systems.

Effect of Drying on Grain Quality

The drying operation must not be considered as merely the removal of moisture since there are many quality factors that can be adversely affected by incorrect selection of drying conditions and equipment. The desirable properties of high-quality grains include:

Low and uniform moisture content;

Minimal proportion of broken and damaged grains;

Low susceptibility to subsequent breakage;

High viability;

Low mould counts;

High nutritive value;

Consumer acceptability of appearance and organoleptic properties.

1. Moisture content

It is essential that the grain after drying is at moisture content suitable for storage. As discussed, the desired moisture content will depend on the type of grain, duration of storage and the storage conditions available.

It is also important that the drying operation is carried out to minimize the range of moisture levels in a batch of dried grain. Portions of under-dried grain can lead to heating and deterioration.

2. Stress cracking and broken grains

Drying with heated air or excessive exposure to sun can raise the internal kernel temperature to such a level that the endosperm cracks. The extent of stress cracking is related to the rate of drying. Rapid cooling of grain can also contribute to stress crack development.

3. Nutritive value

Grain constituents such as proteins, sugars and gluten may be adversely affected when the grain attains excessive temperatures. The feeding value of grains can be lowered if inadequately dried.

4. Grain viability

Seed grain requires a high proportion of individual grains with germination properties. The viability of grain is directly linked to the temperature attained by grains during drying.

5. Mould growth

Many changes in grain quality are linked to the growth of moulds and other microorganisms.

The rate of development of microorganism is dependent on the grain moisture content, grain temperature, and the degree of physical damage to individual grains.

Mould growth causes damage to individual grains resulting in a reduction in value. Under certain circumstances mycotoxin development can be a particular hazard.

6. Appearance and organoleptic properties

The colour and appearance is as perceived by the customer and/or consumer. For example, the colour of milled rice can be adversely affected if the paddy is dried with direct heated dryers with poorly maintained or operated burners or furnaces.

Read Also : Wastewater Management and Treatment Processes


Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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