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plants
Natural methods
Scattered on ground
In this method plants are placed directly on the ground, foliage downwards, so that the pods are exposed to the sun. Conversely, in some areas, foliage is placed upwards with the pods in contact with the moist-soil and protected from the direct sunrays. Plants are left in this position for varied periods of time, which often depends on the beliefs of the individual farmer. There is no criterion of moisture content for determining when the plants should be collected from the ground. In the Eastern province of Northern Rhodesia it was observed that the moisture content reduced from 21 percent to 5 percent in 7 days. For the case of Uganda, the moisture content reduced from about 40 to 25 percent in one day and further diminished to 6 percent after 20 days. The period of time the plant are left on the ground surface is limited to one to two days in Senegal. In South Africa it is recommended that plant should be allowed to lie on the ground surface for a period of three days to allow leaves to dry out before stacking. Curing by this method locally called sun curing has an adverse effect on the quality of kernels and makes it difficult to sell the crop. Grading regulations now discriminate against groundnuts cured in this way. If groundnuts are left too long on the ground to cure, dew and sunlight tend to discolour the nuts. Moulds may develop and cause breaking of pods while overheating may cause splitting of the kernels during shelling.
Windrows
This method is used for curing groundnuts prior to further drying in stacks in South Africa and Israel. After harvest plants are dried in inverted windrows for 2 to 3 days. In these windrows, the pods on top may be exposed to the weather or they may be underneath, next to the ground covered by the foliage. Loose fluffy windrows seem to permit good air circulation, which ensure uniform and fast curing of the pods. Groundnuts ‘dried’ in fairly large windrows with the pods protected from full sunlight by the haulms have been shown to lose moisture more slowly and suffer no apparent damage as compared to the pods exposed to the sun in small, thin windrows. If there are rains before the pods are dry enough to be picked-up, they may become damaged by the moulding and consequent blackening of the shell, together with some blackening of kernels. Heat and too rapid drying usually damage groundnuts exposed to direct sunlight at the top of windrows for more than two days. Pods dried following thin windrows produce hard kernels, excessive skinning and breakage during shelling. The seed also loses their viability rapidly during storage. Consequently, in colder areas freshly dug and windrowed groundnuts, which become frosted, will have impaired viability and reduced vigour of seedlings. When windrow drying is used as a preliminary to all drying, curing may be done for three to four days to attain moisture content between 15 and 20 percent. It is recommended that when windrow drying is used before drying in sacks, the plants should only be left in windrows to wilt for 18 to 24 hours (Blatchford and Hall, 1963). Please see Figure 14.
Figure 14:Groundnut hand-sheller.
DOR Method
In India loss of seed viability is a serious problem in groundnuts produced during the summer season. For drying the pods under shaded conditions, Directorate of Oilseeds Research (1983), Hyderabad, India, developed a method for maintaining seed viability. In this method two big heaps one-metre in diameter are tied near the base with a rope.
Figure 15:DOR-method for the drying of groundnut pods.
One heap is inverted over the other in such a way that haulms of the upper heap cover the exposed peripheral pods from the direct sunlight (Figure 15). Seed of the pods dried following this method retains about 80 percent germinability, even after 8 months of storage. In the regions, where groundnut is grown only in the summer season, pods dried following the DOR method could be stored for the sowing of the next summer season.
There are two major drawbacks to this method. First, in case of cultivars with less height and foliage, if the upper bundle does not completely cover the peripheral pods of the lower bundle by its haulms completely, the peripheral pods are exposed to sunlight and become prone to lose seed viability. Second, if there are rains within 1 or 2 days after the harvest, the seed quality is damaged seriously.
NRCG Method
To overcome the drawbacks of the DOR method as mentioned above, a new method for the drying of groundnut pods was developed at National Research Centre for Groundnut (NRCG). In this method a tripod type structure (pyramid shape) can be raised in the field with the help of three bamboo poles of about 1.5 m long. A coir rope can be wound around the structure starting from the bottom to the top, while maintaining a space of 6 to 8 inches between two loops. Immediately after harvest groundnut plants are hanged on the rope of the structure in inverted position, pods up and haulms down and the structure is filled with groundnut plants in a way that the pods of an upper ring covered the haulms of the lower ring thus forming a sloping structure like the roofing of a thatched house. The plants can be arranged bottom ring upwards (Figure 16). Groundnut pods along with the plants can be allowed to dry in the structure in the field for five days. Seed dried following this method maintains >80 percent germinability, even after 10 months of storage. In India this method was found very useful in maintaining the viability and quality of the seed in the areas where rain-showers are frequent during drying period i.e. the Northeastern parts, state of Orissa and Bengal. This method of drying was demonstrated to the farmers of Gujarat in Kodinar
Taluka, Junagadh (India) in the summer of 2000, while the crop was being dried in the field it experienced rains during 2nd and 3rd day. Seed obtained from the pods dried following NRCG method retained more natural testa colour than the conventional, windrows and DOR methods. If there are heavy rains during the drying period the structures may be covered with polyethylene sheeting, to serve as protection from rainwater.
Figure 16: NRCG-method of drying of pods, the basic structure raised with the help of three bamboo sticks and structure loaded with groundnut plants.
Stacks
Different types of stacks are built in different countries. These may be in a series ranging from those of simple structure, rather like a large heap of plants, to those of complex structure built around a pole and requiring some skill to construct. Blatchford and Hall, (1963) defined the term ‘stack’ as structure formed by grouping a number of plants together. They described four types of stacks: ordinary, ventilated, poled and ventilated poled stacks. The ordinary stack is the simplest type of stack and is formed by gathering the plants into heaps, the dimensions of which often range from 3 feet in diameter and 2 to 3 feet high to about 12 feet in diameter and 5 feet high. The pods may either be scattered throughout the stacks, which is common in the African countries or lie at the stack centre or around the outside of the stack, depending on the area and the type of groundnut grown. In several groundnut-growing areas, pods are picked from the plant after drying for about two to four weeks in these stacks and are made ready for selling or storage. The drying period in these stacks also varies, frequently lasting from 10 to 15 days but in a few cases only two to three days. The small stacks are gathered together into large stacks with the haulms towards the outside and the pods towards the inside. These larger stacks are usually built at the edges of fields and some farmers choose shaded sites for stacking to avoid over drying by the sun. In the Eastern Province of Northern Rhodesia farmers are encouraged to build the freshly lifted plants in to stacks three to four feet high, placing the plants with the pods to the inside of the stack. In ventilated stacks the plants are grouped together so that the centre of the stack remains open to assist ventilation. The pods, as with ordinary sacks, may be scattered throughout the stack or all lie at the stacks centre or around the periphery of the stack. Farmers in Rhodesia and Nigeria build stacks of this type locally known as ‘cocks’. The stacks should be built on small mounds of earth and the pods placed at the inside of the stacks. The central funnel should be covered with grass to prevent the pods at the top from being scorched by the sun. It is also suggested that pieces of plastic sheeting cover the stack would help to shed rain to outer edges. When stacks are built near the farm buildings on an area of cleared ground, ‘ventilation tunnels’ constructed with the help of poles available locally is recommended. In most humid parts of Kenya plants are often built into a stack called ‘poled-stacks’. In this method plants are grouped around a centre pole, which supports the stack and prevents it being blown down by the wind. The pods may be scattered throughout the stack or may lie the stack centre or round the periphery. It is reported that drying in such stacks takes about three to five weeks. The losses are shown in general to be less in the stacking method than in the scattered or windrows methods.
After lifting, plants can be stocked by hand to dry, on some kind of simple frame or pole as used in some parts of Central Africa. Stooking groundnut normally produce excellent quality hay and pods, but are confined to less-developed areas, where machinery is not easily available or where labour is still plentiful. When it is necessary to leave the plants to dry in the field after lifting, some method of keeping the pods off the ground is necessary. This will reduce losses from rotting and termites etc. and allow air to circulate more freely. The simplest method is to invert the plants in lines, so that pods are upward resting on the foliage. Machines which can dig, shake and produce inverted windrows are available and in commercial use. The machine consists of digging blades which span two rows, loosen the soil and cut tap-roots, a pick-up and elevator to lift plants and shake them clean of soil plus a mechanism to invert the plants and place them in windrows. Windrows can be re-turned in wet weather, or re-shaken if large amount of soil remain on pods. Some groundnut varieties produce considerable vegetative growth, which remains green when pods are mature. To reduce the volume passing through lifters and threshers, half the haulms may be cut off before digging. When pod moisture content has fallen to 20 to 25 percent the groundnut combine can be used on the windrows.
Groundnuts required for seed purpose must be handled with greater care than that permitted by normal commercial operations. When groundnut is a major cash crop in the areas, crops should be grown specifically to provide seed. As the extent of mechanical damage and its effect on germination and seedling vigour are not always appreciated, damage to kernels may not necessarily be obvious to visual inspection. Hand-harvested seed can give twice the final stand and twice the yield of mechanically harvested seed. Many countries now have central crop marketing boards and these bodies are able to use the production of pure and improved seed as a method of increasing yields and maintaining standards.
Platforms
Platforms of various heights may be built to raise the plants off the ground during curing and so reduce moisture damage in the bottom layer of pods and avoid damage by the cattle also. In Gambia stacks of groundnut plants are often put straight on to raised platforms immediately after harvesting. In Guinea after curing the plants in small stacks, local farmers sometimes put plants to dry in larger stacks on raised platforms.
Racks
The curing of pods on racks has been referred to in a number of countries. In North Rhodesia to prevent termite damage to groundnuts during the curing period, a horizontal rack is used. The rack consisted of crossed pieces of local wood 36 inches long, 18 inches apart and rose 18 inches off the ground. Plants hung on the rack are protected from termites and could be arranged so that the nuts are shaded from the direct rays of the sun. The moisture content of pods on the rack comes down from 21 percent to 6 percent during the first 7 days of curing.
On ground surface
Drying of pods by spreading them in a thin layer on the soil or woven matting or tarpaulin material is a common practice in many parts of India and Africa (Figure 17). In Uganda where harvesting occurs largely in the wet season a period of four to six weeks is given as the probable time taken for pods to dry to about 10 percent moisture content. In India, the threshed pods being spread out in the farmer’s threshing yard on a hardened mud, cement, or stone slab floor. A layer 1 and 1/2 inches deep needs no string, while one 3 inches deep needs stirring on alternate day. Two major disadvantages were noticed in drying the pods by leaving them spread in a layer on the ground or on areas of concrete or on matting, etc. Initially, there is the problem of moisture in the ground in contact with the pods together with restricted air movement within the produce. The second difficulty is the time and effort required to gather the pods together, cover them during rainshowers and re-spreading the pods as soon as possible to continue drying. A new type of plastic sundryer, which help the farmers to overcome these difficulties, has been designed in the Tropical Stored Produced Liaison Department (Pest Infestation Laboratory) and known as Allegate sundryer.
Figure 17:Thin layer drying of groundnut pods in the courtyard of farmhouse and in the field on polyethylene sheet. A child is playing with the harvested crop in his courtyard.
Trays
In some countries farmers are encouraged to spread their produce on trays, which they leave exposed to sun-drying during the day and shifts into the house at night. In Uganda for example trays, which hold one hundred kilograms of produce consist of a metal mesh base and wooden sides with handles at both ends. These trays can be raised off the ground by supporting the four corners on sticks. The Government subsidizes these drying trays to the farmers.
Platforms
Well-cured pods after removal from the plants are practiced to heap on platforms to complete drying. Very often the pods are left on such platforms for an indefinite period of time and may, in some cases, even be stored there.
In sacks
In cases where the moisture content of threshed unshelled and shelled pods is too high, the pods are sometimes bagged and every day the bags are brought out to the storeroom and left in the open. This is common practice at agricultural stations in Africa.
Racks
Several trials have been conducted on the natural drying of threshed groundnuts in bags in Australia. In the suspended bag trial, groundnuts of 30 percent initial moisture content in open weave bags were suspended vertically from a horizontal wooden rack supported at both ends by strong vertical posts. A galvanized iron roof provided protection from the rain. Staggered hanging of bags at centre distances of 22 inches, two bags rows deep is reported to have permitted safe drying of groundnuts from approximately 30 percent moisture content to safe storage moisture content in 10 days. As a result of this trial it was considered that in humid conditions, it might be necessary to use supplemental heat to achieve safe storage moisture content.
2.4.2 Artificial Methods
The artificial drying of groundnuts on a commercial scale has not been widely adopted, but research on the wide applicability of the artificial drying methods is still being conducted in many developing countries like India. Most of the experience in artificial drying of groundnuts has been gained in the United States of America and only a few experiments have been conducted from time to time in other part of the world. In Kenya work has been done on harvesting in which the plants are lifted, cleaned of soil and stripped of nuts in one operation. In Israel tests have been carried out in which groundnuts were threshed from windrows with mechanical harvesters and dried artificially. Intermittent drying of threshed groundnuts has been investigated and shown to reduce the total time of drying when compared with drying without ‘rest’ intervals. The theory is that by drying the shells and then allowing the nuts to ‘rest’ for some time, moisture will move from the kernel to the shells; the moisture differential between the shell and the drying air will thereby be greater and drying will consequently be more efficient. Investigations, however, on the danger of internal damage to the kernel and economics of such methods using a drier which is idle during the ‘rest’ periods are required. In general, attempts to dry green (uncured) pods on the plant have given poor results in relation to the quality of both the kernels and the haulms. The total moisture percent was such that if drying was at a moderate rate, moulding quickly occurred and if drying was at a fast, quite considerable breakage of the kernels resulted. Unsatisfactory results were also obtained when the haulms were clipped at various periods before curing (Teter, 1954). It has been noticed that an initial period of curing which reduces moisture from about 50 to 60 percent to about 25 percent is necessary, if good quality groundnuts are to be produced by subsequent artificial drying. After partial windrow curing, groundnuts on the haulms have been successfully dried artificially in the United States of America. The best results have been obtained using a temperature of 27 to 32°C and an airflow rate of 10 to 12 c.f.m.cu-1 ft and discontinuing drying when moisture content reaches below 7 percent.
Batch dryers
These consist briefly of a fan and heater unit (oil furnace) connected by suitable ducting to an air chamber, which forms the bottom part of a platform, tray, bin or silo. The threshed groundnuts are contained above the air chamber and the drier is so designed as to ensure that the heated air has to pass through the groundnuts in escaping into the atmosphere. To avoid the possibility of tainting (off flavours) from products of combustion an indirectly fired heater is normally used. In Tanzania test were carried out using a 10-aperture platform or sack drier with uncured groundnuts at about 48 percent moisture content. It was found that in practice using a single layer of sacks and air at 54°C moving at 110 cu.ft per minute (c.f.m) per sack the moisture contend would be reduced from 48 to 8 percent in about 16 hours. During these tests it was also found that the maximum temperature should be limited to 54°C since at 60°C a distinctive smell appeared which may be associated with some form of deterioration. Further drying of nuts at the higher temperature i.e. above 38°C renders the seed unsuitable for the sowings due to loss of viability.
In bulk
Two experimental tray dryers, used for drying cured groundnuts in-shell in Northern Nigeria, consisted basically of a rectangular pit, the laterite soil used to build walls bounding the pit and supporting the drying floor and forming the plenum chamber. Both dryers are of the heat exchange type the wood fuel in oil drums is placed in the pit, being completely gas tight and heating air in the plenum chamber which rise through the drying floor; one drier used the forced draught. One drier takes a maximum charge of 1 600 lb and other 2 600 lb wet kernels in-shell. The need of this artificial drier for groundnuts arose from the fact that the Mokwa and also in the Riverian area of Nigeria nuts are harvested at the wettest time of the year. In Tanzania using an experimental tray drier with a slatted floor, it was concluded that a two ft depth of freshly harvested (threshed) pods could be dried from 48 to 8 percent moisture content in about 42 hours using air at 38 °C at the rate of 55 c.f.m sq-1. ft (27.5 c.f.m cu-1 ft). The output of dry pods in shell using a two ft depth would be about 2 000 lb per 10sq.ft drier floor area in a six-day week. It was emphasized that the drying of wet pods stored in bulk should be started as soon as possible. This work in Tanzania led to design East African Industrial Research Organization to simple twin tray drier capable of dealing with about 75 tonnes of dry pods in shell in a drying season of 33 days. A fan, driven by a diesel engine, blows air heated to 38°C under the floor of perforated metal trays. Each tray is loaded to a depth of about two ft. and drying is completed in about 45 hours. Also in Alabama shelled nuts were satisfactorily dried in tray drier from 11.8 to 7.1 percent moisture content in 5.5 hours at 39°C; the fan was operated for an additional three hours to cool the nuts (Blatchford and Hall, 1963).
Tests were conducted in Israel on the artificial drying of groundnuts in three rectangular wooden bins, 2 m high with a slatted and wire netting floor. Air for drying was supplied by a fan to a plenum chamber below the bins and additional heat could be supplied to the air stream by indirect heating. These tests showed that there was no decrease in the quality of the product if it was dried in bulk in layers of 1.8 m high at an air velocity of about 0.5 m per second (m sec-1) which is equivalent to 98.4 ft per minute (f.p.m.) and an air temperature less than 40°C. Artificial drying had no influence on the colour and appearance of the shells or on the germination of seed. A bin-type drier used in Alabama to dry rain-soaked nuts consisted of an underground air duct and drying bins placed over vents in the air duct. The drying temperature was 46°C and the results were satisfactory (Smyth 1959).
Since work on drying of freshly harvested groundnuts has not been conducted much in the developing countries, due to resource constraint, the work conducted in Australia and the USA is described below. Before implementing the techniques various additions and modifications would be necessary based on the local environmental conditions and drying requirements. During experimental work in Georgia the moisture content of freshly harvested groundnuts on haulms was reduced from 35 to 8 percent in two weeks by spreading the plants in layers in what is described as a barn drier consisting of ducting through which air is blown. Fair results were obtained with this method and the pods and hay maintained very high quality. However, the enormous amount of space required has rendered this method impracticable for drying groundnuts. Column dryers consist of tall towers, the diameter being small in comparison with the height; the floor of the drier is constructed of ducting through which the air is blown. Dryers of this type have been used experimentally to dry groundnuts.
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