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Preface

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GASGA-the Group for Assistance with Systems relating to Grain After-harvest-is a voluntary association of organizations primarily linked with donor operations.

These organizations all have major involvement in most, if not all, of the following:

The association is essentially technical; it is international in character, but informal and limited in membership, so that its deliberations, aimed at the specific objectives indicated below, can take place readily.

GASGA consists of the following organizations:

GASGA aims to stimulate improvement in the technical help given to developing countries in the postharvest handling, processing, storage and transport of grain, and to harmonise activities so that the most effective use is made of members' resources. GASGA seeks to identify and suggest ways of meeting needs for research, development, training and information in this subject field, in the light of existing or planned operations by GASGA members and other organizations.

The Group is also prepared to answer requests for technical advice put to it by developing countries.

GASGA also seeks to facilitate the appropriate dissemination of information about technical developments and activities in the postharvest sector to donors, developing countries, and other interested organizations. The last group includes, for instance, the International Agricultural Research Centres whose commodity-oriented preharvest programs need links with postharvest activities and requirements.

The GASGA Executive meets annually to review progress in its activities and discuss proposals for future work

Since the 19th Executive Meeting, held at Feldafing, West Germany, a technical seminar has been held in association with the annual meetings and the papers presented at the seminar published in the GASGA Executive Seminar Series.

This volume, the second in the series, publishes the papers presented at a seminar during the 20th GASGA Executive Meeting, held at ACIAR Headquarters in Canberra from 31 August-1 September 1988.


Overview of grain drying and storage problems in India

B.D.Shukla

Project Coordinator,
AICRP on Post Harvest Technology,
Central Institute of Agricultural Engineering,
Bhopal-462 018, India and

R.T. Patil

Scientist, Central Institute of Agricultural Engineering,
Bhopal 462018, India

Abstract

India produces about 150 million tonnes of food grain per year and production is rising due to higher cropping intensities and the introduction of high-yielding varieties. However, annual post production losses have remained static at about 10%, which means that about 15 million tonnes of food grains are lost during harvesting, threshing, and storage. Storage losses amount to 6%.

Drying is an important operation that can preserve grain and lower losses during storage. In India, dryers are used mainly in grain processing industries, such as in rice and pulse mills. Some dryers are being used in modern drying-cum-storage complexes. However, 70% of the grain stored is sun dried. The reasons for non-use of dryers at farmer level are: unawareness of the importance of Brain drying; non-availability of dryers within their reach; high initial capital investment required; and lack of incentive for properly dried grain. Establishing drying-cum-storage complexes has been suggested as a possible solution. The use of dryers at rural-level food industries has also been targeted and units suitable for this level of operations are identified and described.

The proper scientific storage of well dried grain is also very important to reduce losses during storage. The grain is stored at farmers, traders and organizational levels by cooperatives and government departments engaged in public distribution of grain. Suitable storage structures at each stage are identified and described. The most promising among them are a coal tar bin developed at CIAE and low-density polyethylene-embedded bin for on-farm grain storage. A new type of multipurpose dryer developed at CIAE has been reported where, by changing the trays, the same drying chamber can be used for drying both grain and food products.

Introduction

India produces about 150 million tonnes of food grains per year. The major components of production are 47 million tonnes of wheat, 64 million tonnes of rice, and 13 million tonnes of pulses (Anon. 1987). Due to technological advances in agriculture and the introduction of high-yielding varieties, this may increase. From this production, an average 10% is lost during postharvest operations between the field and the consume. This means that about 15 million tonnes of food grain, valued at about $A240 million (Indian Rupees 2400 million) goes to waste. The major share of the loss occurs during storage of surplus stock. Among the various causes of losses, the most important one is improper drying before storage.

The preservation of agricultural produce by drying is a long-established technique. Sun drying in the open, on mud-plastered or concrete floors, is the conventional method of drying grain and also cash crops like chillies, and plantation and horticultural crops. The drying time required in the open sun for these crops ranges from 5 to 45 days depending upon the crop to be dried. Unfavourable weather conditions are likely to occur during the drying period and degradation in quality of the final produce therefore becomes unavoidable.

It is well-known that deterioration in quality caused by improper drying cannot be eliminated until improved drying systems based on mechanical dryers have been adopted. However, for many reasons, these systems have not been adopted. The main reason that is encountered is a lack of organizational or government incentive to the farmer to deliver a quality product that might command a premium price. This results in not only a negative attitude, but also leads to the overall quality of the product gathered at market points being alarmingly poor.

A second important reason for not using dryers is their high initial costs. Most of the commercially available dryers are designed to suit the needs of the processing industry and their output capacity is therefore far above the needs of individuals, or even of farmer groups.

An awareness of availability of dryers and of their use and advantages in drying food grain for better storage and marketing is lacking among crop growers. The main reason for this is inadequate extension programs. So far, extension agencies have concentrated on increasing production. The time has now come to see that grain saved is equivalent to grain produced. High technology has led to production targets being achieved, but much less attention has been given to minimising losses which have remained constant since the beginning of the 'Green Revolution'. Annual postproduction losses by crop in India, expressed as a percentage of total production, are estimated to be as follows: wheat, 8%; paddy, 11%; pulses, 9.5%; and all food grains, 9.3%.

This paper describes the use of various types of dryers in the Indian food industry and the efforts of research and development organizations to devise dryers suitable for individuals or small groups in the rural population. However, even with properly dried grain, scientific storage remains important and recent advances in developing various storage structures are also described.

Commercial use of dryers

Dryers are used extensively in grain processing industries such as rice milling, pulse milling, and oil extraction. Here the need for dryers has been realised not only for proper storage of stock but also for timeliness of subsequent operations where wetting of grain and redrying are involved.

In the case of the rice milling industry, parboiling of rice is a common practice. The population of the coastal belt of the country consumes parboiled rice and about 70% of production is processed in this manner. The paddy is soaked in water for variable lengths of time depending on the process used and is then steaming.

High moisture content (m.c.) paddy is dried to 12-14% m.c. for milling. There are about 100 000 rice mills with a total installed capacity of about 40 000 tonnes of paddy per hour. About 30000 dryers of 1-2 t/h drying capacity are in use in the industry. The most commonly used dryer is the LSU type in which air is heated for drying by burning rice hulls. The steam requirement for drying is 21.3% of the total steam produced in the boiler, which consumes about 4% of the total power required by the mill. In this type of dryer, the grain is tempered for 30 minutes after every hour of drying so to equilibrate the moisture and avoid stress cracking in further milling operations.

Dryers are also used in the pulse milling industry. Here both LSU-type and flat-bed perforated-floor dryers are wed, ranging in capacity from 1-4 t/h. There are about 4000 pulse mills in India having an average processing capacity of 10-20 t/day. Dryers are required in industry for the following reasons:

In the LSU-type dryer, steam is used as the heating source, while in the flat-bed dryers, light diesel oil is wed, fuel consumption being 7-12 l/h One such commercial dryer is shown in Figure 1. It consists of a drying tray or platform, an oil-fired burner, and an air blower. The cost of a 3 t/h dryer is about SA13 500 and the drying cost has been reported as $1A/t (Thermax India Ltd. personal communication 1988).

Use of dryers at farmer level or in community drying systems

About 70% of total grain production in India is retained at farmer level: only 30% is sold on the open market. This means that a sizeable quantity of about 105 million tonnes is kept by farmers. The losses here, though perhaps not felt by individuals, on a collective basis have a substantial impact on the country. It is therefore essential that drying technology be disseminated to this group which is a major custodian of the nation's grain.

To promote the use of dryers in rural areas, the concept of a community drying-cum-storage system was put forward by T.P. Ojha in 1984. He suggested that changes in ecological balances and the introduction of high- yielding varieties of field crops necessitated the use of mechanical dryers and other devices to protect the food grains from spoilage due to untimely rains.

If rain-soaked food grains are not dried properly, farmers have to sell their excess stocks at low prices to meet urgent financial needs. A community drying-cum-storage centre would therefore serve them well by way of protecting the grain from spoilage and also by advancing temporary loans on their grain deposits. As soon as grain prices stabilise, stocks can be sold and payments can be made to the farmers after deducting dues such as rental and service charges, loans paid, and interest on advances. Such a system would no doubt benefit farmers. They would not be required to make forced sales of their produce and, as a result, storage losses would be minimised.

For such complexes, selection of a dryer of the correct design is very important. The largecapacity dryers used in grain-processing industries are not economical or feasible for most farmer groups. In India, the average village has a population of about 1000 and the small amount of surplus grain available for drying at this level suits dryers of 2-4 t/day capacity operating for 60 days per year. In India, many research organizations have developed, or are currently developing dryers for village groups, but so far with little success.

The main considerations for selection of a grain dryer suited to this level are:

Grain dryers suitable for rural level use

Some important grain dryers developed at different R&D institutions and agricultural universities are described in the following sections.

Small-Capacity, Continuous Grain Dryer

Fig. 1. Commercial batch dryer.

The dryer developed at G.B. Pant University of Agricultural and Technology, Pantnagar, is a continuous type consisting of a frame, grain column, plenum chamber, feed hopper, discharge hopper, heating unit, and blower (Fig. 2). The grain column consists of two vertical columns sandwiched between two vertical screens. Each screen wall consists of an expanded metal netting and wire mesh screen on the inner and outer sides. The plenum chamber has been provided between the two grain columns in order to distribute the air uniformly and at right angles to the direction of grain movement. A centrifugal blower forces the air at 37 m3/min airflow against 2.5 cm of Hg. The air is heated from 40°C to 70°C by 20 kW electric heaters. The dryer has capacities of 0.8 t/h for wheat, 0.8 Ah for paddy, 0.96 t/h for maize, and 0.8 t/h for red gram. The cost of the dryer, excluding the heater and fan, is about $A330 (Indian Rupees 3300), based on 1982 estimates, and the cost of drying is around $A0.50/t (Singh et al. 1982).

Fig 2. Continuous grain dryer (dimensions are in mm)

Cup and Cone Dryer for Paddy Drying

A somewhat different design of dryer has been tested at the Paddy Processing Research Centre (PPRC) at Thiruwarur in India. It consists (Fig. 3) of five cups each having a diameter of 1070 mm and a slope of 52°. Each cup and cone is made of mild steel rod and wire mesh. A hot-air duct of 300 mm diameter passes through the centre of the dryer from the top; the bottom end of the duct is closed.

Individual cups and cones are mounted one above the other by means of bolts and nuts. Paddy passes through the inner surfaces of the cups and exteriors of the cones. To divert the flow of paddy from the outer surfaces of the cones to the inner surfaces of the cups, cylindrical retainers of mild steel sheet are provided. A two-way valve is provided just beneath the bottom of the dryer. A slide is provided at the junction of the two-way valve and the bottom of the cup. This is kept completely open during recirculation and bagging. The hot air generated in a husk-fired furnace passes through a vertical duct and enters the paddy column by means of a central duct having perforations at points covered by cones and the top portion. A circulation rate of 3.5 to 4.5 tonnes per hour is maintained while drying.

Trials conducted during high humidity weather indicated that 950 kg of parboiled paddy of 30% m.c. can be dried to 14% in 2 hours at a drying temperature of 120°c and an airflow rate of 127.5 m3/min. The drying cost is calculated at $A2.10(Rs 21.33)/t (Pillaiyar et al. 1982).

Fig. 3. Cup and cone paddy.

IGSI Batch Dryer

A batch dryer of 1 tonne capacity has been developed at the Indian Grain Storage Institute (IGSI), Hapur (Fig. 4). The unit consists of an electric fan, semi-fluidised bed furnace firing agricultural waste, and three vertical columns (250 mm each) in a paralleloid drying chamber. The drying chamber has provision for manual tilting. The dryer can accommodate 1.5 t of paddy per batch which takes 4 hours to dry. A tempering time of 20-30 minutes is incorporated between drying operations. The cost of the dryer is about $A1350 (Rs 13500) and the operating cost about $A3.20 (Rs 32)/t of paddy (G. Shankar, personal communication 1988).

Community Grain Dryer

The dryer developed at the Central Rice Research Institute (CRRI), Cuttack, is useful for village communities or small-scale rice millers (Fig. 5). It consists of a drying chamber, a husk-fired furnace (inclined grate 0.5 m² at 45° inclination, horizontal revolving grate 0.15 m², and fluted roller-type husk feeding mechanism), solar collector (flat plate, black painted galvanised iron corrugated sheet with 23° slope towards south, 43.5 m² surface area provided with 40 mm thick insulation of paddy straw and tar-felt sheet combination), blower {backward curved fan operated by a 5 hp motor with airflow capacity of 160 m³/min at 25 mm water gauge static pressure), and a bucket elevator (2 t/h capacity operated by a 0.5 hp electric motor). Generally, it takes 6.5 hours to dry 1 t of paddy from 24% to 1496 m.c. The cost of the dryer is estimated at $A5000 and the cost of drying $A10.6/t (Kachru et al. 1986).

Fig. 4. IGSI 1 tonne batch dryer.

Solar cum Husk-Fired Paddy Dryer

This dryer has been developed at the Indian Institute of Technology (IIT.), Kharagpur (Fig. 6). The system consists of an unglazed flatplate collector which houses an inclined-type husk-fired grated furnace, a 3 hp electric blower and a batch-type dryer. Dampers are provided to put either the collector or the furnace into operation, depending on weather conditions. The absorber surface is a corrugated galvanised iron sheet coated with ordinary blackboard paint. A false roof of bamboo functions as an insulator. Two sides are closed to form channels for airflow. The blower assembly forces the heated air to pass it onto a batch dryer. The capacity of the dryer is I t/day. The cost of the dryer has been estimated at $A2000 and drying cost at $A8.8 /t (Kachru et al. 1986).

Recirculating Batch Dryer

A batch-type recirculating grain dryer has also been developed at IIT, Kharagpur. The unit consists of a dryer and a husk-fired furnace (Pig. 7). The dryer has two concentric perforated cylinders (the inner cylinder has a diameter of 900 mm and a height of 1500 mm and the outer cylinder has a diameter of 1350 mm and a height of 2250 mm), a 6000 mm-high bucket elevator with a capacity of 2.5 t/h, and a fan of 85 m³/min capacity at a static pressure of 5 cm of water.

The husk-fired furnace consists of a hopper, a combustion chamber with inclined grate, and a curtain wall. It can burn 20 kg of paddy husk per hour. The fan draws air through the combustion zone of the furnace and blows the hot air to the inside chamber which acts as a plenum chamber. Air from this chamber travels through the paddy column and the outside cylinder. Paddy is fed in at the top of the inside cylinder and comes in contact with hot air while flowing downwards between the two cylinders. The feed rate of paddy is controlled by closing or opening the gate provided to the discharge hopper of the dryer. Paddy is circulated till the moisture content falls to 14%. The capacity of the dryer is 1.25 t/batch. The cost of the dryer is $A4000 and the drying cost $A4.5/t (Kachru a al. 1986).

Dryers for cottage-level industries

There is plenty of scope for setting up cottage-level food industries in the villages in developing countries. Recent trends indicate that the rural population is migrating to urban areas at an alarming rate in search of jobs. Due to the immediate sale of food grains or crops produced, the primary as well as secondary processing operations are in the hands of middle-men. This has created a social problem in rural areas.

Fig. 5. Community grain dryer.

Recently, the Government of India created a separate department to promote food processing industries at the rural level. Only some two centuries ago Indian villages were self sufficient, not only in food grains but also in value-added commodities based on those crops. Since the produce was processed at rural level, gainful employment was also available in rural areas. This discussion may appear irrelevant to the topic, but there is tremendous scope in developing agro-processing industries at the rural level. This would greatly reduce the present problem of unemployment in the country.

Pulse processing, rice processing, producing badi-like products from pulses (a cooked and dried product from pulses which is used in India for making curries), producing snacks like chakali, kurdaya, papad, etc. from pulses and cereals are all operations which can be done in villages at the domestic as well as collective level. Thus, there can be a viable processing industry in villages due to the labour-intensive nature of this work. Most of these operations are of the wet-processing type, where raw material is soaked either at the start or at an intermediate stage. However, where this is currently done, drying in the open is used because of ease of operation and also because it is the most economical method. Even fruit and vegetable drying is profitable at the grower level provided a proper drying system is available. The main considerations for adoption of dryers for these industries are:

Many R&D organisations in India have developed dryers suited for the above purposes but, for various reasons, they are not being used

Two types of dryers solar and mechanical) have been developed to meet the requirements of agro-industries:

Fig. 6. Solar cum husk-fired paddy dryer.

Solar Dryers

As India is a tropical country, solar energy is abundant. Broadly speaking, there are three main types of solar dryers:

· Direct type: the material to be dried is placed in an enclosure with a transparent cover and side panel. Heat is generated by radiation absorption on the product itself as well as on the internal surface of the drying chamber. These dryers are used for crops which are not sensitive to sunlight.

· Indirect type: solar radiation is not directly incident on the material to be dried. Air is heated in a solar collector and then inducted to the drying chamber.

· Mixed type: the combined action of radiation incident on the material to be dried and air preheated in the solar air heaters provides the heat required to complete the drying operation. These dryers are used for materials which are not sensitive to light, and are quicker drying and have higher capacity ranges than the direct type of dryer.

Some promising solar dryers developed in India that can be used for food processing industries at cottage level are described in the next part of this paper.

Fig. 7. Recirculating batch dryer (dimensions in mm.)

CIAE Solar Cabinet Dryer

The CIAE solar cabinet dryer is a tray-type natural-convection dryer which can dry 30-50 kg per batch. The dryer can be constructed from materials such as wood, glass, plywood, wire mesh, and mild steel sheeting. It is simple in design and does not require any mechanical prime mover (Fig. 8). The dryer has been found suitable for drying chillies and potato chips. The cost of the dryer is $A150 and the cost of drying $A94/t (Kachru et al. 19863.

CPCRI Solar Dryer

This is a cabinet-type dryer developed at the Central Plantation Crops Research Institute (CPCRI), Kasaragod. The frame of the dryer is made of wood. The drying surface (1.06 m²) is made of 22 gauge corrugated galvanised iron sheet to give an absorber area of 1.19 m². The surface is inclined at an angle of 12.5° and is fitted on a 30 mm-thick wooden board with a 2 kg coir-fibre insulation between. The drying chamber is made up of 3 mm window glass on the sides, and 3 mm acrylic plastic on top to reduce the risk of breakage during handling.

The drying chamber is uniformly 40 cm high from the inlet to the exhaust end. The inlet and exhaust are covered with wire mesh. Commercial-grade aluminium foil mounted on hardboard is used for reflectors on three sides. The inlet opening is covered with a blackpainted galvanised iron sheet hood inclined at 45°. Castor wheels are provided for changing the direction and for moving the dryer short distances. The galvanized iron rod on the top is used to determine the direction for sun tracking (Fig. 9). The cost of the dryer is $A220 and it can dry the material in half the time it would take using open drying, with double the spreading capacity. The dryer has been found suitable for drying copra, arecanut, and black pepper (Patil 1984).

Fig. 8. Solar cabinet dryer (CIAE) (dimensions in mm)

Fig. 9. CPRCI dryer (dimensions in mm)

Mechanical Dryers

Though solar dryers are economical, their use is limited. They are entirely weatherdependent, and the volume of the structure limits the quantity that can be handled. Solar dryers are less flexible when there is a range of crops to be dried. For these reasons, there is a need for a dryer that is low in cost, simple to operate, and uses locally available sources of fuel. The tray-type dryers are the most suitable for such types d operations. The commercially available tray dryers used in large food industries are not suitable for cottage industries because of their high capital and operating costs.

Some low-cost mechanical tray dryers developed by R&D organizations in India which have the potential to strengthen the agro-food industries are described in the following sections.

Mechanical Copra Dryer

This dryer (Fig. 10) has been developed at the Central Plantation Crops Research Institute (CPCRI), Kasaragod. The dryer can be made from materials such as wood, galvanised iron sheeting, AC sheet, asbestos rope, and mild steel sheet. The dryer consists of drying chamber, air distribution unit, plenum chamber, heating unit, and blower. The drying chamber has an air distribution unit in the centre with copra trays on both sides. The heating unit has 8 kW air heaters and the blower is equipped with a 1.5 hp motor. The hot air is distributed over the trays by the air distribution unit and the exit of the air is provided such that the hot air then passes through the material to be dried. This ensures uniform mixing of air and material being dried. The capacity of the dryer is 1000 nuts/batch and drying time is 30 h. The cost of the dryer is $A1200 and the operating cost $A65.3/t (Patil and Singh 1983).

Fig. 10 Mechanical copra dryer developed by CPCRI

Fig. 11 Natural convection food dryer for soybean (dimensions in mm).

Natural Convection Tray Dryer for Soybean

A tray dryer using agricultural waste as fuel and working on the principle of natural convection of hot air currents has been developed at CIAE (Patil and Shukla 1988). The dryer is made in two parts (Fig. 12). The bottom portion (i.e. plenum chamber) is made of mild steel angle frame and is covered with asbestos sheet on the sides and wire mesh on top. A drum-type, combustion heat-exchange unit with fins for effective heat transfer is located at the centre of this chamber. A chimney with a regulator valve has been provided at the other end to allow the smoke to escape.

A drying chamber made of softwood and plywood which can hold 20 trays is kept on the plenum chamber. The bottom of the drying chamber which rests on the plenum chamber is open and the top portion is provided with an air vent with an adjustable opening. Fuel is burned in the burning chamber at the rate of 3 kg/h. The dryer can take 100 kg of wet material. The average temperature in the dryer at this rate of combustion is 49°C at no load, and 46°C while drying. The trays are interchanged and the material is mixed/stirred at one-hour intervals. The drying time for soy split to reduce moisture content from 60% to 10% was 15 hours. To dry flakes from 30% to 10% required 6 hours. The cost of the dryer is about $A570 and the cost of drying $A25/t.

A new approach to developing a dryer for cottage level food industry

Given a proper sized, low-cost dryer, food processing can proceed uninterrupted in rural areas. At the village level, an industry to process surplus crops into acceptable and marketable food items needs a dryer for two kinds of operations.

Firstly, to dry the grain to maintain its storability until it is processed, and secondly to dry the finished product. Grain drying requires a dryer that can facilitate better mixing and contact between grain and air.

For food-product drying, drying in thin layers on trays is preferable. To have two different dryers for these operations is not feasible due to the limited space available and, in any case, would be uneconomical.

The plenum chamber and heating unit are generally common to all the dryers whether they are used for food or grain drying. Only drying chamber design varies according to the type of material to be handled. The drying performance of the dryer is then decided on the optimum quantity of material processed and required air characteristics. By making use of this fact a simple but unique dryer has been developed at CIAE Bhopal.

The dryer has a 2 hp fan, a 10 kW electric heating unit, and a drying chamber as shown in Figure 12. A trapezoidal plenum chamber is located in front of the drying chamber to ensure uniform mixing of hot air. The drying chamber is a simple short tunnel with panelling on both sides, insulated with rockwool. Mild steel angles are provided on the panels as runners for wire mesh trays or baffled trays.

When baffled trays are put on the runners, it forms a 200 kg holding capacity LSU-type dryer for drying food grain in small cottagelevel pulse processing or soy processing plant. When the dryer is needed to dry food products made after processing of raw material such as wet, scratched pulses, soy snacks, blanched soybean splits to make soyflour and soybean flakes, trays with plywood sides and wire mesh bases are used. This makes a tray dryer suitable for drying 100 kg of wet or soaked material per batch. The wooden trays are kept open on one side and closed on the other allowing variation in the drying chamber as required by the particular material being processed. If all the open portions are on the heater side, then the material is exposed to more air. If other combinations of open ends are used, the quantity of air entering and coming in contact with the material and its residence time can also be varied.

The dryer has been successfully tested for drying soybean grain and also blanched soydal or soysplits. It can dry 200 kg of soybean grain from 2496 m.c. wet basis to 10% in 4 hours.

Intermittent removal of dried grain raises the capacity of the dryer to 500 kg per day. When used as a tray dryer, it can dry 100 kg of blanched soysplits in 6-8 hours with intermittent mixing of materials. The cost of the dryer is about $A800 (Rs 8000), including a 2 hp blower.


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