Seed Processing for Quality Upgradation

Abstract

Pre-harvest and post-harvest operations are integral parts of the quality seed production system. If due attention is not paid during the first phase, probably therewill not be any means left to obtain quality seed. Similarly, good quality seed produced with utmost care may lose much of its value if proper management is not followed during the post-harvest period. The seed quality attributes are genetic purity, appearance and physical purity, germination potential, vigour and seed health. Proper attention should be given during both the phases of seed production, regarding all the attributes of seed quality. Genetic purity is primarily linked with production or pre-harvest phase. Whereas, physical purity is primarily achieved in post-harvest phase. The harvested raw seed mass consists of various materials other than seed (MOS). MOS may be trash, plant parts, inert matter, weeds, other crop seeds and non-viable seeds, e.g. cut grains, insect pest damaged, off sized or physically damaged kernels, etc. Such contaminants reduce seed quality and increase the volume of harvested seed mass that requires additional space during storage. Moreover, these contaminants often attract pest infestation, which can further cause seed loss both in terms of quality and quantity. Proper storage of processed and clean seed is also an essential component of seed programmes as the sowing season normally falls at least six to nine months after the production season.

1 Introduction

Soundness, well-filled size and uniformity of physical attributes of seed (e.g. size, shape, density, colour, appendages) are significantly linked with its quality, as well as the ease of mechanical planting and for better crop establishment. Seed processing is an effective and efficient process to upgrade the quality of harvested seed and prepare it for safe storage until used. The seed processing has to be carried out with a goal to enhance seed lot quality by exploiting the physical attribute of seed as well as constituents of harvested seed mass. The three basic processes, conditioning, grading and application of protectants or enriching with nutrients, are accomplished during seed processing, following standard principles and procedures (McDonald and Copeland 1997; Agrawal 1996; Jorgensen and Stevens 2004). Operational modifications and machine adjustments are made as per the specificities of the crops and prevailing conditions.

2 Conditioning

The conditioning enables the raw seed lots to be handled safely in subsequent operations in seed processing and harnessing maximum efficiency of implied machines. It may comprise of moisture conditioning, removal of appendages, removal of major dockage or a combination of these in order to make the lot free flowing and without any mechanical abuses in further processing operations. In general, moisture content ~10% is considered safe for mechanical processing. Too low or too high moisture content may induce mechanical injury especially in pulses. Thus, drying or humidification of seed lots are recommended seed conditioning operations for maintaining safe moisture content before further processing operations.

2.1 Moisture Conditioning

It may require drying to reduce seed moisture content (if too high) or humidification to increase moisture content (if too low), with the ultimate aim to prepare the raw seed lots suitable for safe mechanical handling.

3 Seed Drying

The process of decreasing the seed moisture content to safe moisture limits is called ‘seed drying’, which is an important operation, as.

• it permits early harvest and reduces losses due to bad weather

• it is an aid for efficient and economical labour management

• it reduces the mechanical damage during handling and processing

• it reduces loss in seed quality and quantity during storage

The moisture in seed is distributed in two ways. The first is surface moisture and second one is internal moisture. Hence, during seed drying, movement of moisture is also in two ways, i.e. transfer of moisture from seed surface to air around the seed, and movement of moisture from the inner parts of the seed to the seed surface. The drying is controlled by: (i) air flow rate, (ii) drying temperature, (iii) drying rate and (iv) drying time.

The transfer of moisture from the seed surface to surrounding air is a function of the gradient in vapour pressure between the seed surface and the surrounding air. Free air movement is essential to progress the drying operation. Drying rate is a function of air movement. As the air flow increases, the drying rate increases up to a point at which the air absorbs all the moisture that is available to it. The required minimum air flow rates for efficient drying are given below:

Drying with non-heated air		Drying with heated air
Moisture content (%)	Air flow rate m3/min per m3 seed	Seed type	Air flow rate m3/min per m3 seed
25	7.5	Light	2.8
22	6.0	Heavy	4.6
18	4.2
15	2.8

The vapour pressure gradient between the seed surface moisture and the air can be increased by increasing the temperature of the air. The higher air temperature can dry seeds more rapidly to low moisture levels. But the high temperature can cause heat injury to the seed. In case high moisture content seed is dried with high temperature air, the seed viability deteriorates significantly. Hence, the seed cannot be dried safely with high temperature air, especially when seed moisture is high. In general, seed drying temperature should not be higher than 43 °C. The critical temperature for drying varies with the moisture content of the seed. Hence, it is advisable to dry the seed depending on its moisture level.

Seed moisture content (%)	Drying temperature (°C)
Over 18	32
10–18	37
Under 10	43

1. (Sinha et al. 2010)

Sometimes the situation may be such that the seed cannot be dried to the required level at safe drying temperature condition if the relative humidity (RH) of the air is high. In such situations, the drying air should be dehumidified chemically or by refrigeration. Certain chemicals, e.g. silica gel, calcium chloride, activated alumina, activated charcoal and anhydrous calcium sulphate, that have a strong affinity for moisture are used to dehumidify the air in dryers. These chemicals are reactivated in dehumidification system to sustain the drying process. An alternate method of dehumidifying air is to employ refrigeration to drop the air temperature below its dew point. The dehumidification process either raises or lowers the air temperature abruptly. In the case of chemical dehumidification, the temperature of air rises, while in refrigeration dehumidification, air temperature declines. In order to attain optimum drying rate, cooling or heating arrangements are used in combination with chemical and refrigeration methods of dehumidification, respectively.

Vacuum drying is another way to remove moisture from the seed. It is somewhat like drying with dehumidified air. When the total pressure in the vacuum drying chamber is lowered, the component of this pressure due to vapour is reduced proportionately and drying of the seed is attained. In this case, the heat for evaporation is supplied mainly by conduction or radiation.

Dehumidified air or vacuum drying is costlier than heated air drying, but these methods are useful to dry heat-sensitive seed as well as for safe drying of seed to very low moisture contents.

The removal of moisture from the seed surface to the surrounding, i.e. the first phase of drying, can be regulated by considering the above facts. But the movement of internal moisture to the surface, i.e. the second phase of drying, is more critical. The rate of moisture movement in the second phase should be at least equal to the first phase of drying. It is important to note that the second phase of drying greatly varies with the species and cultivar. If this phase is slow, the cultivar should be dried in multi-stages. Otherwise, cracking or splitting of the seed coat may occur, which will reduce the seed quality or destroy it completely.

The drying rate is an important aspect in seed drying. Though fast drying helps in controlling moulds, but if the rate is too fast, seed coats of some species (such as many legumes) tend to shrink or split and/or they may become impermeable to moisture (developing into hard seeds), especially in large-seeded legumes, even though the inner parts of the seed might still remain moist. Rapid-dried seeds with cracks and splits are more prone to disease infection and insect pest attacks. Slow drying, on the other hand, increases the chance of mould growth which lowers the seed quality significantly. Hence, an optimum drying rate is adjusted by applying the rule of thumb, so that about 0.3% of the moisture can be removed per hour with an air flow rate of 4 m³/min/m³ of seed at 43.5 °C. However, some adjustments are needed considering the kind of seed (structure, composition), temperature and initial moisture content. The rate will be less if the initial moisture content is low, and if the temperature of drying air is below 43.5 °C. The time required for drying a seed lot depends on its initial moisture content, final moisture content (MC) to be attained, drying rate, air flow rate, relative humidity (RH) and temperature of the drying air.

The seed drying systems are mainly of three types:

• 1. Batch drying system

• 2. Continuous drying system

• 3. Heated air-drying system

3.1 Batch Drying System

This is a system in which products are kept in a bin and the heated air or drying air is ventilated through them. The different types of batch drying are bin dryer, tray dryer and tunnel dryer.

3.1.1 Bin Dryer

This type of dryer is useful when a small quantity of seed is to be dried. The air may be pushed or pulled through the seed bed and may go from top to bottom or vice versa. The depth of the seed bin is limited because the power required to force air through the seed is proportional to the depth. In deep bins, the entering air picks moisture from the seed, cools and may deposit moisture on the last layer of seed before it is released out. This causes mould growth and results in deterioration of seed in this layer.

3.1.2 Tray Dryer

In a tray dryer, many small trays are kept one above the other with a gap in between the drying chamber. The trays normally have perforated bottoms for better air flow. Seed is kept in thin layers in the trays and these may be manually shifted to allow uniform drying in all trays. The gap between the stack of trays allows ventilation.

3.1.3 Tunnel Dryer

It is similar to tray dryer. The only difference is that the group of trays is kept moving in a tunnel. The flow of heated air in a tunnel dryer may be co-current or counter-current.

3.2 Continuous Drying System

This is a system in which the products are kept moving either by gravity or by some mechanical means and the drying air is ventilated through the moving column/layer of seed.

3.2.1 Rotary Dryer

Rotary dryers are fitted with a drum normally of 1–3 m diameter and 3–6 m length, which rotates on its axis. The product flows downward through the rotating drum and is periodically lifted by inclined flights, then dropped, ensuring good air contact. Such dryers are indirectly heated and are suitable for seeds with relatively low moisture content.

3.2.2 Column Dryer

Column dryers are usually for continuous flow of large lots of seed with two columns 20–35 cm thick. The seed flows through the air chamber in a solid column and is turned by baffles as it descends the column to allow uniform drying.

3.2.3 Belt Dryer

Seeds are spread in a thin layer over a perforated belt and the heated air is allowed to pass through the perforations. Such dryers are suitable for seeds with poor flowability.

3.2.4 Fluidized Bed Dryer

In this method of drying, products are dried under fluidized condition in a dryer. The seed is fluidized by the use of drying air with high velocity to cause suspension. In this process, higher rates of migration of moisture take place. Since every surface of the product is in contact with the drying air, uniform drying takes place. This method is normally used for seeds which have high moisture content and need to be dried quickly, such as vegetable seeds.

3.3 Heated Air Dryers

The drying air can be heated mainly in two ways.

3.3.1 Direct Fired Type

The combustion gas goes directly from burner into the drying air stream and then to the drying bin. Such driers are less safe because air burner may release particles of hot soot if it is not properly adjusted, and may cause carbon deposits on seeds. The thermal efficiency of this type of drier is higher than the indirect fired drier.

3.3.2 Indirect Fired Type

This type of dryer consists of the heat exchanger and the fire chamber. In the fire chamber, fuel is burned and heat is generated which heats the heat exchanger where drying air takes the heat. The heated air is forced into drying chamber by a blower (Fig. 1). The thermal efficiency is lower than the direct fired type, but is safe for seed drying.

Fig. 1

Batch type hot-air dryer. (Photograph courtesy, Seed Processing Unit, IARI Regional Station, Karnal, India)

4 Tempering

Besides drying, seeds of certain crops, e.g. soybean and peas, need tempering under very dry conditions to raise the moisture content to about 10%. Tempering is needed for crops which are sensitive to impact damage at a lower level of moisture content. With the help of tempering, the moisture level can be increased to a safe level of ~10–12%.

5 Removal of Appendages

Seeds containing awns, beards, hairs, glumes and other appendages tend to interlock and cause clustering. These should be removed for improving the flow properties, cleaning characteristics and quality of the seeds. The general machines used for this operation are hammer mill, debearder and pebble mill.

5.1 Hammer Mill

It is a thresher with hammer type beaters in closed cylinder casing, concave, a set of oscillating sieves and an aspirating blower. Seeds that can be processed successfully in hammer mill are grasses, e.g. bluebunch wheat-grass, blue wildrye, and species like tall oat grass, bulbous barley, squirrel tail (Sinha and Srivastava 2003) and species with similar kind of seeds. Mechanical processing of Pusa Basmati 1, a popular long-awned paddy variety, is problematic due to the entangling of its awns. In order to make the seed mass suitable for mechanical processing, de-awing is practised with the hammer mill at optimized operational parameters by adjusting the cylinder speed, screen opening and feed rate (Sinha et al. 2010).

5.2 Debearder

It has a horizontal beater with arms rotating inside a steel drum. The arms are pitched to move the seeds through the drum. Stationary posts, adjustable for clearance with the arms, protrude inward from the drum. The seeds are rubbed against the arms and also against each other. By regulating the discharge gate, the processing time can be regulated. The degree of action is determined by the processing time, beater clearance and beater speed. Larger capacity, simple operation and lesser damage to seeds are its advantages over the hammer mill.

This machine can be used to remove cotton webbings from Merion bluegrass, remove the clip seed from oat seed, debeard barley seed, thresh white caps in wheat, split grass seed clusters, remove awns, beards and hull from some grass seeds as well as polish the same to upgrade the quality.

5.3 Pebble Mill

It has a drum rotating about a shaft inserted off-centre at opposite ends. The mill is loaded with seeds and smooth, half-inch pebbles and turned at a slow speed until the rubbing action of the pebbles rolls the fuzz from the seeds into small, round balls. The mixture of pebbles, seeds and matted fuzz is then run over a scalper to remove the pebbles. These types of machines are much effective in removing seed hairs and fuzz, e.g. for removing the cobwebby hairs from bluegrass and similar seeds.

5.4 Scalper

Seeds from a thresher or combine brought to a cleaning plant may contain a larger amount of trash, leaves, weed seeds, cut grains and infected seeds. Because of these materials, seed mass cannot be handled efficiently in cleaning or separation machines. Its removal is generally accomplished by scalpers. In general, two types of scalpers are in use: (a) a reel of perforated metal screen, which is inclined slightly and turns on a central shaft and (b) seeds fed into the higher end tumble inside the reel until they drop through the perforations. However, longer trashes remain in the reel and are discharged separately. In small capacity plants, the scalper is combined with the pre-cleaner air screen machine.

5.5 Huller and Scarifier

The hull or seed coat of some seeds are too hard to permeate water during germination. Such types of seeds require removal of the hull/husk, or scarification of the seed coat to absorb water and sprout properly. Hullers or scarifiers usually abrade the seeds between two rough surfaces, such as sandpaper. The severity of the abrasion or impact must be controlled carefully to prevent damage.

Commercially available scarifiers scarify the seed by forcing them against an abrasive material such as carborundum. Either an air stream or a centrifugal force may be used to bring the seeds in contact with the abrasive surface. Some scarifiers use a rubber abrading surface instead of carborundum to prevent damage, especially for fragile seeds. Seeds with high moisture content are hard to undergo hulling or scarification, due to the damaging effects induced by a huller or scarifier upon such seeds.

Seeds with long viability after hulling and scarifying can be processed immediately after harvest and stored until the following season. Others that lose viability rapidly may be stored and be hulled and scarified only before the planting season. Hulling and scarification may be performed separately or jointly, depending on the presence of unhulled seed, hard seed or both. Seeds such as that of Bermuda grass, bahia grass, buffalo grass require only hulling, whereas seeds of wild winter peas, hairy indigo, alfalfa, crotalaria, subclover and suckling clover may require scarification only. On the other hand, some seeds may require both scarification and hulling such as sweet clover, Sericea lespedeza, crown vetch, black medic and sour clover (Sinha and Srivastava 2003).

6 Removal of Dockage

Occasionally the harvested seed mass possesses more than 20% dockages by volume. Such lots cannot be subjected to mechanical processing as it will reduce the processing machine efficiency with increased seed loss through the reject port. In this condition, such lots are subjected to winnowing first, which exploits the differential aerodynamic behaviour of dockages to isolate them from the seed lot. An adjustable air flow with human safety winnower is advisable to deploy for the purpose. Various types of winnowers are available commercially.

7 Cleaning and Grading

The harvested seed mass may contain considerable amounts of moderate impurities like plant parts, broken kernels, soil clods, dust, stone etc., which have to be removed. Apart from these impurities, the seed mass comprises seeds with variation in shape, size, density, colour, texture etc., which reduce the seed quality with respect to biological value as well as commercial value. Cleaning and grading are required to upgrade both the physical and biological quality of seeds using the following units.

7.1 Air Screen Cleaner Cum Grader

It removes fine impurities and undersized seeds from the seed lot, which improves seed lot’s physical purity as well as the flow of material in subsequent operations by the combination of air stream (aspiration) and screens (sieves). Basically, the difference in aerodynamic behaviour between seed and seed lot impurities is employed for cleaning and that of thickness of seed as a grade factor. This machine is also called Air Screen Thickness Grader cum Cleaner. It is important to note that it grades only on the basis of thickness, not the length of the seed.

An aspirating system is also installed in the machine providing air streams at two locations: (a) pre-suction channel—to remove lighter impurities before seed mass reaches to screens and (b) after suction channel—to remove impurities as well as lighter seed components that remain after the grading of seed mass.

There are two sets of screens for scalping and grading used in the Air Screen Cleaner cum Grader. By increasing the number of sets of screens the cleaning efficiency increases, however the throughput capacity decreases. The performance of the machine is a function of operational parameters, i.e. selection screen (size and type.), volume of air blast, oscillation speed, pitch of screen and feed rate. The size of the scalping (top) screen should be large enough to pass all the seed components and scalp off the coarse impurities, while the size of the grading (bottom) screen should be smaller than the optimum thickness/diameter of good seed so as to pass materials other than good seeds and ride over all good seeds. The selection of the type of screen should be according to the shape and size of the seed to be processed.

Shape of seed	Screen	Opening of screen
For round seeds	Scalping	Round hole
	Grading	Slotted hole
For oblong seeds	Scalping	Oblong hole
	Grading	Oblong hole
For lens-shaped seeds	Scalping	Oblong hole
	Grading	Round hole

However, the selection of size of opening of scalping and grading screens shall be done on the basis of sieve analysis for the specific seed lot with reference to available standards for the crop seed in respect of scalping and grading screens. Screen specifications for the processing of some important crop species as followed in India (Trivedi and Gunasekaran 2013) are presented in Table 1.

Table 1 Screen aperture sizes for processing seeds of major field crops

Oscillation speed, pitch and slope of screen cradle should be optimized so as the seed turns and tumbles at its own axis vertically and is subjected to grading by screen opening with cleaning and grading efficiency as well as a good throughput of processing. In general, the scalping screen is set at steep slope for fast removal of trashes and weed seeds, while the grading screen is set to flat slope so as to hold the seed longer for close separation. The pitch should be steeped for chaffy seeds and flattened for round seed in order to prevent bouncing over the screen, which affects the grading efficiency.

The feed rate of the seeds over the screens should be properly adjusted so as to get better cleaning and grading. It should be adjusted according to seed mass dockage level and shape of seed. Lesser feed rate should be maintained for higher dockage seed mass. Feed rate should be optimized as maximum the three-layer seed remain on the screen.

The screen cleaning is also an important mechanism of the machine to harness maximum efficacy. There are three types of screen cleaning used in air screen machines, namely, (a) knockers or beater, (b) rubber balls and (c) brushes. Amongst all, the rubber ball system is the best as it induces least tear and wear to screen as well as minimizes the chance of mechanical mixing of seed (Fig. 2).

Fig. 2

Line diagram of air screen cleaner cum grader (Source: Sinha, J.P. and A.P. Srivastava (2003)

Screening efficiency increases as screen length increases, but the structural strength reduces with increasing the length of the screen. Hence, two Air Screen Cleaner cum Grader are deployed in seed processing in order to maximize the service life of the machine and cleaning and grading efficiency. The first machine primarily works for cleaning and the second one mostly for grading. It is important to note that the air screen machines are also effective in removing dockages and dead seeds (Sinha et al. 2001; Sinha et al. 2002).

7.2 Indented Cylinder Separator

It is also called length separator, as it utilizes the difference in length between the seed and impurities (especially lengthwise broken seeds) of seed mass which are either longer or shorter than the crop seed. The indented cylinder separator consists of rotating cylinder almost horizontal with adjustable horizontal separating trough mounted inside. The inside surface of the cylinder has small closely spaced semi-spherical indents (cells or pockets), hence the name.

The fed seed mass moves inside the bottom of the cylinder from the feed end to the discharge end. The cylinder revolves, turning the seed mass to be assessed by the recessed indents of the cylinder. Short seeds/impurities are lifted out of the seed mass and are dropped into the lifting trough. Long seeds remain in the cylinder and are discharged out to a separate spout at the end of the cylinder.

The rotation motion and recessed indents of the cylinder lift the shorter impurities to a certain height until the gravitational force exceeds the centrifugal force due to rotation and falls down in the separating trough. The length, centre of gravity, surface texture and size of the seed determine how they fit into the indent so that it can be lifted out of the seed mass. The indented cylinder separator can be used in two different ways.

• 1. Right Grading: Short seed/impurities are lifted and dropped into a separating trough and larger seed lot in the cylinder and discharged separately.

• 2. Reverse Grading: The crop seed is lifted and larger impurities are left in the cylinder. In this case, the capacity of the cylinder should be larger because all crop seed has to be lifted out. There is a higher risk of losing good seed with this application. To keep up with the capacity of the material passing through the air screen machine and right grading, multi-reverse grading indents are required.

The efficiency of separation depends on the operational parameters, cylinder speed, size of indent, feed rate, pitch of cylinder and retarder position. An increase in the cylinder speed increases the centrifugal force which holds the seed in the indents; hence, it will lift the seed longer or higher helping its separation. Conversely, a decrease in cylinder speed will decrease the centrifugal force and the seed will fall out of the indents at the lower level. The speed of the cylinder is so adjusted that the liftings should fall in the separating tray (Fig. 3).

Fig. 3

Exploded view of indented cylinder. (Source: Sinha, J.P. and A.P. Srivastava (2003)

The size and shape of the indent has to be selected as per requirement of the specific seed lot. The position of the trough also should be adjusted so that the trapped material in the indents falls in the trough. The longitudinal tilt or pitch of the cylinder and retarder position should be adjusted for fine separation.

7.3 Specific Gravity Separator

The differences in the density of good and poor quality seeds and trash are exploited by a gravity separator in separating the unwanted seed components or impurities from the seed mass subjected for processing. If the seed mass cleaned and graded by Air Screen and Indented Cylinder still possesses high-density material such as clods, stones or lower density grain components (immature, insect damaged, diseased seed) which are not separated by earlier operations, it has to be processed for density grading.

The machine employs the principle of floatation, in which seeds are vertically stratified in layers on the deck according to their density by vibration and inclination of deck the different density grain components or impurities of lot takes separate paths.

Seeds are fed onto the specific gravity separator in layers of three to five seeds thickness. The shaking deck and the differential air stream from the deck push the heavier seed uphill; while the lighter seeds follow the path to downhill. The machine consists of different adjustable grates to separate materials according to their density. Stones or clods are pushed to transverse edge where adjustable grates direct the materials to different outlets.

The performance of the machine is closely linked with the operational parameters: volume of air, end slope, side slope, oscillating speed and feed rate. Stratification of seed mass on the deck is achieved by differential air volume adjustment from feed to discharge end. It should be adjusted so that heavy seeds lie on the deck and the lighter seeds are lifted into the upper layer of the seed mass. The air volume on the feed side should be more than the outlet side.

The slope of the deck from the feed hopper to the discharge end controls the speed at which seed mass moves across the deck. The flat end slope is required when the difference between the seed and contaminants or seed to be separated is slight to hold the seed mass longer time on the check. On the other hand, the end slope can be increased if the differences in seed and contaminants are high in respect of specific gravity. Side slope is the tilt or inclination of the deck from low side to high side of the discharge end. It facilitates the light seed layers riding on a cushion of air slide downhill to low side of the cheek, while deck oscillation moves heavy seed uphill to the opposite side of the deck. Feed rate is critical and should be adjusted. Seed loss in reject port is inevitable during processing, but it can be minimized considerably by operating the machine at optimized parameters for the particular seed kind and lot subjected to processing. About 1 to 3% seed loss has been reported with specific gravity separation, and higher seed loss for poorer seed lots (Sinha et al. 2002) (Fig. 4).

Fig. 4

Line diagram of the functioning of the specific gravity separator (Source: Sinha, J.P. and A.P. Srivastava (2003)

7.4 Pneumatic Separator

It exploits the difference of terminal velocity in air stream of seed and impurities. Density, shape and surface texture are physical parameters which affect the resistance of particle to air flow and exhibit difference in terms of its terminal velocity. The seed mass is fed into the confined rising air stream where all particles with different terminal velocity are lifted to different levels. Particles of higher to lower terminal velocities are separated in different fractions.

In the pneumatic separator, the air system forces the air through the machine by creating pressure greater than atmospheric. On the other hand, in the aspirator, the fan is at the discharge end and induces a vacuum, which allows the atmospheric pressure to force the air through the separator. It consists of a control unit for variation in pneumatic pressure as per the requirement of seed mass which has to be processed (Fig. 5).

Fig. 5

Schematic diagram of pneumatic separator. (Source:https://www.usgr.com/seed-processing/wall-mount-air-seed-separators/)

7.5 Electric Separator

It separates seed mass constituents on the basis of differences in their electrical properties. The degree of separation depends on the relative ability of seeds in the mixture to conduct electricity or to hold a surface charge. A thin layer of seed is conveyed on the belt through a high-voltage electrical field, where it is given a surface charge. This charge is like the one that is picked up by a comb passed through the hair. As the belt rounds a pulley, seeds that have quickly lost their charge fall in the normal manner from the belt. Seeds that are poor conductors and slow to lose their charge will adhere to the belt and fall off gradually. Dividers in the drop path can then be positioned to collect any fraction of the distribution desired.

The operational parameters of the machine are feed rate, belt speed, electrode position, voltage and divider position. As the moisture content of the seed affects the conductance of electricity, all adjustments are linked with it.

Seeds of watercress from rice, ergot from bent grass, and Johnson grass from sesame can be separated by the electronic separators efficiently.

7.6 Spiral Separator

It classifies seed mass according to particle shape which is linked with rolling resistance. It consists of sheet metal strips fitted around a central axis in the form of spiral. The unit resembles open screw conveyor standing in a vertical position. The seed is fed at the top of the inner spiral. The round seeds roll faster as their rolling resistance is less than the flat or irregularly shaped seeds. The orbit of the round seed increases with speed on its flight around the axis, until it rolls over the edge of the inner flight into the outer flight where it is separately collected. On the other hand, the flat or irregularly shaped seed slide or tumble and move slowly and does not build up enough speed to escape from the inner flight. Hence, they are collected from inner flight outlet at the bottom separately.

The spiral separator is very useful for the separation of damaged seed in brassicas, vetch, lentil, peas and soyabean. In general, the spiral separator is useful to remove broken grains or texturally different seeds or impurity mixed in seed mass. It is also suitable to separate round seeds or damaged seeds (in respect of shape) from a mixture of round and flat seeds or damaged seeds (Sinha et al. 2008) (Fig. 6).

Fig. 6

Line diagram of a spiral separator (Source: Sinha, J.P. and A.P. Srivastava 2003)

7.7 Inclined Draper

It separates the materials on the basis of differences in rolling or sliding characteristic, specific gravity and surface texture. The seed mass is fed from a hopper which distributes the mixture uniformly across the middle area of an adjustable tilted velvet, canvass or plastic-covered flat surface. The belt moves upwards, resulting the smooth or round seeds move to lower the end, while rough-surfaced or oblong or flat materials remain on the belt.

The rate of seed flow and the belt angle and speed can be adjusted according to the properties of the seed surface harnessing maximum separation efficiency. Multiple inclined drapers can be used for higher capacity processing.

The machine is useful to separate the fruits or seed clusters or plant debris and to remove buckhorn plantain (Plantago lanceolata) from red clover (Trifolium pratense) and to clean flower seed (Fig. 7).

Fig. 7

Line diagram of an inclined draper. (Source: ‘Guide to Handling of Tropical and Subtropical Forest Seed’ by Lars Schmidt, Danida Forest Seed Centre 2000)

7.8 Magnetic Separator

It also exploits the differences of surface texture of the seed. The seed lot is treated with iron fillings; rough seeds pick the fillings, but smooth do not. The treated seed is passed over a revolving magnetic drum. The seeds containing the magnetic materials are attracted to and fall near the drum or remain adhered to the drum. The adhering materials are brushed off at the end of a batch. The other materials fall farther to the drum and thus are separated.

The magnetic separator is used to remove Stellaria media (chickweed) from clover and alfalfa, dodder (Cuscuta pentagona) from clover, Lucerne (alfalfa), and red clover, Sinapis arvensis (wild mustard) from brassicas (Sinha and Srivastava 2003) (Fig. 8).

Fig. 8

Line diagram of a magnetic separator (Source: Sinha, J.P. and A.P. Srivastava (2003)

7.9 Horizontal Disk Separator

It utilizes the difference in shape and surface texture with rollability characteristics and centrifugal force to separate different fractions. Seeds, confined to the centre of a flat rotating disk by a stationary circular plastic fence, are metered to the outer part of the disk through adjustable outlets. Centrifugal force causes round or smooth seeds to roll or slide off the disk. Irregular or rough seeds remain on the disk and are moved into a different hopper.

The feed rate through the outlets has to be adjusted so that each seed moves independently. The horizontal disk is similar to the spiral separator, but it is more selective because it has disk speed control that can change the proportion of seed retained or thrown off.

An added capacity can be obtained by mounting many disks on a single vertical shaft. The horizontal disk separator can separate dodder seeds from lucerne (alfalfa), curly dock from red clover, and other mixture in which one seed has a greater tendency to roll or slide than another.

7.10 Colour Sorter

The colour sorter is mainly used to separate discoloured seeds from the good seed lot when the other physical parameters like thickness, length, density and surface texture cannot differentiate them. The discolouration of seed occurs due to mechanical abuse of seed coat or pest infestation or adverse weather conditions. The machine uses photoelectric cells/sensors to compare the seed colour with the background filter. The background filter is selected to reflect the light of similar wavelength as that of good seeds. The seed mass to be colour sorted is metered by vibratory feeder so as single seed file is fed to the sorting chamber. Seed which differs in colour is detected by the photoelectric sensors by assessment of the reflected wavelength of light from the discoloured seed, which generates an electric impulse and activates an air jet to blow out the discoloured seeds from its normal path. As the discoloured seed has deviated its normal feed path, it is separated from the good seed which follows the normal path.

The colour sorter should be used for a seed lot only after the basic cleaning and grading. The efficiency of the machine depends upon the operational parameters, such as vibration of the feeder, speed of feeding belt, feed rate, position of discharge point, background colour filter, colour range, ejector timings and lag time of ejector (Fig. 9).

Fig. 9

Schematic view of a colour sorter (Source: Sinha, J.P. and A.P. Srivastava (2003)

The electronic colour sorting significantly improves the purity, germination (%), vigour index and true density of the lot. However, colour separation process is effective only in the seed lots of colour purity level of around 70% or more (Sinha and Modi 2000; Sinha and Vishawkarma 2001).

7.11 X-Ray Sorter

A combination of radiography and image processing is utilized to process seed lots on the basis of physical parameters, i.e. size, shape, density, colour, texture as well as the anatomy of individual seeds. X-ray-based imaging provides a method for the non-invasive analysis of the internal structures even of treated, coated or pelleted seed.

Seeds in batches are placed in a special holder and exposed to long-wave X-rays, from where digital radiograph is generated of individual marked seeds. The radiograph is further converted into a 3D image, which subsequently is subjected to digital image processing (Fig. 10).

Fig. 10

Schematic view of an X-ray sorter. (Source: https://www.seedquest.com/technology/from/Incotec/upgrading/xray.htm)

The processed image data are digitally analysed and compared with reference to the digital data of quality seed database. It is important to note that the X-ray imaging and sorting is based on internal morphology and not biological value.

8 Seed Treatment

In general, seed treatment refers to the application of a single pesticide, or a combination of two or more, to disinfect/disinfest seed from seed-borne pathogenic organisms or to protect the seed from storage pests. It also provides a safe microenvironment to planted seeds for developing into healthy seedlings. In some cases, coating or pelleting is carried out also with the objective of delivering essential nutrients to seeds for better nourishment and production of vigorous young plants or increasing the size and shape of seeds for enhancing the ease of mechanical planting (see chapter “Seed Quality Enhancement”).

The equipment used to apply chemicals to seed in any form or method are classified as seed treater and these are of three types, namely, (a) dry seed treater, (b) slurry seed treater and (c) Mist-o-matic seed treater.

8.1 Dry Seed Treater

The seed is simply mixed by any means, e.g. shovels, scoops, rotating drum with treatment chemicals which are in the powder/dust form. The dusty condition that prevails during treatment and handling of the seed subsequent to treatment is the main disadvantage of this method. The low chemical use efficacy and non-uniformity of treatment are other limitations of this method.

8.2 Slurry Seed Treater

The chemical is mixed with water to form a thick suspension. The machines are equipped with an adjustable hopper to control the flow of seed into the machine, a slurry tank with mechanical agitator to stir the mixture constantly, a positive seed slurry metering device and a short mixing auger that mixes the fungicide slurry and seed and also moves the treated seed to the discharge spout of the machine. It overcomes some limitations of the dry seed treater, e.g. dusty environment and non-uniformity in coating of chemical. The average efficiency of this machine is about 60–70%.

8.3 Mist-O-Matic Seed Treater

This type of treater is designed to apply low doses of liquid/water soluble chemicals. It has an adjustable hopper that regulates the flow of seed into the machine; a positive metering device; a seed dispersing cone; and a rapidly spinning disk, which breaks up the liquid/water solution of chemicals into droplets. The dispersion cone causes the seeds to fall in a layer through the droplets. Further, the material is moved through a mixing chamber where the material is mixed through rotating auger or brushes and is moved to the discharge spout. The efficiency provided by the treater is in the range of 80–95%, as well as more uniformity in coating of chemical is associated with this method.

It is important to note that the solution of chemicals (in case of slurry seed treatment and mist-o-matic seed treatment) should be prepared so as the moisture content of treated seed does not increase by more than 1%. To attain it, chemical solution should be prepared according to the dose specified by pathological test and treatment method (Fig. 11).

Fig. 11

Schematic view of a mist-o-matic seed treater (Source: Sinha, J.P. and A.P. Srivastava (2003)

8.4 Thermoseed Treatment

Developed by Lantmännen, Sweden, this is an innovative patented technology of thermal seed treatment for effective and economically competitive alternative to chemical seed treatment. Hot and humid air is used as heating medium and fluidized bed technology for seed mass movement in the system to ensure even exposure of thick seed layers. This system is capable to treat 30 tonnes of seed per hour effectively. The treatment plant consists of heating and cooling systems, sensing devices, system control software and transportation facilities. The exposure of seed for precisely conditioned hot humid air renders harmful pathogens present with seed without affecting seed quality https://www.lantmannen.com/.../thermoseed).

9 Seed Coating and Pelleting

Primary functions of coating /pelleting are to act as a binder and carrier for actives (pesticides or fungicides, nutrients, growth regulator, repellent), reducing the hazardous dust formation during handling, enhancer flowability, booster efficacy of actives and cosmetic improver. It improves seed-soil microenvironment (see chapter “Seed Quality Enhancement”). There are two components to a seed pellet: bulking (or coating) material and binder. The bulking material can be either a mixture of several different mineral and/or organic substances or a single component. The coating material changes the size, shape and weight of the seed. Desirable characteristics of a good coating material include: uniformity of particle size distribution, availability of material and lack of phytotoxicity. The second component, the binder, holds the coating material together. Binder concentration is critical because too much binder will delay germination. Too little binder will cause chipping and cracking of pellets in the planter box, which can cause skips and/or wide gaps in the plant rows. Many different compounds have been used as binders, including various starches, sugars, gum arabic, clay, cellulose and vinyl polymers.

The critical parameters for efficient coating and pelleting are: physico-chemical properties of the material, pan rotation speed, curvature, surface, angle of rotation, drying mechanism, temperature and operational parameters of machine with respect to seed to be given the treatment (Fig. 12).

Fig. 12

External views of seed coating/pelleting machine. (Source: http://www.seedpelletingequipment.com/coatingpan.htm)

10 Seed Packaging

The main functions of packaging are to contain the product and protect it against a range of hazards which might adversely affect its quality during handling, distribution and storage. Seed packaging is the last unit operation, but also of utmost importance after physical upgradation of seed lots to keep it safe until its use at planting. Selection of packaging material has to be done considering its tensile strength, bursting strength and tearing resistance to withstand the normal handling procedures, type of the seed, seed moisture content, expected duration of storage, threat of pathogens and storage environment. Most commonly used packaging materials are cotton cloth, paper film, hessian cloth, laminated polythene, metal, glass etc. Jute/cotton cloth bags are used when the seeds are to be stored under controlled conditions. Pulse seeds are packed in smooth but tough surface packaging material, as the bruchids (major storage insect pest) if infested, cannot lay eggs on it (Sinha 2000). Metallic cans, glass bottles or laminated poly-lined bags are used for smaller amounts of seed where high moisture proofing is required or low moisture seed is to be stored for long periods, such as the germplasm. The ultra-dry seed is only stored in hermetically sealed container. Contrarily, high moisture seeds should not be packed in moisture-proof containers (see chapter on Seed Storage and Packaging for more details). Special attention is required regarding mechanical mixing at the time of packaging operations. The mechanical mixing must be avoided by packaging only one lot at a time and cleaning the total processing area after completing each lot. The packed materials should also be labelled properly, with full lot identification information, e.g. crop, variety, lot no, production year, moisture content, seed quality parameters, weight etc.