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Seed processing



Seed processing


HARVESTING AND SEED EXTRACTION

Harvesting is one of the agronomic management practices that require technical knowledge on maturation of the crops. This knowledge is much important in seed production than in commercial production.


  1. Advantages on correct method of harvesting

    • Seed quality will be more

    • Seed yield will be protected without loss due to shattering

    • Processing loss will be reduced

    • Seed storability will be more

    • Above aspects will improve the seed marketability

  2. Physiological maturation

    • The correct stage of harvesting for seed crop is termed as physiological maturation i.e., only after the attainment of physiological maturation crop should be harvested.

    • Physiological maturation is the stage at which the dry matter accumulation of the seed will be the maximum and it will be associated with maximisation of seed germination and seed vigour. The moisture content of the seed will be in decreasing order.


Physiological maturity characters

Physiological maturation is the stage at which seeds have maximum dry matter, germination and vigour.

  • The attainment of physiological maturation is represented by sigmoid curve of growth pattern.

  • The physiological maturation can be represented both as duration and visible symptoms.

  • Physiological maturation during is the days taken for maturation after the anthesis / flower opening / fertilization.


  • Fixing the days and harvesting is difficult. Hence for physiological maturation is identified based on visual symptoms that occur in fruit and seed characters. The visual symptoms are: Physiological maturity symptoms in different crops Crop Symptoms of Maturity

    Brinjal Yellow colouration of fruits

    Tomato Reddish colouration of fruits

    Bhendi Hairline crack formation in dried fruits

    Ashgourd Ashy coating over the fruit

    Pumpkin Yellowish browning of fruit rind

    Ribbed gourd Rattling sound of seed in fruit.

    Browning of dried fruits.

    Snakegourd & Bittergourd Change of fruit colour yellow

    red at distal end or any part of fruit.

  • These symptoms are easy to identify even by illiterates and alarms for harvesting crop with good seed quality.

Harvesting the crop of physiological maturation is difficult at physiological maturation based on days is calculated more or less based individual seed and physiological maturation based on visible characters, all fruits of a population will not come to physiological maturation storage at single phase which will be more common in plants with indeterminate flowering habit than with determinate type. The solution for the problem is harvesting of crop at harvestable maturity.


George (1980) divided vegetable seed crops into three broad groups, depending on the state of seed at harvest time.

  1. Dry Seed: seed is usually dried on the plant before harvesting eg: brassica, peas, beans onion

  2. Fleshy fruits: ripened fruits are picked form the plants and dried first, the dried fruits are then opened later to remove the dried seeds eg: chilli, okra, gourds pumpkin and eggplant

  3. Wet fleshy fruits: Fruits are harvested at high moisture level, the seeds has gelatinous or mucilaginous coating adhering to it. This has to removed after seed extraction by a fermentation process or treatment with dilute acids. Eg; Tomato and cucumber Methods of harvesting

  • Harvesting of crop can be done either mechanically or manually.

  • Mechanical harvesting can be done only as single or once over harvest.

  • Care should be at mechanical harvesting to avoid mechanical injury by adjusting blade size, gap between blades, speed of operation etc.

  • Manual harvesting is done in two methods which is single harvest or periodical harvest.

  • Either single or periodical harvest termed as picking is depending upon the growth habit of the crop. In determinate type where flowering is confined to a shorter duration once over or single harvest is recommended.

  • In crops with indeterminate growth habit, periodical harvest or picking is recommended.

  • Picking is harvesting the crop as and when a part of the population attains HM.

  • The number of picking varies with crop depending on the growth habit of plant.

  • Usually seeds of later picking are not considered for seed purpose.

  • The type of harvest recommended for different crops is as follows.


Harvesting methods of vegetable corps

Amaranthus:

A lightening or yellowing of foliage colour is an indication that seeds are beginning to mature. Types with an apical inflorescence are usually harvested once. Types with several side shoots are harvested several times as the seeds mature. The harvested seed stalks are placed on a clean tarpaulin or into very fine mesh nylon bags and allowed to dry in the shade.

Processing

Seeds are easily threshed by hand. The threshed seed is cleaned by winnowing.

Bean

Pods are harvested when they have turned yellow but are not yet completely dry. The inner seeds will be firm, well developed, and beginning to loosen inside the pods. Harvesting is often done in the morning to avoid losses due to shattering. Dry the pods in the sun and then place in a shelter for 1–2 weeks of curing. Pods are threshed by hand, being careful not to injure or split the seeds (injured seeds will germinate without primary leaves and grow poorly). Seed is further cleaned and dried after threshing. Beet Family (beet, spinach and Swiss chard):
Seed does not ripen uniformly on the plant and sheds easily when mature. Cut stalks when most flowering clusters have turned brown and stalks have turned yellow and dried—the first seeds are shed at this time.: Store stalks in a cool dry location for 2–3 weeks to encourage further seed ripening. Do not heap stalks on top of one another since this causes seeds to ferment. Handle stalks as little as possible since seeds shed easily. Small quantities of seed can be stripped by hand as seed matures. Large numbers of stalks can be put into a bag and beat with a stick. Chaff is winnowed away. Brassicas (broccoli, Brussels sprouts, cabbage, Chinese cabbage, kohlrabi, mustard and turnip)
Brassica seed shatters easily. Harvesting is done carefully when 60–70% of the pods have turned brown and most of their inner seeds are light brown and firm. Harvested seed stalks are cured for 1–2 weeks. Pods are then threshed with sticks and sifted by hand. Seed is brittle and should not be crushed when handled. Seed is dried in partial sun, then cleaned and stored.

Carrot

The seed turns brown 6 weeks after pollination. Before the seed shatters, cut and place umbels into paper bags to dry completely. Late-season rains will reduce seed quality. For small amounts, handpick each umbel as it dries brown. Large amounts of seed can be harvested by cutting the entire stalk as umbels begin to dry. Allow seed to mature in a cool, dry location for an additional 2–3 weeks. Seeds can be removed by hand-beating or rubbing umbels between hands. Winnow to clean. Remove spines from dry seed by rubbing.

Cucumber Family

The fruits should be left to fully ripen and turn colour. The fruits of luffa and bottle gourd should be left on the plant until they dry. For cucumbers, fruits will turn brownish colour. Bitter gourd fruits will turn orange. Some wax gourds will be covered with a pale-white powdery wax on the surface of the fruit. After harvest, the fruits can be kept in a shed for a couple of weeks to allow the seed to further ripen. For ‘wet seeds’ such as cucumber, wax gourd, bitter gourd and melons, cut the fruit lengthwise and scrape seeds out with a spoon. Allow seeds and the jellylike surrounding liquid to sit in a container at room temperature for 1–2 days. Fungus may start to form on top. Stir daily. The jelly will dissolve and good seeds will sink to bottom while remaining debris and immature seeds can be rinsed away. Spread seeds on a paper towel or screen until dry. For ‘dry seeds’ such as luffa and bottle gourd, keep the seeds in the fruit until they naturally separate from the flesh. This can be identified when you shake the fruit, the sound of seeds moving inside is heard. Cut off the bottom of the fruit and shake the seeds out, winnow to clean the remaining chaff, then place the seed on a screen for further drying before storage.

Eggplant

Harvesting is done when fruits are fully ripe (the skin of fruit turns brownish-yellow in green varieties or brownish in purple varieties). Harvest and store the fruits in a shed for a week until the fruits get soft. The outer covering is peeled off and the flesh with the seeds is cut into thin slices. These are then softened by soaking until the seeds are separated from the pulp. If the material is allowed to stand overnight in this condition, the separation of seeds from the pulp becomes easier. After separation, the seeds are dipped into water. The plump seeds will sink to the bottom. The seeds should then be dried on a mesh for a couple of weeks in a cool, dry place before storage. Processing to improve seed quality after the harvest, the fruits not confirming to the varietal character, small fruits and pest and disease infested fruits must be removed and only graded fruits are to be used for seed extraction. Seed extraction methods: The seed viability depends on the method by which the seeds are extracted and hence, it is more important to choose proper method of seed extraction.The selected fruits are to be cut into pieces and put in the cement tank with water for 10-12 hours for fermentation. The fruits can be allowed for over ripening for one or two days. This facilitates for easy pulping of fruits. Then fruits are to be made into pulp by manually. Add excess quantity of water and after ½ hour, remove the floating pulp fraction and collect the seeds settled at the bottom. For large scale seed extraction we can use the brinjal seed extractor.. The seeds extracted by this machine may again be treated with concentrated HCl @ 2-3 ml/kg seed with equal volume of water for 3-5 minutes with constant stirring and then seed should be washed with water for three to four times. It is easy to dry the seeds extracted by acid method and also remove the fungus growth over the seed coat, thus seeds possess golden yellow colour and high vigour.

Okra

The okra pods mature in a sequence from the base of the plant toward the top. The pods have tendency to split along the suture when they are dried out. Exposed seeds may be damaged by rain or may drop to the ground; therefore, the pods must be harvested as soon as they have become fully mature (brown color) and before shattering. Pods are easily hand threshed.

Onion

Clip umbels as soon as most flowers have dried and before seeds begin shattering. Some growers harvest when the seeds are exposed in 10% of the umbels. Fully dried flowers will drop clean seeds naturally. For small amounts, rub remaining flowers to free seeds. For larger amounts, rub heads over screens. Winnow to remove remaining debris. Allow to dry in cool, dry location for up to 2–3 weeks. Artificial drying is often used.

Chilli

Harvest mature, fully-ripe peppers for seed. Most peppers turn red when fully mature. Pepper seeds may be extracted from fresh fruits or from fruits that have been dried in the sun for a few days. Seeds may be removed by hand or extracted by grinding the fruits and separating the seeds from fruits with a series of water rinses. Spread the seeds on a screen for drying under shade for 2–3 days but bring them inside every evening.

Radish

Threshing seeds from pods are difficult. Allow the plants to mature fully; then bring them to the threshing floor for drying. Threshing is done by hand-beating with sticks. The seed is then dried further.

Tomoto

Allow tomatoes to completely ripen on the plant before harvesting for seed. Seeds from green, unripe fruits will be most viable if extracted after allowing the fruits to turn colour, but this is not advisable. Cut each tomato into half at its equator, opening the cavities that contain the seeds. Gently squeeze out the jelly-like substance that contains the seeds. Place the jelly and seeds into a small container for fermentation; add a little water if you are processing only one or two small tomatoes. Loosely cover the container and place in a warm place (around 25–30 °C) for 1–2 days, stirring daily. A layer of fungus will begin to appear on the top of the mixture after a couple of days. This fungus not only eats the gelatinous coat that surrounds each seed and prevents germination, it also produces antibiotics that help to control seed-borne diseases such as bacterial spot, canker, and speck. After fermentation, fill the seed container with water. Let the contents settle and begin pouring out the water along with pieces of tomato pulp and immature seeds floating on top. Viable seeds are heavier and will settle to the bottom of the container. Repeat this process until water being poured out is almost clear and clean seeds line the bottom of the container. Pour these clean seeds into a fine-mesh strainer. Let the excess water drip out and invert the strainer onto paper towel, fine mesh, or newspaper. Allow the seeds to dry completely in an oven or in partial shade. Break up the clumps into individual seeds, label and store for later use.

    Fermentation method

    The ripe fruits are crushed well in a nonmetallic container to make slurry. Do not drain out the juice. Keep the entire material till ferment. When fermentation process completed profuse foam forms on the upper surface of the material and tomato flesh separated from seeds completely. The liquid is decanted off and seeds are washed at least for 8-10 times with clean water. Spread the seed on cement floor thinly and sundry.

    Alkali method

    Add 300 grams of washing soda to 4 liters of boiling water. Take 4 liters of extracted pulp and add the washing soda solution. Allow the mixture to stand overnight in an earthen pot. Next day all the seed will settle down at the bottom of the container. Decant the liquid. Wash the seed thoroughly with clean water and allow it to dry under sun.

    Extraction with Hydrochloric acid

    Add commercial Hydrochloric acid @ 5ml per kg pulp and mix thoroughly by stirring and leave for 30 minutes. Again stir the mixture and wash the mixture with clean water repeatedly for 8-10 times and allow the seeds to dry under sun.
    Dry the seeds up to 8% moisture. Treat the seed with Thiram or Captan 75% WDP @ 2grams per kg seed and store the seeds in a moisture proof container which will help in retaining 80% germination at the end of 2 years.


Cowpea

Allow pods to dry brown before harvesting. The first harvest will be 8–10 weeks after sowing, followed by 2–3 harvests per week during the 6–8 week season. Cut pods with a sharp knife to minimize plant damage. Pods that are harvested 20 days after pollination will give the maximum seed quality. Pods are dried in the sun for approximately 3 days (Fig. 52). For small amounts, pods may be opened by hand. For large amounts, hang the pods in a burlap bag and beat them with a stick, or put on the floor and walk on them. Remove large chaff by hand or winnowing. Discard blemished and shrivelled seeds. Place remaining seeds under shade for 1–2 weeks for further drying.

SEED PROCESSING

Seed processing is to narrow down the level of heterogeneity of the lot by using suitable processing methods.
This heterogeneity can be narrowed down in the processing of seeds by eliminating the undersized, shrivelled, immature, ill filled seeds using appropriate sieve size. So if grading is done to obtain a particular range of size, shape, length and density of the seeds the quality of the lot is upgraded.

Requirements in seed processing

  1. There should be complete separation

  2. Minimum seed loss

  3. Upgrading should be possible for any particular quality shape

  4. Efficiency

  5. It should have only minimum requirement

Physical characteristics used to separate seeds are

  1. Size : Based on size it can be separated with air screen cleaner cum grader

  2. Length : Disc or indented cylinder separator

  3. Weight : Specific gravity separator

  4. Shape : Spiral separator or draper separator for round and flat seeds

  5. Surface texture : Rough from smooth surface seed- dodder mill

  6. Colour : Electronic colour separator

  7. Electrical conductivity: Seed differing in their ability to conduct electrical charge can be separated with electronic separator.

  8. Affinity to liquid : The seed coat of seed will absorb water, oils etc., which provides a means of separating seed on the magnetic separator.


Seed processing equipments

  1. Air Screen cleaner

  2. It consists of the following parts

    • A hopper in the top for seed filling

    • A flutted roller below the hopper to regulate the seed flow to the screen.

    • A flutted roller below the hopper to regulate the seed flow to the screen.

      • Rectangular perforations for paddy and

      • Round perforations for seed other than paddy

    • Screen shaking unit: for oscillating the sieves to move the seeds on the screens

    • Screen brushes to remove the blocked seeds

    • Air blower with adjustments for air outlet

    • Collecting outlet

    • Air duct for directing the blown up light particles to outside

    • Collecting bins.


Types of seed cleaner cum grader

  1. Crippen model cleaner cum grader

  2. Clipper model cleaner cum grader

  3. Petkas cleaner cum grader

  • Disc separatorl

  • It consists of a series of discs, which revolve together on a horizontal shaft inside the cylindrical body. Each disc contains many under cut pockets. The seed enter the intake end of the separator and move through the open centers of the discs towards the discharge end of machine. As the discs revolve through the seed mass the pockets lift out short seed but rejects longer seed. Longer seeds are conveyed by flights on the disc spokes towards the discharge end of the machine where they go out through the tailings gate. The rate of seed travel through the open disc centers is controlled by conveyor or blades attached to the spokes of the discs. The discs separator makes a very precise separation. No factor other than seed length and shape affects its separation. Flexibility is obtained by varying the size of the pockets.

  • Indented cylinder separator

  • The indented cylinder separator is a rotating, almost horizontal cylinder with a movable, horizontal separating adjustments which are mounted inside it. Indents line are there inside the surface of the cylinder. The indented cylinder revolves ,turning the seed mass to give each seed a chance to fit into indent. Short seeds are lifted out of the seed mass and are dropped into the lifting through long seeds remain in the cylinder and are discharged out via., a separate spout at the end of the cylinder.
    As the cylinder revolves, it creates centrifugal force which helps to hold the seed in the indent. Short seeds are held in the indent until the cylinder turns to the point where the indent is inverted enough for gravity to cause the seed to fall out of the indent. The length, surface texture and size of seeds all determine how they fit into the indent, so that it can be lifted out of the seed mass. The speed of the cylinder creates centrifugal force which holds the seeds in the indent as it is lifted upward. Thus the shape and size of the seed to cause some seeds to fall out after being lifted only a short distance, while other seeds are lifted closer to the top of the cylinder before they fall out.
    As the seeds enter the cylinder, the small, short, easy to separate seeds are quickly removed. The center cylinder section removes the intermediate sizes of seeds still in the cylinder. All indents in a cylinder are the same size, only the progressively declining amount of material to be lifted causes this difference in separating action.

Adjustments

  1. Cylinder speed

  2. Size of the indent

  3. Through setting

  4. Tilt of the cylinder

  5. Adjustable retarder.

Specific gravity separator

Seeds of the same size and general shape can often be separated because they differ in specific gravity or relative weight. This difference is very useful in removing light, immature seeds or heavy sand and rocks to improve the purity and germination of crop seeds.
If seeds which differ in specific gravity (relative weight / unit of volume) are placed on substrate of intermediate density, seeds of higher specific gravity will fall down through the substrata, while seeds of lower specific gravity will be buoyed up the substrata. Here air is used as a separation substrata.
As seeds flow on the deck of the gravity separator, they enter a column of air coming up through the porous surface of the deck. The pressure of terminal velocity of the air rising through the deck can be controlled very closely to separate two kinds of seeds differing in specific gravity, the air is adjusted so that only the lighter seeds are lifted up off the deck surface. These lighter seeds are held up by air pressure and tend to float on the deck surface.
The heavier seed possess a velocity greater than that of the air column's so they are not lifted and so will lie on the deck surface. The air column thus stratifies the seed mixture into vertical zones of relative weight with the heavier seed lying on the deck and the lighter seeds lifted up to the top of the seed mass.


Adjustments

  1. Feed rate

  2. Air flow

  3. End slope

  4. Side slope

  5. Deck oscillation speed

  6. Deck speed

SEED TREATMENTS

Based on the purpose of seed treatments it can be classified in to two groups

  • Pre - sowing seed treatments

  • Pre-storage seed treatments

Pre - sowing seed treatments

It is the treatments given to the seeds before sowing to improve the germination and vigour potential and as well as to improve the health of the seed.

Pre sowing seed treatments includes the following

  • Dormancy breaking treatments

  • Chemical treatments improve germination and vigour potential.

  • Insecticidal and fungicidal treatment.

  • Accelerating the speed of germination treatments (other special treatments)

other special treatments

  • Dormancy breaking treatments: Refer seed Dormancy chapter

  • Chemical treatments to improve germination and vigour potential.

  • Insecticidal and fungicidal treatment.


  • Benefits of the Insecticidal and fungicidal treatments.

    1. Prevents the spread of plant diseases

    2. It protects the seed from seed rot and seedling blights.

    3. It improves the seed germination

    4. It provides protection from storage insects.

    5. It controls the soil insects.


    Types of fungicidal and insecticidal seed treatments, they are 3 types they are

    1. Seed disinfection

    2. Seed disinfestation

    3. Seed protectants

    Seed disinfection

    it refers to the eradication of fungal spores that have become established with in the seed coat (or) in more deep seated tissues. For effective control, the fungicidal treatments must be penetrate the seed in order to kill the fungus that is present.

    Seed disinfestation

    It refers to the destruction of surface borne organisms that have contaminated the seed surface but not infected the seed surface. Application of chemicals through chemical dips, soaks, fungicides applied as dust, shiny or light have been found successful.

    Seed protection

    The main purpose is to protect the seed and young seedlings from organisms in the soil, which might other wise cause decay of the seed before germination.

    Fungicidal / insecticidal seed treatment materials are available in the form of dusts, wettable powders and liquids.

    1. Dusts : It is usually applied @ 200-250 gms / quintal of seed. Main dis-advantage is dusty condition will prevail during the seed treatment and after handling.

    2. This type of fungicide is applied to the seed along with soap like water suspension which can be mixed with seed by using special slurry treater

    3. Liquids : The use of liquid solution is known as the "quick wet ' method. Here a volatile fungicide is applied to the seed and it throughly mixed with them.


    Accelerating the speed of germination treatments (other special treatments)

    • Seed hardening treatment

    • Seeds can be hardened for 2 purposes I) Drought tolerance ii) Cold tolerance Seeds has to be imbibed in solution. Once the seeds imbibes water, the germination process takes place. At the end of soaking period the seeds should be dried back to its original moisture content. Chemicals used : CaCl2, KCl,

    • Seed fortification

    • Main aim is supply nutrients to seeds. The main objective is to have the high vigour, to over come unfavourable soil reactions. eg.) seed fortification with MnSO4 @ 0.5 to 1 %. will improve oxidation - reduction potential of seeds, which ultimately leads to higher germination.

    • Seed pelleting

    • Here the nutrients are coated on the seeds. This technique is very much adopted in forest tree seeds.


    Importants

    • Normally in small seeds this technique is adopted .

    • By pelleting we can increase the size of sees and we can make it free flowing one.

    • Through this we can able to reduce the seed rate.

    • It is also important for aerial sowing (gum arabica) in tree seeds.


    Materials used : Nutrients , adhesive, filler material.

    Inert materials: Lime, CaCO3, Chalk powder.


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