Everything you need to know about vegetable seeds processing. Learn about:- 1. Dry Seed Processing 2. Wet or Flashy Seed Processing 3. Cleaning 4. Seed Grading 5. Seed Treatment 6. Adjusting Moisture Content for Storage 7. Maintaining Identity during Processing.
Seed crop received from the field after harvesting is never pure. Mixed with it are the undesirable materials namely light and chaffy seeds, off-sized seeds, damaged or deteriorated seeds, soil clods, seeds of other crops, weed seeds, soil particles and other inert materials. These contaminants must be removed after harvesting, extraction and threshing to get pure crop seeds for replanting.
Contaminants removal is done in seed conditioning plant through the use of various separating equipments in series that take advantage of the physical property differences between the crop seeds and contaminants. These physical properties include length, width, thickness, shape, density, terminal velocity, surface texture and electrical conductivity.
Seed separators are thus designed to utilize the difference in a single physical property or a combination of physical properties. Thus the function of seed processing is to remove or minimize contaminants to a level that meets the minimum seed certification standards.
The objective of seed processing is to achieve clean and pure seeds of high physiological quality which can be stored and easily handled during succeeding process. Processing includes a number of handling procedures, which can be grouped into- harvesting, extraction, cleaning, grading and adjustment of moisture content for storage. Any step in the processing procedure must be carefully adjusted to the particular seed type.
Seed processing methods can be divided into two- dry processing and wet processing. Dry processing involves harvesting seed that has already matured and dried within the seed bearing portion of the plant. Examples of dry processed seed plants include peas, beans, broccoli, lettuce, okra, onion, and turnips. Wet processing is used when the mature seed is enclosed within a fleshy fruit or berry.
Examples of wet processed seed plants include cucumbers, melons, and tomatoes. Some vegetables can be either dry processed or wet processed, for example, peppers and squash.
Dry Seed Processing:
When seeds are ready to be processed, the entire seed pod, capsule or seed head will become brown and dry. During the maturation process, the ripening pods and capsules change color from green to yellow green to yellow to light brown to a darker brown or dark gray. Ripening and maturation may be uneven within the pod or capsule, uneven on the plant, and uneven within the stand of plants.
For that reason, the pods of many plants are harvested individually. Seeds of legumes and brassicas often develop a split along one side of the pod. This is the best time to collect the seed, before the pods start to open and scatter their seed. Some seed may mature in the capsule or pod, even before the pod has turned completely brown. Most seeds turn darker in color as they mature. Seeds may initially be white, turning green and then brown or black.
Once the seed pods, capsules and seed heads start to mature, it is important to check the crop on a daily basis. Rain or seed predators can destroy a good seed crop in a short period of time. Plants that produce umbels (members of Umbelliferae family like carrot) can usually be left in the field to harvest until the umbels are dry. Some members of this family mature their seed unevenly causing seed to scatter, while other seeds in the umbel continue to mature.
One method of dealing with crops that mature their seed unevenly is to pull the plants and hang them upside down to dry under cover. This allows the seed to continue to mature on the plant while the plant dries. This procedure is often used for lettuce. Confidence in knowing when to harvest comes both with experience and familiarity with different species and crops.
After harvest, seeds are threshed to remove the seed from the surrounding plant material. A period of air-drying is important before seeds are threshed. Plant material should be spread out in thin layers until all plant material is dry; otherwise, mold, decay, and heat from decay will cause damage to the seeds. As the plant material dries, seed pods may split open or shed seed. Harvested material should be stored in a well-ventilated room with low humidity. During this time we should be aware of insects, especially weevils that feed on the seeds.
Threshing is best done outside on a dry day. The threshing process involves application of mechanical force using a controlled pressure and a shearing motion, and is accomplished by hand or by machine. There are different methods for threshing seed. Plants that have pods, such as beans and okra, can be threshed by placing the pods in a large feed sack, which is tied shut securely, and then placed on the ground where it is flailed, stepped on, jogged on, or danced on with a twisting motion.
The sack is turned often to redistribute the plant material for further threshing. When using this method, it is best to use running shoes or other soft-soled shoes because seeds can develop hairline cracks and splits from too much pressure. To separate seed from flower heads, plant material is spread on a concrete slab and then gently walked on with a twisting motion to break open the flower heads. Care must be taken not to apply so much pressure that the seed is broken.
To thresh seeds of brassicas, place plant material in a large wheelbarrow or on a large tarp, and while wearing gloves twist and wring the plant material through your hands until the seed breaks free from the pods. Another method for extracting seed is to place seed heads on a piece of plywood. The seed is extracted by placing a wooden cement float above the material and then pressing and twisting until the seed breaks free. For threshing small lots of seed, a threshing box may be used.
This consists of a wooden box, with sides slanting outward, open at the top and on one end, and the long sides and bottom covered with corrugated-rubber floor matting. Seed is placed in the box and a rubbing board (also covered with corrugated rubber), is moved back and forth across the seed heads to separate the seed from the heads.
Wet or Flashy Seed Processing:
Wet seed processing is used with seed crops that have seeds in fleshy fruits or berries.
There are three steps to the process:
(1) Extraction of the seed from the fruit,
(2) Washing the seeds
(3) Drying.
(1) Extraction:
Extraction process depends on the species. Soft fruits such as tomatoes are cut up, mashed, and then fermented. Cucumbers and melons are cut in half, the seed scraped out along with the fruit pulp surrounding the seed, and then fermented. In watermelons, the entire fleshy fruit is fermented along with the extracted seed. These types of fruits have a gel surrounding the seed that contains germination inhibitors. The presence of the gel also makes handling and drying of the seed difficult.
Fermentation is a natural process that occurs to a small extent as fruits decompose. When fermentation is done in a controlled manner, the microorganisms, principally yeast, break down the gel thus releasing the seed while killing bacteria and fungi that cause most seed-borne diseases. The temperature and length of fermentation are important. If the mash is not fermented long enough, seed borne diseases will not be eliminated, but if fermented too long, the seeds may sprout prematurely.
The length of the fermentation is dependent on temperature. At 70 to 75°F (21°C to 24°C) three days are necessary for fermentation. Length of fermentation may also depend on the variety itself. For example, varieties with high sugar content may take longer time to ferment, up to four days. With few exceptions, fermentation periods longer than three days risk damaging the seed.
There are different fermentation techniques for different crops, for example, pepper seeds are extracted from the fruits by mashing, but the fermentation process may last only 24 to 48 hours. Though eggplant isn’t a watery fruit, it can be mashed and fermented for about 48 hours. Small peppers can be put directly into the mill, whereas larger fruits such as eggplant are cut into small cubes before processing.
Before processing another variety, the mill has to be disassembled and cleaned thoroughly to remove bits of small seed caught in the plates and other parts of the mill. Though some people use a food processor and blender set on low speed to extract the seed of hot peppers and eggplant, but it damages the seed.
There are some differences of opinion about adding water to the fermenting mixture, the concern being that water slows the fermentation process causing premature sprouting. As long as the ferment is not diluted more than 10 to 20 percent by volume, this isn’t a concern. The issue of whether to add water depends on how thick the mash is and it depends on the variety.
Some varieties make a very thick mash that is hard to stir, and others make a watery mash that stirs easily. The mash should be stirred three times a day, once in the morning, once in mid-day, and once in the evening. If the mash is too thick to stir easily, the nutrients are not going to circulate easily. Stirring is also important for better control of seed-borne diseases.
In addition, when the mash is not stirred, a foul-smelling, white mold forms on top of the mash. This mold can discolor (darken) and damage seeds at the top of the mash. These seeds will later have to be hand picked out of the dried seed lot. A properly fermenting mash should not have a foul smell, and there should be little or no white mold on the top. A small amount of white mold is not harmful and can be stirred back into the mash, but a heavy overgrowth should be removed.
Alkali method is relatively safe can be used for small quantities of seed in cooler temperature areas where the fermentation method is not used. The pulp containing the extracted tomato is mixed with an equal volume of a 10% solution of sodium carbonate (washing soda).
The mixture is left for upto 48 hours at room temperature and there after washed out in a sieve and subsequently dried. This method is used by vegetable breeders and other workers for maintaining breeding material and inbred parent lines. But not suitable for commercial seed production as sodium carbonate tends to darken the testa of the seed.
Acid method is often favored by large commercial seed producers as it produces a very bright clean seed. Addition of 10-12 ml of hydrochloric acid per liter of seed and pulp mixture, stirred properly and left for half an hour is successful.
In commercial seed extraction, system may be completely mechanized i. e. picking of fruits automatically before passing into a crusher or the fruits are picked and put directly into the crusher or transported to the stationery crusher. The crushed material of the fruit pulp and gelatinous seed is passed through a screen to separate the gelatinous seed from the bulk of the remaining material.
The crushed material is usually passed into a revolving cylinder screen which allows the seeds and juice to pass through the mesh, while the fruit debris passes through the cylinder screen to drop in the field. The derbies are disposed- off later. The juice and seed mixture collected in a separate container and finally separated from each other.
(2) Washing of Seed:
After fermentation & extraction, the seeds are washed to remove pulp, pieces of fruit and debris, and low quality seed. Before washing the seed, it is useful (especially for washing tomato seed) to first scoop out pieces of pulp floating on top of the mash. This is done by straining the mash with your fingers, pulling out the larger chunks. Whether or not there is floating pulp depends on the variety or how thoroughly the fruit was processed.
Add a volume of water at least equal or twice the volume of mash. It is important to dilute the mash sufficiently because more the dissolved solids in the mash, the higher the specific gravity. If the specific gravity is high (lots of soluble solids) it will be more difficult to wash the seeds properly. As a general rule, good seeds are heavy and sink to the bottom, whereas poor quality seeds are light and tend to float off with the wash.
The washing process is repeated until the wash water becomes clear. Although most good seeds sink to the bottom, some vegetables have very light seed and require extra care during the washing process. For example, it is common that a significant amount of good pepper seed floats rather than sink during washing.
This can be avoided by adding the wash water slowly, so as not to create tiny air bubbles that adhere to the seeds, making them buoyant. Even with this precaution, there are a few varieties of watermelon for example, where the good seeds tend to float rather than sink, thereby requiring special care in washing.
(3) Drying of Seed:
Seeds should be dried fairly quickly after washing. Slow drying may result in mold growth or premature sprouting of the seed. Dark colored seeds are especially vulnerable to damage when sun dried. Instead, seeds should be dried in a climate-controlled environment using fan ventilation. A combination of ceiling fans and air conditioning dries seed safely and very quickly.
Seed should be spread out in thin layers (no thicker than ¼” for small seeds) and then stirred several times a day until dry. Once the seeds feel dry, they should cure for another two to three weeks. Curing is the final stage in the drying process. As the seed moisture content declines it comes into equilibrium with the relative humidity. After the seeds are cured they can be placed in a container.
When drying seeds, choose plywood, window screen, or any hard, non-stick surface. Avoid using paper towels, newspaper, cardboard, or cloth because seeds will tend to stick to the surface making them difficult to remove. Beginning growers often make the mistake of drying squash seed on newspaper, which adheres permanently to the seed coat. However, now-a-days seed dryers with different temperature and time settings are available in the market which can be used for efficient and proper drying of seeds.
Cleaning:
After extraction the seed lot typically consists of seed mixed with inert matter such as twings, leaf and fruit fragments, soil particles, foreign seeds, dust chaff and like. The aim of seed cleaning is to eliminate all this foreign material to reduce bulk, improve storability and make seeds easier to handle during subsequent process. An ideal clean seed lot consist of all viable seeds of the target species, and is free from any other matter.
The degree to which this is achieved is called the purity, usually measured in percentage. Some cleaning methods are also used during the process of grading or upgrading in which both germination and uniformity of the seed lot are increased by removing a fraction of seeds of the same species usually the smallest and those with no or poor viability.
Seed Grading:
While the objective of seed cleaning is to improve purity by eliminating non- seed material and foreign seed from the seed lot, the purpose of grading is to improve the average physiological quality of the seed lot by removing seed of the same species with low quality. Such seed may be empty immature seed, damaged or dead seed.
In the latter case the removal also serves to improve the genetic quality of the seed lot. Sometimes a larger fraction of small yet viable seed is deliberately removed from the seed lot based on an assumed correlation between seed size and vigour.
A positive correlation between seed size and seedling size/vigour has been documented for several crops. Often large seeds tend to germinate faster and produce larger and more vigorous seedlings than small seeds of the same species.
Grading according to size can be useful to assure a more uniform germination speed and seedling growth within each grading class.
Seed Treatment:
The application of fungicide, insecticide or a combination of both, to the vegetable seeds can be useful to disinfect them from seed borne or soil borne pathogenic organisms and storage insects.
Adjusting Moisture Content for Storage:
After having obtained clean pure seed the final processing procedure is to adjust moisture content for seed that is to be stored for any length of time. The appropriate moisture content varies with species and potential storage period. Orthodox seed, which includes most dry-zone species plus most humid zone pioneers.
A seed moisture content of 6-8% is only appropriate for long term storage at very low temperature. Recalcitrant seed does not tolerate much desiccation and must be stored with high moisture content for the shortest possible time. A large intermediate group (semi-orthodox, semi-recalcitrant or intermediate seeds) shows varying levels of tolerance.
Most orthodox seed can be stored safely for at least 1-2 years at a moisture content of 8-10%. For long term storage at sub-zero temperatures a moisture content of 4-6% is desirable. Drying may be carried out by normal sun drying or artificial heating with a dry air current. If the initial moisture content is high e.g. after moisture extraction or cleaning, care should be observed not to overheat seed during the initial drying stage.
The following drying temperatures may be suggested:
1. Air temperature 30-35°C moisture content has lowered to 10-12%.
2. Air temperature 35-45°C until moisture content has lowered to 5-10%.
3. Air temperature 45-55°C until moisture content has lowered to 3-5%.
Once the seed have been dried to appropriate storage moisture content, they should be stored in air-tight container as soon as possible to avoid re-gain of moisture from the air.
Maintaining Identity during Processing:
During processing the seeds pass through a number of processes. They are unloaded and loaded into different containers and processing equipment, and often handled by a number of people. The risk of losing or accidentally mixing labels is obviously high, especially when handling a number of minor samples of the same species. A system must be created to minimize the risk of losing seed identity. Handling of labels is in many cases, as important as handling of seed itself.
Simple routine procedures are recommended:
i. Two labels should always follow the seed lot during harvesting. One is placed outside the container and another is put inside together with the seeds. The labels should be written with water-repellent ink and they should be resistant to some degree of moisture.
ii. Labels that are no longer valid should be discharged to avoid later confusion, e. g. if new labels are written because the old ones become difficult to read, or if several seed lots are mixed.
iii. When seeds are poured into trays, cleaning machines where the label cannot be kept with the seeds or where it would be easily lost by wetting or blowing away, the labels should be clipped or stuck to the processing equipment. Once the particular processing part has been concluded, the label is replaced together with the processed seeds.
iv. Invalid labels should be removed completely from the processing site so that they will not later be confused with valid labels.