Lesson 48. Extraction and Storage of Vegetable Seeds

48.1 EXTRACTION OF VEGETABLE SEEDS

The vegetable seeds are separated from the mixture having impurities by various processes as described below:   

48.1.1 Separation based on weight (or specific gravity):

Cleaning seed by differences in specific gravity is one of the oldest seed cleaning techniques. When done by hand in the wind it is commonly referred to as winnowing. On the simplest scale, seed and materials are dropped before a wind source (either natural wind or a fan). The heavier materials fall closer to the wind source while lighter materials are carried further from the wind source. On a small-to-medium scale this is a very effective method to quickly clean seed. Many screen cleaners have a fan to assist in blowing off some dust and chaff.

Gravity tables are probably the most widely used machines. They separate seeds by differences in seed weight. Gravity tables operate by blowing air up through the body of the machine which holds a fine-screened, tilted "deck" on the surface. The seed is fed by a hopper and passes over the deck. The screen openings are small, so no seed or material passes through, and the screening material has a slight nap that 'holds' the seed. The air moving through the screening lifts lighter material - it then 'floats' down the deck; the heavier material remains in contact with the screening material because of the nap - it moves up the deck.

48.1.2 Cleaning wet-seeded crops:

Wet seeds are seeds that are produced in a fruit such as tomatoes, cucumbers, and squash. They are processed by crushing the fruits by hand or with a wet seed extractor. Some wet-seeded seed crops benefit from a fermentation process following the extraction. Others can simply be washed thoroughly and dried.

a. Wet processing of seed: It involves following three steps:

  1. Removal: For seeds located inside fleshy fruits and vegetables such as squash, cucumbers, and tomatoes, wait for the fruit to fully ripen on the plant. Typically, fruit is harvested at a stage that is much more mature than the maturity at which the fruit would be harvested for eating. Wash the fruit, then break open the fruit and remove the seeds. Clean pulp residue from seed by washing or fermenting (see below).
  2. Drying: Spread the seeds out on a tray in a warm, dry place, and let dry. Seeds should not reach temperatures over 95 F.
  3. Fermentation: Seeds such as tomato, pepper, cucumber, and squash are typically processed by fermentation as it facilitates removing the pulp from the seed and in some cases kills certain bacterial seed borne pathogens.

b. Fermentation of wet-seeded crops: It involves following nine steps:

  1. Rinse dirt and debris from fruit.
  2. Mash the fruit and pour the mixture of seeds, pulp, and juice into a large container (garbage cans work well).
  3. Place bucket in a location at 75-80 F (24-27 C). Ferment tomatoes for up to three days and squash for up to 2 days, depending on the ambient temperature. At 75 - 80 F, fermentation may require 42 to 72 hours.
  4. Stir the fermenting mixture two or three times a day to aerate the mixture and facilitate even fermentation.
  5. In two to three days a white scum may appear on the top of the mixture. This is normal and indicates that fermentation is successfully taking place. After the two- to three-day period, seed is ready to be rinsed. Pour off the top layer of scum and pulp. Pour water into the remaining mixture so that the volume is doubled.
  6. Stir, allow the mix to settle again, and pour the top layer of pulp and debris off the top. Some lighter, less viable, seed may be poured off with this top layer.
  7. Repeat this washing process from 3 to 6 times, until the water is fairly clear.
  8. Pour the remaining contents (seed) through a large strainer retaining the seed and draining off the remaining water.
  9.  Spread the drained seed out onto a fine screen to dry. Fine mesh window screening works well for most seeds. Distribute the seed on thescreen so that the seed layer is as thin as possible (less than 1/4 inch). Avoid placing seed on paper, as drying seed may stick to the paper and paper may hold moisture. Stir the seed frequently to encourage even drying. If possible, place seed in front of a fan or gentle breeze to facilitate drying.

 48.2 SEED STORAGE

The loss of viability impairs the biological value or function of seeds, which is to protect and nourish the living cells of the embryo, until seedling is established. The cultivator is concerned with the phenomena of seed longevity because he wants high germination and vigorous seedlings from the seed which he has planted. Nursery men concerned with seed viability to get maximum return of it. While scientist accept high longevity of this is breeding stocks and not to loose the value able material through death of seed. For all mankind it is concerned with seed longevity since the genetic material concerned in seed bank may be essential to our survival in changing ecology of this world in future.

In the tropical climates where both temperature and humidity are high, seed storage often present problem which are lacking in temperate conditions. Seed longevity is affected by several factors as follows.

  1. Seed factor: Seed storage begins immediately after maturity regardless of where or seeds are held. Viability of seed varies with the crop and it is short lived in onion, beans, cowpea, and capsicum seeds. In heritance of seed longevity is not limited to the species but also to cultivars. Generally healthy, pulpy and well matured seed stored better than immature seed. The moisture content of the seed during storage is no doubt the most influential factor affecting their longevity. It is important to harvest mature relatively dry seeds or to reduce the moisture content of freshly harvested seed soon after harvest.
  2. Storage environment: The storage requirements for the maintenance of viability vary for different type of seed. Storage temperature and seed moisture content are the two storage condition both relative humidity and temperature are kept low of the two seed moisture content in relatively more than temperature. Temperature are also play in important role in of seed although it does not appear to be a controlling with normal range of biological activity of seed, insect and mold increase as temperature increase. Higher the moisture content of the seed the more they are adversely affected by the temperature. Decreasing temperature and seed moisture content there for is an effective means of maintaining seed quality in storage. Harrington (1972) stated that life of seed halves (1) for every 50°C increase in storage temperature (2) for increase in every one percent of seed moisture. This holds well between 5-14 percent of seed moisture and 0 to 50°C temperature most of the vegetable seed ideal stored at 6-7% moisture content.
  3. Storage container:  A storing of seeds in the containers made up of suitable packing material will prevent the direct contact of seed with the storage environment and this is another approach for retaining viability. The packaging materials used are paper, cotton, metal, plastic, glass and laminated foil. These are selected according to kind and amount of seeds to be packed, during of storage etc.
  4. Storage fungi: The storage fungi comprise mainly aspergillum and pencillium spp which affects the seed storability. These fungi will grow successfully at moisture content equivalent to RH of 85% or above. Storage fungi affect the seed by decrease in germination, discoloration, production of mycotoxins, heating and total decay. Chemical treatment with fungicides prevents the attack of storage fungi.
  5. Physiological and biochemical changes in seeds  during storage:  Among many physiological manifestation of seed deterioration are changed in seed colour, delayed germination, decreased tolerance to sub optimal environmental conditions during germination and storage conditions, reduced germination and seeding growth and increased number of abhoma/seedlings. Biochemical changes includes 1) increase or decrease in enzyme activity, 2) decrease in oxygen up take, 3) increase in leaching of organic and inorganic constituents from seed, 4) increase in free fatty acid, 5) decrease in total soluble sugar, 6) increase in reducing sugar and decrease in total soluble sugar, 7) decrease in protein and increase in amino acid, 8) changes in carbohydrate, organic acid and protein metabolism.

To conclude, a quality seed is required for better establishment of seeding in field as well as for higher crop production. This could be achieved through

a. Production of seed under ideal climatic and enrich soil condition

b. Supplementation of seed through seed treatment

48.2.1 Proper handling and storage of seed for higher longevity.

The proper handling and storage of seeds for higher longevity depends upon various factors namely storage temperature and relative humidity. The recommended temperature and relative humidity for storages of some vegetable seeds are listed in Table 48.1 and Table 48.2.    

          Table 48.1: Recommended temperature, relative humidity and approximate storage life of various vegetable seeds:

==========================================================                                           

       Vegetables      Temperature (ºC) Relative humidity (%)  Approximate Storage life

===========================================================

Beans: Lima in pods        40-45                         95                                       7-8 days

     Lima shelled                       37                           95                                       10-14 days

 Dolichos lab in pods        32-35                        90                                        2-3 weeks

   Snap bens                         38-42                        95                                       10-14 days

    Winged                                32                       90-95                                      2-3 months

   Beet root                           32                       90-95                                      2-3 months

   Bitter gourd                    33-35                    85-90                                       1 month

   Brinjal                            50-55                   90-95                                        2-4 weeks

   Cabbage, early                  32                      95-98                                        3-6 weeks

   Cabbage, later                31-32                   95-98                                         4-5 months

   Carrot topped                   32                        95                                             5-6 months

   Cauliflower                    32-35                   85-95                                         5-8 weeks

   Celery                             31-32                   92-95                                         8 weeks

   Coriander leaves             32-35                  90                                               5 weeks

   Chow-chow                    52-55                  90                                                3 weeks

   Cucumber                       50-55                  95                                               10-14 days

   Garlic (bulbs) dry             32                   60-65                                              7-8 month

   Ginger                               55                      65                                                5-6 months

   Gourd, bottle                     45                  85-90                                              4-6 weeks

   Gourd, snake                      65-70            85-90                                               2 weeks

   Green, various leafy             32                 95                                                10-14 days

   Knol, knol                         33-34               90                                                  3 months

   Mushroom                         32                  95                                                    1 weeks

   Muskmelon, cantaloupe   35-38             85-90                                                10 days

   Muskmelon honeydew     45                    85                                                   4-5 weeks

   Okra                             45-50                   90-95                                             1-2 weeks

   onion, white                   34                      70-75                                               4-5 months

   onion red                       32                  70-75                                                 5-6 months

   pea, green                        32                    88-92                                              2-3 weeks

   pepper, sweet(green)   45                     85-90                                                3-5 weeks

   potato                         38-40                 85                                                     7-8 months

   pumpkin                     35-60               70-75                                                  3-5 months

   radish, topped            32                   88-92                                                      3-5 weeks

   spinach                       32                     95                                                      10-14 days

   squash, summer           50                   95                                                        1 week

   squash, winter            50-60            60                                                         3-4 months

   sweet potato               55                90                                                          4-5 months

   tapioca root                32-35          85                                                          5-6 months

   tomato, mature green  50-55           85-90                                                   5-6 weeks

   tomato,  ripe               35-45          85-90                                                     5-7 days

   turnip                          32               90-95                                                   2-4 months

   watermelon              45-60             80-90                                                    2 weeks

   yam                          80                  60-70                                                   3-5 weeks

----------------------------------------------------------------------------------------------

 

Table 48.2: Recommended temperature and relative humidity approximate storage life of fresh fruit in commercial storage.

Commodity    Temperature     Relative Humidity       Approximate storage period

                          (ºC)                          (%)                       

Apples                   30-40                          90                                    3-8 months

Apricots                31-32                           90                                    1-2 week

Avocados              40-55                           85-90                              2-4 week

Banana                  56-58                           90-95                              -

Strawberry            32                                90-95                             5-7 days

Cherries                32                                90-95                             3-7 days

Coconuts               32-35                          80-85                          1-2 months

Dates                   0 or 32                        75 or less                       6-12 months

Figs fresh             31-32                            85-90                           7-10 days

Grapes vinifera     30-31                         90-95                                3-6 months

Guavas                 45-50                              90                                  2-3 weeks

Lemons                  -                                85-90                                 1-6 months

Limes                48-50                           85-90                                 6-8 weeks 

Litchies                  35                             90-95                                  3-5 weeks

Mangos                 55                              85-90                                  2-3 weeks

Oranges            38-48                             85-90                                  3-8 weeks

Papayas              45                                85-90                                  1-3 weeks

Peaches             31-32                              90                                      2-4 weeks

Pears                  29-31                           90-95                                   2-7 months

Pineapples        45-55                            85-90                                   2-4 weeks

Pomegranates      32                                90                                       2-4 weeks

_____________________________________________________________

Last modified: Tuesday, 13 August 2013, 5:08 AM