A variety of seeds are commonly stored for consumption or processing in the future. The primary concern in such storage is to protect the seed from damage by fire, water, insects, and rodents. However, seeds must also be preserved for sowing at a later date or for genetic experiments. When stored for such purposes, the seeds must also be protected from loss in germination potential.
The process of elimination of moisture from the seed is called drying. Seed drying should reduce the seed moisture content to safe moisture limits to maintain its viability and vigor during storage, which may otherwise deteriorate quickly owing to mold growth, heating, and enhanced microbial activity.
It is necessary to dry moist seeds before processing and storing them. Seeds with high moisture content are more susceptible to physical damage during processing. This reduces viability and encourages the formation of mold.
In addition, the germination of moist seeds that are stored can be severely reduced. In this condition, the respiration of the seeds and of the microorganisms present in and on the seeds may produce enough heat to kill the seed. Excess moisture favors the infestation of insect pests. It also increases the respiration of the seeds, consuming the stored food of the seeds and resulting in weak seedlings. Seeds that are not well dried have high respiration rates, causing them to rot. Usually, the moisture content of seeds after harvesting is high, especially when they are cleaned by washing.
If the air is humid, dry seeds absorb the water from the air. If the air is dry, it absorbs water from wet seeds. This is why air-drying can dry wet seeds. This is also the reason seeds are stored in air-tight containers after they have been properly dried.
Factors that Affect the Longevity of Seeds During Storage:
- Moisture Content of the Seed — Even if seeds are thoroughly dried, improper storage can still enable them to absorb water. To avoid damage caused by excessive moisture content, (1) store seeds in air-tight containers (bottle with tightly closed metal cover, tin can, sealed thick plastic); (2) keep seeds dry by including desiccants or materials that absorb moisture (example: dry charcoal, dry ash, toasted white rice, lime, silica gel) inside the storage container, and, (3) replace desiccants, such as dry charcoal, dry ash and toasted white rice, each time the container is opened. The moisture content of the seeds can also be kept low if the seeds are sun-dried from time to time.
- Temperature — The life of vegetable seeds during storage is prolonged when the storage temperature is low or cold (but not freezing). If a refrigerator or air conditioner is not available, choose a cold place (for example: near the river, under trees, underground, inside a clay jar). Ensure that the seeds will not get wet.
As a general rule:
The life of seeds doubles when the moisture content is lowered by 1% or when the storage temperature is lowered by 5°C.
- Pests — Storage weevils, fungi, and bacteria shorten the life of seeds during storage. Storage weevils begin to multiply when the moisture content is 10%. Fungi infestation becomes a problem when the moisture content is 13%. Bacteria become a problem when the moisture content is above 20%. To prevent pest infestation, choose only pest-free seeds during storage. Pest problems can also be prevented if the seeds are maintained dry. Materials that prevent or stop the growth and multiplication of pests can also be used. These are: Dry ash and charcoal, Sand, Cooking Oil, Lime, Dried and powdered leaves or seeds of different aromatic plants, etc.
- Other factors — The storage life of seeds can become shorter if the seeds are over mature, if they came from plants that have been attacked by pests and diseases or if the seeds were damaged during seed processing.
Things to Remember in Drying Seeds
- Do not allow the seeds to come in contact with the soil or ground. This will prevent the seeds from getting in contact with soil microorganisms that will lower the quality of the seeds. Use a wedge so that the seeds can be dried above the ground.
- Use a drying material with holes (example: sack, winnowing basket, mat) to allow air to pass through, giving fast, even drying.
- Do not dry the seeds rapidly because it will lower seed germination. Rapid drying can also harden the seed coat, making the seed impermeable to water when planted. If the initial moisture content of the seeds is high, air-dry the seeds in a shady area for one to two days before sun-drying. Do not dry seeds under the sun from 1:00 p.m. to 2 p.m. when the heat of the sun is intense because it will kill the seeds.
- Spread the seeds thinly and stir and turn them occasionally (at least 4 to 5 times a day) to make drying fast and even.
- Before it rains or gets dark, cover the seeds and take them indoors to prevent their moisture content from increasing.
How to Determine if Seeds are Well-dried
1.Seeds that were harvested dry have enough moisture content when they are dried under the sun for 2-3 days. If seeds were harvested wet or were washed before drying, 3-5 days sun-drying is enough after they have been air-dried for 1-2 days.
2. Seeds have distinct sounds when their moisture content is already low enough for storage.
a) Large, thin seeds will break with a “snapping” sound when twisted between the fingers.
Example: squash, bottle gourd
b) Large, thick seeds will break with a “cracking” sound when bitten between the front teeth. Do not do this for very hard seeds because it might damage your teeth. Also, avoid this if the source of the seeds is unknown since they might have been applied with chemicals.
Example: ladyfinger, cowpea
c) Small seeds will break with a ”cracking” sound when squeezed between the fingernails.
Example: mustard, pechay, amaranth
3. Seeds have a distinct tinkle when they are well-dried.
4. If possible, use an oven that can reach a temperature of 100°C or higher. Weigh the seed sample before placing it inside the oven. Weigh the seeds again after drying for 17-20 hours inside the oven. The lost weight indicates how much water was lost after the seeds have been dried. From there, the percent moisture content of the seeds can be computed. The seeds are dried enough for storage when they reach a moisture content of or less than 10%.
Example: Before oven drying – the weight of seeds is 10 grams After oven drying – the weight of seeds is 9 grams
Lost water 1 gram
% Moisture Content of the Seeds = 1 / 10 x 100= 10%
However, it is not easy to obtain an oven to determine the moisture content of the seeds so the practical methods above are recommended.
