It is necessary to dry moist seeds before processing and storing. Seeds with high moisture content are more susceptible to physical damage during processing. This reduces viability and encourages the formation of molds.
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 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 which are not welldried 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 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.
Things to Remember in Drying Seeds
1. 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.
2. Use a drying material with holes (example: sack, winnowing basket, mat) to allow air to pass through, giving fast, even drying.
3. 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 11:00 a.m. to 2 p.m. when the heat of the sun is intense because it will kill the seeds.
4. Spread the seeds thinly and stir and turn them occasionally (at least 4 to 5 times a day) to make drying fast and even.
5. 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 which 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 these, 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 - weight of seeds is 10 grams After oven drying - weight of seeds is 9 grams
Lost water 1 gram
However, it is not easy to obtain an oven to determine the moisture content of the seeds so the practical methods above are recommended.
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