Drying is a normal part of the seed maturation process. The amount of moisture in the seed is probably the most important factor influencing the longevity and germination capacity of the seed. Seeds of fleshy fruits such as tomato, cucumber and melons, have much higher moisture content at harvest and may absorb more water during their wet extraction process. On the contrary, the seeds, which become desiccated during the ripening process, are relatively dry at the time of harvest, e.g., onion, amaranthus, brassicas, etc.
Some seeds must dry down to minimum moisture content before they can germinate. Low seed moisture content is a pre-requisite for long-term storage and is the most important factor affecting longevity. Seeds lose viability and vigor during processing and storage mainly because of high seed moisture content (seed moisture greater than 18%).
Lowering down the vegetable seeds moisture content to safe moisture limits is very important in order to maintain seed viability and vigour and save from mould growth, heating and increased micro-organisms activity.
By the time the vegetable seeds are separated from the mother plant, the seed moisture content is below fifty per cent and there after the moisture content is in equilibrium with the storage atmosphere. It is frequently necessary to dry vegetable seeds immediately after threshing and cleaning or when they first arrive at the processing plant from the field, after extraction from fruit or possibly after processing but before storage and packaging.
Mainly three methods of seed drying are followed viz. natural drying, sun drying and artificial drying. The natural drying helps the seeds to loose water naturally in standing seed crop at and after maturity and needs no help of farmers. The extent of natural drying is controlled principally by weather and maturity at harvest.
In sun drying, seeds are dried by spreading the seed material on floor under diffused sun light or under shade. Stirring of lots should be done frequently to facilitate rapid drying. On large scale seed production, natural and sun drying becomes inadequate and need to depend on artificial drying.
Problems of High Seed Moisture:
i. Moisture increases the respiration rate of seeds, which in turn raises seed temperature. For example, in large-scale commercial seed storage, respiring seeds may generate enough heat to kill the seeds quickly.
ii. Mould growth will be encouraged by moisture, damaging the seeds either slowly or quickly, depending on the moisture content of the seeds. Some moulds that don’t grow well at room temperature may grow well at low temperatures causing damage to refrigerated seeds. In such a case there may be no visual sign of damage.
iii. Unless seed moisture is at least eight percent or below, insects such as weevils can breed causing rapid destruction of seeds in a short period of time.
Drying Seeds for Long-Term Storage:
Silica gel is the most effective desiccant (moisture absorbing material) for drying seeds. Powdered milk has been recommended as a desiccant in older seed- saving literature, but is less than ten percent as effective as silica gel. Silica gel is a highly porous form of silica that absorbs moisture. It is available as a powder or as beads in different sizes.
The best size bead for drying seeds is approximately 1/16″ to 1/8″ in diameter. “Color-indicating” silica gel is a form of silica gel that has been treated with a small amount of cobalt chloride which acts as a moisture indicator.
When the indicator gel is completely dry it is a deep blue color. As the gel absorbs moisture from the air, it gradually changes in color from deep blue to light pink. Silica gel can be repeatedly reactivated (re-dried) after it has absorbed moisture.
The procedure involves heating the gel and driving off the moisture, and as it dries the color gradually changes to deep blue. Drying must be done in a controlled fashion, otherwise the beads will turn black, and the moisture-absorbing capacity of the beads will be destroyed.
There are two methods for reactivating silica gel:
This method gives the best results, but it takes longer time and uses more energy. Set the oven for a temperature of at least 200°F, but not higher than 275°F. Remember, that some ovens may run hotter than the set temperature. Place the silica gel in a thick-walled Pyrex dish, no deeper than one inch, and continue heating until the beads turn deep blue.
When drying large quantities (a pound or more), the gel should be stirred occasionally. The oven drying method takes 1 – 1/2 hours per quart of gel (with the oven temperature set at 275°F). One quart of silica gel weighs approximately 30 ounces (1.9 pounds).
The microwave method works much faster but must be monitored more closely to avoid overheating the gel. Use only a thick-walled Pyrex container for heating the gel. Set the microwave on medium or medium high and dry for approximately three to five minutes. The colour change in the gel can be monitored through the over door, but the gel should be inspected and stirred at the end of each heating cycle.
If the gel has not dried, heat again for another three to five minutes. Approximate drying time is eight to twelve minutes per pound of gel, though actual heating time will depend according to the type of microwave. There are some fine points and cautionary notes about using silica gel. When using a Pyrex dish, the glass should be thick. Do not use plastic microwave containers that will melt on contact with the gel.
Silica gel gets very hot, and the glass container may shatter if it is too thin, of the wrong type, or if unevenly heated some time may notice a slight odour during heating. This is due to either overheating the silica gel, or to the evaporation of organic seed volatiles absorbed by the silica gel during the drying process. Silica gel itself is chemically inert, non-toxic, non-corrosive, and odorless, but breathing the dust can be hazardous under prolonged and repeated exposure. For that reason, do not use the finely ground or powdered form.
Instructions for Drying Seed with Silica Gel:
To dry seed, determine the weight of the seed to be dried, including the packets or envelopes that contain the seed. Measure out an equal weight of silica gel and place the seeds and silica gel in an airtight container for seven days. When drying seed it is important to keep the container size small in relation to the volume of seeds being dried.
At the end of seven days, remove the packets of seed from the drying container and transfer into another airtight container, such as a Mason jar, or barrier pouch. Because seeds can re-absorb moisture from the air quickly, they should be transferred quickly.
When used as directed, silica gel dries seed from 12% typical moisture content to a desired moisture content of approximately 5% for small seeds and 7% for large seeds. As a “rule of thumb”, seeds will not be damaged provided the drying time doesn’t exceed seven days. This drying time applies to humid climates. Longer drying may drop the moisture content below 3% for small seeds and below 5% for large seeds, levels which may damage seeds or force them into dormancy. Legumes, such as beans, are especially are injured by over drying.
Artificial Drying Using Seed Dryers:
Drying rate at which a seed lot can be dried artificially depends on the packing character of the species and the initial moisture content of the sample. The artificial unthreshed seeds of garden pea and bean material be dried by air at 3.50 m2 of floor area and should not exceed 2 m2. For radish, it should be upto 4 m2 when the unthreshed moisture content was at 30-45 per cent.
Lettuce and cucurbits are generally considered to be relatively quick dryers, carrot, beet and tomato medium dryers but legumes, brassicas and onion are slow dryer. Generally at 43.5° C, 0.3% seed moisture content is removed per hour by an air flow rate of 4 m3 per cubic meter of seed.
Usually the rotary paddle dryers are used for the drying of tomato, chilies, bell pepper, brinjal and cucurbits seeds which have been extracted from fruits by a wet extraction process. The wet seed is placed over a finely perforated surface and dried by a current of hot air which blows through the perforations and layer of seed from below.
As the hot air passes between the seed the moisture is removed to the outside air. The air temperature is controlled between 37 and 40 °C at the start but when the moisture content beings to lower, the air temperature is reduced to 32 – 35 °C for the wet extracted solanaceous and cucurbit seeds.
The total time taken for drying a batch of wet extracted seed depends on the vegetable seed crop but it is approximately 8 hours for brinjal, chillies and bell pepper, 10 hours for cucumbers and tomatoes and 12-16 hours for the larger seeded cucurbits such as squashes, depending on the cultivar’s seed size. Drying is repeated if required until complete drying of seeds. For commercial drying continuous drying systems designed either as vertical seed flow or a horizontal bed on a perforated belt is in use.
Moisture Equilibrium between Seed and Air:
Air temperature, air flow and relative humidity are the controlling factors for drying of vegetable seeds. Usually higher temperature with lower, humidity and steady flow of air surrounding the seed surface would facilitate withdrawal of water from the seed.
On the other hand, dehydration tends to hold on more strongly to the residual moisture in seed tissues. When moisture retaining capacity of the seed is in balance with the moisture withdrawing capacity of the air, the seed neither loses nor requires moisture and is said to be in equilibrium with the air.