IV. Seed Drying

Prediction of drying period
Seed drying procedures

What is meant by seed drying?

This is the reduction of seed moisture content to the recommended levels for seed storage, using techniques which will not be detrimental to seed viability.

Why are seeds dried?

Seeds which are dry will retain their viability for longer periods of storage in genebanks. It is recommended that, in general, seeds should be dried to between 3-7% moisture content for long-term storage, except in certain cases where it has been shown that low moisture content causes problems e.g. soyabean should be dried to about 8% moisture content. Drying has an independent effect from temperature on viability during storage and adequate drying could prolong viability for reasonably long periods without cold storage.

When should seeds be dried?

The drying process should begin as soon as possible after the receipt of the seeds to avoid unnecessary deterioration. It can take up to several weeks to lower the moisture content to the low levels required for good storage.

How should seeds be dried?

Several methods are available for drying seeds. Some are more suitable in certain environments and safer for the viability of the seeds than others. Drying seeds in an atmosphere of reduced relative humidity is recommended. The lower the humidity the faster the seeds will dry and the lower their final moisture content. A relative humidity of 10 - 15% and a temperature of 15°C are recommended by the IBPGR Advisory Committee on Seed Storage: try to obtain these conditions as closely as possible. The following methods can be used with safety and one should be chosen depending on the equipment available.

Prediction of drying period

If there is no previous experience of drying seeds of a particular species it may be necessary for you to do some experimental work to predict the approximate drying period. Seeds dry at an exponential rate until the equilibrium moisture content is reached. The rate of drying of different seed lots of the same species is fairly constant for seeds dried under the same environmental conditions. The length of the drying period can be predicted in one of two ways:

Prediction of the correct drying period by weight loss

1. Predict the current percentage moisture content and the percentage moisture content required for storage.

2. Weigh the seed sample.

3. Use these three values to calculate the weight of seeds at the required moisture content by using the following formula:

4. Weigh the seed sample at regular intervals during the drying period until the weight of the seeds has reached this calculated value.

Notes and Examples

The prediction of the drying period will be more accurate if the initial moisture content is determined. However, this is a waste of seeds when moisture content can be adequately predicted.

1000 g of seeds with 12% moisture content were dried to 5% moisture content. What would be the weight of these seeds after drying?

Substitute the values in the equation on the facing page:

Therefore when the initial 1000 g of seeds have been dried to 926.3 g, their moisture content will have decreased from 12% to 5%.

Coarse balance

1. Do not waste valuable seeds for this, but use either excess seeds or those which are being discarded because they have lost viability.

2. Take at least two lots of seeds of the species and place to dry, using the method that you will use in practice.

3. Remove a sample of seeds and do an accurate determination of moisture content for each seed lot as described in Section III.

4. The mean of the two tests can be used as a guide because other seed lots of the same species should dry at a similar rate.

5. Repeat the determination daily and plot a graph of the drying curve (mean percentage moisture content against time) for that species under these drying conditions. Your graph will be similar to the one on page 44.

6. The work can be repeated with seeds of all species which are of interest and their drying curves plotted for different conditions.

Notes and Examples

If the seeds are very small and are dried quickly, the moisture content can be determined more frequently during the drying period to plot a more accurate graph.

Grinder
Heat resistant dishes with covers
Analytical balance
Forced draught oven
Desiccator
Silica gel
Tongs and oven cloth

1. Use the graph that you have prepared for seeds of a particular species being dried under those conditions.

2. Predict the current percentage moisture content of the accession by using the table of equilibrium moisture contents. Select the final percentage moisture content that is required for storage of this species.

3. Find the value for each of the percentage moisture contents on the vertical 'Y' axis, follow this across to the curve and read off the day on the horizontal 'X' axis for each of the moisture contents. The difference between these two values is the drying period in days.

Figure 2. Typical drying curve

Notes and Examples

Example of how to predict the drying period by using a typical drying curve for small seeds of crops such as onion or cabbage, dried quickly under low relative humidity conditions:

Figure 3. Typical drying curve of small seeds

Predict the moisture content by using the table of equilibrium moisture contents for your genebank which you have prepared. As an example take this as 12% and the desired moisture content after drying as 5%. Using the graph, the intersects from the curve to the time axis show the values 10 and 2.25 days.

Therefore, the time to dry the seeds to 5% moisture content under the same conditions will be the difference between 10 and 2.25 days. This can be approximated to the nearest day, in this case 8 days.

In practice it is unlikely that using the graph will give you a whole number of days, therefore you should approximate to the nearest day.

Graphs of mean drying curves in your genebank

Plot graphs in the space below to use for future reference:

1. Species:
Temperature:
Relative humidity:

2. Species:
Temperature:
Relative humidity:

3. Species:
Temperature:
Relative humidity:

4. Species:
Temperature:
Relative humidity:

5. Species:
Temperature:
Relative humidity:

6. Species:
Temperature:
Relative humidity:

7. Species:
Temperature:
Relative humidity:

8. Species:
Temperature:
Relative humidity:

Seed drying procedures

STEP 1. PREDICT MOISTURE CONTENT AND DRYING PERIOD

1. Use the experience and techniques already described in Section III to predict the moisture content of the accessions.

2. Predict the length of drying period required from your previous experience of drying seeds of the same species with similar moisture contents, as already described. Information on drying periods can also be found in Cromarty, Ellis and Roberts (1982).

Notes and Examples

Predict the moisture content of each accession and the number of days to reach the required moisture content. Remember that seeds will dry more slowly as the moisture content decreases towards the equilibrium moisture content. At low moisture contents a difference of one or two days of drying will not have a large effect on seed moisture content.

1. Place each accession in labelled paper envelopes or cloth bags.

2. Do not put too many seeds in the same bag. One accession can be split into several labelled bags to help rapid drying.

Notes and Examples

Make sure that the bags are porous. Seeds will not dry in plastic bags!

Several methods are available for drying seeds. The safest methods rely on leaving the seeds in an environment of low relative humidity and allowing the seed moisture content to reach equilibrium with this at relatively low temperatures. Seeds will equilibrate with the relative humidity at different rates, depending on species, seed size and conditions. Seeds dry quicker when first placed in the room and more slowly as the low moisture contents suitable for storage are approached. Two common and safe methods are described here:

Notes and Examples

Choose the best drying method for your own genebank. In tropical countries with high relative humidities, it will be more difficult to hold a drying room at very low relative humidity. A combination of methods is possible. Silica gel can be used after the first drying period if the relative humidity is too high to allow the seeds to equilibrate to low moisture contents.

Do not use sun drying because it is believed to affect long term seed viability of some species. Heated air drying should also be avoided for the same reason.

Aim for 10 - 15% relative humidity and 15°C as the optimum drying conditions.

1. Use a dehumidifier dryer to dry a small volume of air in a drying room or other limited space. The humidity of this room will depend on its size, the ambient humidity of the area and the efficiency of the dehumidifier.

2. Place the seeds which have been packaged in labelled porous containers in the drying area.

3. Do not stack the bags too closely and use open racks in a drying room or cabinet with a fan to allow air to circulate.

4. Measure the relative humidity and maximum and minimum temperatures of the room daily.

5. Leave the seeds in the drying area until the moisture content is predicted to be in the range required for storage.

Equipment

Dehumidifier dryer or cabinet
Envelopes
Porous boxes or containers
Maximum minimum thermometer
Aspirated hygrometer
Racks
Coarse balance
Spatula/spoons

1. Use deep blue silica gel in an enclosed space, such as a desiccator, can or glass jar with an air-tight seal.

2. Place either the silica gel or the seeds in a porous bag.

3. Use a weight of silica gel equal to the weight of seeds for rapid drying.

4. Place the required weight of silica gel in a desiccator, jar or can with the correct weight of seeds. When using cans or jars make sure that the silica gel is in close proximity to, but not touching, the seeds.

5. Place the containers in a room held at approximately 15 °C whilst the seeds are drying.

6. Change the silica gel daily or when the colour changes from deep blue to pale blue or pink.

7. Heat the pale blue or pink silica gel in an oven above 100 °C until it turns deep blue again, when it is ready for re-use. Store in an air-tight container.

8. Leave the seeds with fresh changes of silica gel in the container until the moisture content is predicted to be in the range required for storage.

Notes and Examples

Desiccators are useful but expensive. Cans and jars with air-tight seals are cheaper and adequate and also allow separation of seeds of different accessions. Silica gel is cheap and easy to use. It can be made into permanent packages of fixed weight in fine cloth bags. These can be heated in the oven without damage and are quicker and easier to handle than loose silica gel.

The silica gel is used to reduce the relative humidity of the atmosphere surrounding the seeds. The seeds then dry by equilibration with their surroundings.

Silica gel
Desiccators, air-tight cans or bottles
Forced draught oven from 50 - 200°C
Coarse balance
Spatula/spoons

1. When you consider, from experience, that the seeds are dry enough, remove a sub-sample from each accession and carry out a moisture content determination as explained in Section III.

2. When the moisture content is between 3-7 % for most species, go to packaging (Section VI). Low moisture contents are detrimental to the viability of the seeds of some crops. These seeds should be treated with care and not dried to low moisture levels, e.g. soyabean should not be dried below 8% moisture content.

3. If the moisture content is not low enough, continue to dry for a further period as described above. When the moisture content is predicted to be in the correct range for seed storage, carry out an experimental determination as described in Section III.

Notes and Examples

At this point it is necessary to do an experimental determination of moisture content to ensure that predictions of storage life of this accession are as accurate as possible.

Make sure that seeds which have been dried are kept in the drying area or moisture-proof containers whilst tests are carried out to prevent absorption of moisture from the surroundings.

Grinder
Heat resistant dishes with covers
Analytical balance
Forced draught oven
Desiccator
Silica gel
Tongs and oven cloth

Table of approximate drying times in your conditions

Fill in this table for your future reference:

Species	Mean initial percent moisture content	Days required to reach 5% moisture content