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III. Seed Moisture Content


Prediction of seed moisture content
Determination of seed moisture content

What is seed moisture content?

The moisture content is the amount of water in the seed and is usually expressed as a percentage. It can he expressed on either a wet weight basis (where it is expressed as a percentage of the fresh weight of the seed) or on a dry weight basis (where it is expressed as a percentage of the dry weight of the seed). For genebank work it is usually expressed on a wet weight basis and all determinations and calculations should follow this rule.

The seed moisture content can either be accurately determined experimentally by scientific techniques, or it can be predicted approximately from the information available. The determination is destructive to the seeds used and since in many cases a prediction is sufficient, determination should only be carried out where essential.

Why is it important to determine moisture content?

A small change in seed moisture content has a large effect on the storage life of the seeds. Therefore it is important to know the moisture content in order to make a reasonably accurate prediction of the possible storage life of each accession.

When should moisture content be determined and when should it be predicted?

Accurate determinations should be kept to a minimum because for many activities a prediction is sufficient. In most cases this means one accurate determination after the drying period, so that the moisture content of the stored seeds is known and can be used to make better predictions of storage life.

How should moisture content be determined and how should it be predicted?

The methods outlined in the following pages can be used as a guide to prediction but experience in dealing with a crop in your own genebank will be necessary for reasonable predictions to be made. The methods for determination are modified from those of the International Seed Testing Association (ISTA) as recommended by the IBPGR Advisory Committee on Seed Storage. Other methods are available and can also be used provided that the results give an accurate determination of moisture content and are calculated on a wet weight basis, to enable comparisons to be made between materials and between genebanks.

Prediction of seed moisture content

The possibility of making reasonably accurate predictions of moisture content will improve as you gain experience with seeds of different species and use your knowledge of the origin of the seeds and the conditions in which they have been stored. The moisture content of a seed will equilibrate with the relative humidity of the air surrounding it. This moisture content is called the equilibrium moisture content and because it is constant for any species at known temperature and relative humidity, it can be used as an approximation of the actual seed moisture content.

Moisture contents will differ from seed to seed within the same accession, but should not be sufficiently different to make more than about 1% difference in moisture contents between seeds of the same accession and different accessions of the same species. This is sufficiently close for a prediction.

A list of a few common species and their equilibrium moisture contents is provided for guidance (Table 1). Further information is available from Cromarty, Ellis and Roberts (1982). For species not on the list, it is possible to experimentally determine equilibrium moisture contents for these species at the temperatures and relative humidities at which you are working and prepare your own table to use for future predictions.

Determination of seed equilibrium moisture content

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

2. Take two lots of seeds of the species of known weight and leave them in open containers in the environment of your genebank to equilibrate. This may be several days for small seeds and longer for large seeds. Seeds will have reached equilibrium with the moisture content of the air when there is no change in weight on different days.

3. Measure the relative humidity of the atmosphere where the seeds have been held for the past few days by using an aspirated hygrometer.

4. Measure the mean temperature.

5. Remove a sample of seeds and do an accurate determination of moisture content for each seed lot.

6. The mean of the two tests can be used as a guide because other seed lots of the same species should equilibrate at a similar moisture content under these conditions.

7. Repeat the determination the following day to make sure that the moisture content is stable and that the equilibrium has been reached,

8. The work can be repeated using a number of species which are of interest at a range of relative humidities that are common in your country at different times of the year.

9. Make a table of the equilibrium moisture contents at different temperatures and relative humidities and use this to refer to in the future.

Table 1. Equilibrium moisture contents of some common crop seeds at 25°C 1

Species

Percentage relative humidity

15

30

45

60

75

90

Barley (Hordeum)

6.0

8.4

10.0

12.1

14.4

19.5

Beet (Beta)


5.8

7.6

9.4

11.2


Buckwheat (Fagopyrum)

6.7

9.1

10.8

12.7

15.0

19.1

Cabbage (Brassica)


5.4

6.4

7.6

9.6


Carrot (Daucus)


6.8

7.9

9.2

11.6


Cucumber (Cucumis)


5.6

7.1

8.4

10.1


Egg plant (Solanum)


6.3

8.0

9.8

11.9


Flax (Linum)

4.4

5.6

6.3

7.9

10.0

15.2

Groundnut (Arachis)

2.6

4.2

5.6


9.8

13.0

Lettuce (Lactuca)


5.1

5.9

7.1

9.6


Lima bean (Phaseolus)


7.7

9.2

11.0

13.8


Maize (Zea)

6.6

8.4

10.2

12.7

14.4

18.8

Mustard (Brassica)


4.6

6.3

7.8

9.4


Oat (Avena)

5.7

8.0

9.6

11.8

13.8

18.5

Okra (Abelmoschus)


8.3

10.0

11.2

13.1


Onion (Allium)


8.0

9.5

11.2

13.4


Radish (Raphanus)


5.1

6.8

8.3

10.2


Rice (Oryza)

5.6

7.9

9.8

11.8

14.0

17.6

Rye (Secale)

7.0

8.7

10.5

12.2

14.8

20.6

Sorghum (Sorghum)

6.4

8.6

10.5

12.0

15.2

18.8

Soyabean (Glycine)

4.3

6.5

7.4

9.3

13.1

18.8

Tomato (Lycopersicon)


6.3

7.8

9.2

11.1


Turnip (Brassica)


5.1

6.3

7.4

9.0


Watermelon (Citrullus)


5.1

6.3

7.4

9.0


Wheat (Triticum)

6.5

8.5

10.4

12.1

14.6

19.8

Winter squash (Cucurbita)


5.6

7.4

9.0

10.8


1 Information derived from:
Roberts, E.H. (ed.) (1972) Seed Viability. Chapman and Hall, London.

Table of equilibrium moisture contents of seeds stored in your genebank

Do the experimental work and fill in this table to use as your future guide:

Species

Temperature

Percentage relative humidity

Seed moisture content






















How to predict seed moisture content

STEP 1. MEASURE ENVIRONMENTAL CONDITIONS

1. Measure the relative humidity of the atmosphere where the seeds have been held for the past few days by using an aspirated hygrometer.

2. Measure the mean temperature where the seeds have been held for the past few days.

3. If the seeds are from a different environment, they must be allowed to equilibrate with the current one before any predictions are made.

Notes and Examples

Wet and dry bulb thermometers can also be used to measure relative humidity. They do not give as accurate a result because the air around the bulbs is static.

Allow sufficient time for the seeds to reach equilibrium. Large seeds take longer to equilibrate.

Equipment
Aspirated hygrometer
Maximum and minimum thermometer
STEP 2. CHECK THE EQUILIBRIUM MOISTURE CONTENT IN THE TABLE

1. Find the name of the crop in the left hand column of the tables of equilibrium moisture contents.

2. Read off the equilibrium seed moisture content at the relative humidity measured in Step 1 above.

3. Use this figure of approximate moisture content as your predicted value in making estimates of drying period or when an exact value of moisture content is not necessary.

Determination of seed moisture content

Seed moisture content can be experimentally determined by various methods. The IBPGR Advisory Committee on Seed Storage have advised that, for use in genebanks, the ISTA methods are accurate and acceptable. However, considering that the cost of germplasm is high and moisture content determination is destructive, it has been recommended that the ISTA methods should be modified by reducing the weight of seeds required for each replicate whilst increasing the accuracy of the weighings.

The ISTA has only recommended standard methods for common crop seeds (Table 2). There are therefore no standard methods for many of the species stored in genebanks. The ISTA methods can be applied to seeds of these other species where appropriate as explained in the following pages.

Table 2. Species whose moisture content should be determined by ISTA methods

Modified low constant temperature oven method

Brassicas
Castor (Ricinus)*
Pepper (Capsicum)
Cotton (Gossypium)*
Eggplant (Solanum)
Falseflax (Camelina)
Flax (Linum)
Groundnut (Arachis)*
Onion (Allium)
Radish (Raphanus)
Sesame (Sesamum)
Soyabean (Glycine)*
All tree species

Modified high constant temperature oven method

Alfalfa (Medicago)
Asparagus (Asparagus)
Barley (Hordeum)*
Bean (Phaseolus)*
Beet (Beta)
Bentgrass (Agrostis)
Bermuda grass (Cynodon)
Black salsify (Scorzonera)
Bluegrass (Poa)
Brome (Bromus)
Buckwheat (Fagopyrum)*
Canarygrass (Phalaris)
Caraway (Carum)
Carrot (Daucus)
Chervil (Anthriscus)
Chickory (Cichorium)
Chickpea (Cicer)*
Clover (Trifolium)
Cocksfoot (Dactylis)
Cress (Lepidium)
Crested dogtail (Cynosurus)
Cucumber (Cucumis)
Cumin (Cuminum)
Dallisgrass (Paspalum)
Fescue (Festuca)
Foxtail (Alopecurus)
Lettuce (Lactuca)
Lupin (Lupinus)*
Maize (Zea)*
Millet (Panicum)
Oat (Avena)*
Parsley (Petroselinum)
Pea (Pisum)*
Rhodes grass (Chloris)
Rice (Oryza)*
Rye (Secale)*
Ryegrass (Lolium)
Sainfoin (Onobrychis)
Serradella (Ornithopus)
Sorghum (Sorghum)*
Squash (Cucurbita)
Sweetclover (Melilotus)
Tall oatgrass (Arrhenatherum)
Timothy grass (Phleum)
Tomato (Lycopersicon)
Trefoil (Lotus)
Tufted hairgrass (Deschampsia)
Velvetgrass (Holcus)
Vetch (Vicia)*
Watermelon (Citrullus)*
Wheat (Triticum)*

* Indicates grinding is required for moisture content determination

How to determine seed moisture content

STEP 1. DETERMINE HOW MANY SEEDS ARE AVAILABLE FOR THE TEST

1. The total weight of seeds used for a moisture content determination should be sufficient to make the test accurate and yet not be wasteful of seeds. The sample should be fully representative of the accession and a minimum of ten seeds should be used to prepare the samples for the test.

2. The ISTA rules for seed testing recommend that two replicates of 4 g of seeds are used for the determination of moisture content for official seed testing (ISTA, 1976b). However, this weight of seeds is not appropriate for use in genebanks because large numbers of small seeds are needed, unnecessarily depleting the accession sample size. Therefore, it is suggested that approximately 0.5 g of seeds per replicate should be used for the moisture content determination of seeds in genebanks.

3. The lower the weight of seed used, the more accuracy is required to achieve a true result. Small samples should be weighed with an analytical balance to four decimal places using light-weight dishes, so that the ratio of the weight of the seeds and the dish is not too disproportionate.

Notes and Examples

Great care is required especially with small samples when doing the determination to ensure that the results are valid. In order to reduce experimental error, light-weight dishes can be made from strong aluminium foil, but these are fragile to handle and covers are not easy to make.

Always work with care and finish one sample at a time. Do not leave the dishes open in the laboratory between weighings because the seeds will either lose or absorb water from the air and small changes in weights can result in large differences in the calculations when the amount of seed used is small.

STEP 2. DETERMINE IF PRE-DRYING IS REQUIRED

1. Use your experience to predict moisture content as explained in the previous Section.

2. If the seeds are very moist, pre-drying may be necessary. ISTA requires pre-drying for seeds of the species which require grinding listed in Table 2 of more than 17% seed moisture content, for seeds of soyabean of more than 10% moisture content, seeds of rice of more than 13% moisture content and seeds of other species of more than 30% moisture content.

STEP 3. DETERMINE IF GRINDING IS NECESSARY

1. Check the ISTA list of species to see if the seeds require grinding (Table 2).

2. If the seeds are not on the list but are larger than wheat seeds, grinding is required.

Notes and Examples

Grinding should be done carefully to prevent errors in moisture content determination through water loss or absorption during the process. Hard seeds can be difficult to grind and a pestle and mortar may be useful. Very moist seeds will also be difficult to grind and a sharp knife and chopping board can be used to cut them into small pieces of the recommended sizes.
STEP 4. DECIDE WHICH IS THE APPROPRIATE METHOD TO USE

1. Two methods are recommended: one for oily seeds and one for all others.

2. If there is a recommended ISTA method for seeds of this species (see Table), use the modified version of this method.

3. If the species is not included in Table 2, but has oily seeds use the modified ISTA Low Constant Temperature Oven Method.

Notes and Examples

High temperatures cannot be used to determine the moisture content of oily seeds because the oil will also vaporize and give a false result of water plus oil content. Temperatures of just over 100 °C allow evaporation of water and minimal vaporization of oils. If you are in doubt as to which method to use, choose the low temperature method for safety.
STEP 5. PROCEED WITH THE DETERMINATION OF MOISTURE CONTENT

The standard ISTA determination methods have been modified for use in genebanks in this manual by decreasing the weight of seeds required for each replicate and the use of balances which weigh to more decimal places.

Modified ISTA low constant temperature oven method

1. If pre-drying is not required, weigh out approximately the total weight of seeds to be used, as determined in Step 1.

2. If pre-drying is required, weigh out accurately two sub-samples of between 1 - 1.5 g of seeds. Pre-dry these samples in a warm dry place overnight. Weigh again, writing all the weights in the note book.

3. If grinding is necessary, grind the seeds of each sample, directly before weighing, into pieces not larger than 4 mm for legumes and tree seeds and 0.5 - 1 mm for cereals.

4. Keep the ground seeds in a covered dish and go directly to the weighings.

5. Weigh one clean numbered dish and cover accurately to 4 decimal places using an analytical balance. Write the weight in the notebook.

6. Add approximately 0.5 g of the prepared seeds distributed evenly over the base of the dish, replace the cover and accurately weigh the dish and cover and write this weight in the notebook.

7. Place the dish in a safe place and continue to do the second replicate and additional samples in the same way.

8. When all samples have been weighed into numbered dishes, place each dish on top of its numbered lid in the oven at 101 - 105 °C.

9. Wait for the oven to reach this temperature and heat the samples for 15 - 17 hours.

10. Remove the dishes from the oven, replace their covers and place in a desiccator to cool for 30 to 45 minutes at room temperature.

11. Remove the dishes one by one from the desiccator and immediately weigh each dish and cover and note the weight in the notebook. Do not leave the desiccator open during the weighings.

Notes and Examples

Use a standard table form to record the results and simplify your work and calculations.

After heating, make sure that the dishes are put directly in the desiccator so that the dry seeds do not absorb more moisture. Do not try to close a desiccator with hot dishes inside until the air has expanded, or the expansion of air can break the seal and cause the lid to slide off. After cooling there will be a slight vacuum in the desiccator and the lid may be difficult to remove. Grease the lid of the desiccator to make it easier to remove. The use of a desiccator with a tap in the lid can prevent these problems.

Equipment
Grinder
Heat resistant dishes with covers
Analytical balance
Forced draught oven
Desiccator
Silica gel
Tongs and oven cloth
Modified ISTA high constant temperature oven method

1. If pre-drying is not required, weigh out approximately the total weight of seeds to be used, as determined in Step 1.

2. If pre-drying is required, weigh out accurately two sub-samples of between 1 - 1.5 g of seeds. Pre-dry these samples in a warm dry place overnight. Weigh again, writing all the weights in the note book.

3. If grinding is necessary, grind the seeds of each sample, directly before weighing, into pieces of not larger than 4 mm for legumes and tree seeds and 0.5 - 1 mm for cereals.

4. Keep the ground seeds in a covered dish and go directly to the weighings.

5. Weigh one clean numbered dish and cover accurately to 4 decimal places using an analytical balance. Write the weight in the notebook.

6. Add approximately 0.5 g of the prepared seeds distributed evenly over the base of the dish, replace the cover and accurately weigh the dish and cover and write this weight in the notebook.

7. Place the dish in a safe place and continue to do the second replicate and additional samples in the same way.

8. When all samples have been weighed into numbered dishes, place each dish on top of its numbered lid in the oven at 130 - 133 °C.

9. Wait for the oven to reach this temperature and heat the samples for 2 hours for cereals, 4 hours for maize and 1 hour for all other species on the ISTA list.

10. Remove the dishes from the oven, replace their covers and place in a desiccator to cool for 30 to 45 minutes at room temperature.

11. Remove the dishes one by one from the desiccator and immediately weigh each dish and cover and write the weight in the notebook. Do not leave the desiccator open during the weighings.

STEP 6. USE THE RESULTS OF THE WEIGHING TO CALCULATE MOISTURE CONTENT

1. Moisture content is calculated on a wet weight basis and expressed to one decimal place.

2. Replicates should not differ by more than 0.2%.

3. For samples tested without pre-drying use the following formula:



4. For samples which have been pre-dried, use the following formula:

where S1 = Percentage moisture content from first stage of drying
and S2 = Percentage moisture content from second stage of drying

STEP 7. ENTER THE DATA INTO THE DATA FILES

1. If the standard method was used, enter the percentage moisture content of each accession into the data file for that accession.

2. If the standard method was not used, also enter a note of the method used to determine it.

Example 1. One stage drying

Material

Dish number

Wt. of dish (g)

Wt. of dish and fresh seed (g)

Wt. of dish and dry seed (g)

C124 rep. 1

35

10.3245

14.8668

14.6356

C124 rep. 2

29

10.1442

14.9948

14.7485


Calculation:

Rep. 1:

Rep. 2:

Example 2. Two Stage Drying (Pre-drying)

Material

Dish number

Wt. of dish (g)

Wt. of dish and fresh seed (g)

Wt. of dish and dry seed (g)

First stage of drying (pre-drying)

Z099 rep. 1

07

20.4112

35.6615

33.2419

Z099 rep. 2

08

21.3121

36.7446

34.2689

Second stage of drying

Z099 rep. 1

61

10.2323

14.8839

14.6532

Z099 rep. 2

62

10.4132

14.9647

14.7475


Calculation:

Rep. 1

Rep. 2

Table of moisture content determination methods for seeds stored in your genebank

Fill in this table to use as your future guide:

Species

Pre-drying

Grinding

Oven temperature

Drying period



























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