International rules for seed testing 1985

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Adopted at the Twentieth International Seed Testing Congress, Canada 1983, to become effective on 1 July 1985. International Seed Testing Association, Seed Sci. & Tech., 13: 299-355,(1985)

Chapter 9: DETERMINATION OF MOISTURE CONTENT

9.1. Object

The object is to determine the moisture content of seed by methods suitable for routine use.

9.2. Definition

The moisture contents of a sample is the loss in weight when it is dried in accordance with these rules. It is expressed as a percentage of the weight of the original sample.

9.3. Principle

The methods prescribed are designed to reduce oxidation, decomposition or the loss of the other volatile substances while ensuring the removal of as much moisture as possible.

9.4. Apparatus

The following apparatus is necessary, depending on the method used:

(a) An adjustable grinding mill;

(b) Constant temperature oven and accessories which shall include containers and a desiccator;

(c) Analytical balance;

(d) Sieves

Suitable apparatus is described in the Annex.

9.5. Procedure

9.5.1. Precautions

The submitted sample shall be accepted for moisture determination only if it is in an intact, moisture-proof container from which as much air as possible has been excluded. The determination shall be started as soon as possible after receipt. During the determination, exposure of the sample to the atmosphere of the laboratory shall be reduced to the absolute minimum. For species that do not require grinding no more than two minutes may elapse from the time the sample is removed from the container in which it was received until the working sample is enclosed in the drying container and weighed.

9.5.2. Weighing

Weighing shall be in grams to three decimal places.

9.5.3. Working sample

The determination shall be carried out in duplicate on two independently drawn working samples each of the following weight, depending on the diameter of the containers used:

Less than 8 cm diameter-- 4 to 5 9. 8 cm diameter or larger-- 10 9.

Before the working sample is drawn, the submitted sample shall be thoroughly mixed by one of the following methods:

Either, (a) Stir the sample in its container with a spoon, or, (b) Place the opening of the original container against the opening of a similar container and pour the seed back and forth between the two containers.

Each working sample shall be drawn by a method prescribed in Rule 2.7.2 in such a manner that the sample is not exposed to the air for more than 30 seconds.

9.5.4. Grinding

Large seed must be ground before drying unless their high oil content makes them difficult to grind or liable to gain in weight through oxidation.

It is obligatory to grind seed of the species indicated in Table 9A.

The grinding shall be done on a sub-sample before drawing the working sample. The required fineness of grinding is indicated in the Annex.

9.5.5. Pre-drying.

If the species is one for which grinding is necessary and the moisture content is more than 17% (or, 10% in the case of Glycine max (Soybean) and 13% in the case of Oriza saliva (Rice)), pre-drying is obligatory. Two sub-samples, each weighing at least 259 to the nearest 2.0 mg, are placed in weighed containers. The two sub-samples, in their containers, are then dried to reduce the moisture content to less than 17% (or, 10% in the ease of Glycine max (Soybean) and 13% in the case of Oriza sativa (Rice)). Pre-drying methods are indicated in the Annex.

After pre-drying, the sub-samples are reweighed in their containers to determine the loss in weight. Immediately thereafter the two partly dried sub-samples are separated ground and the ground material subjected to the procedure prescribed in Rule 9.5.7. or 9.5.8. as appropriate.

9.5.6. Prescribed methods.

1. The Low Constant Temperature Oven method as prescribed in Rule 9.5.7. shall be used for seed of the species indicated in Table 9B.

2. The High Constant Temperature Oven methods as prescribed in Rule 9.5.8. shall be used for seed of the species indicated in Table 9C.

9.5.7. Low constant temperature oven metho.

The working sample, drawn according to Rule 9.5.3., must be evenly distributed over the surface of the container. Weigh the container and its cover before and after filling. Place the container rapidly, on top of its cover, in an oven maintained at a temperature of 103 ± 2°C and dry for 17 ± 1 hours. The drying period begins at the time the oven returns to the required temperature. At the end of the prescribed period cover the container and place it in a desiccator to cool for 30 45 minutes.

After cooling weigh the container with its cover and contents. The relative humidity of the ambient air in the laboratory must be less than 70% when the determination is carried out.

9.5.8. High constant temperature oven method.

The procedure is same as described in 9.5.7., except that the oven is maintained at a temperature of 130 -133 °C, the sample is dried for a period of four hours for Zea mays (Maize), two hours for other cereals, and one hour for other species and no special requirement pertains to the relative humidity of the ambient air in the laboratory during determination.

9.6. Calculation of results.

9.6.1. Constant temperature oven methods

The moisture content as a percentage by weight shall be calculated to one decimal place by means of the following formula:

M₁ — is the weight in grams of the container and its cover,

M₂ — is the weight in grams of the container, its cover and its contents before drying, and

M₃ — is the weight in grams of the container cover and contents after drying.

If the material is pre-dried, the moisture content is calculated from the results obtained in the first (predrying) and second stages of the procedure. If S. is the moisture lost in the first stage, and S2 is the moisture lost in the second stage, each calculated as above and expressed as a percentage, then the original moisture content of the sample calculated as a percentage is:

9.6.2. Tolerances

Take as the result the arithmetic mean of the dupilcate determinations carried out on a sample if the difference between the two determinations does not exceed 0.2%. Otherwise, repeat the determination in duplicate.

9.6.3. Reporting results (omitted)

Annex to chapter 9

9.4.A. Apparatus

9.4.A.1. Grinding mill

The mill used for grinding must meet the following requirements:

(a) Be constructed of non-absorbent material.

(b) Be so constructed that both the seeds to be ground and the resulting ground material are protected from ambient air during grinding, to the maximum extent possible.

(c) Grind evenly at a speed that does not cause heating of the ground material, and minimises air currents that might cause loss of moisture.

(d) Be adjustable so as to obtain particles of dimensions indicated in Annex 9.5.4.A.

9.4.A.2. Constant temperature oven and accessories

The oven may be of gravity-convection or mechanical-convection (forced draught) type. It should be electrically heated with thermostatic control, well insulated and capable of maintaining a reasonably uniform temperature throughout the chamber and the specified temperature at shelf level. It should be equipped with removable perforated or wire shelves with a thermometer which has been tested to be accurate to 0.5°C placed near the upper shelf in the vicinity of the samples. The heating capacity should be such that after preheating to the required temperature, followed by opening and loading with containers the oven will again reach the required temperature within 15 minutes.

Containers must be made of non-corrosive metal or glass of approximately 0.5 mm thickness, and have snug fitting covers to minimise gain or loss of moisture, and should have sides rounded at the base, a flat bottom and level edges. Both the container and its cover should be identified by the same number. Before using, dry the container for one hour at 130°C, or by an equivalent drying procedure, and cool in a desiccator.

The effective surface must enable the working sample to be distributed so as to give not more than 0.3 9/ cm².

The desiccator should be fitted with a thick metal plate to promote rapid cooling of the containers and contain a suitable desiccant such as phosphorus pentoxide, activated alumina, or molecular sieves type 4A, 1/16 (11) pellets.

9.4.A.3. Analytical balance

The analytical balance must be quick weighing and capable of weighing to 0.001 9.

9.4.A.4 Sieves

Wire sieves are required with meshes of 0.50 mm, 1.00 mm. and 4.00 mm.

9.5.4.A. Grinding

For cereal and cotton seeds fine grinding is necessary; at least 50% of the ground material shall pass through a wire sieve with meshes of 0.50 mm and not more than 10% remain on a wire sieve with meshes of 1.00 mm. For leguminous and tree seeds coarse grinding is necessary; at least 50% of the ground material shall pass through a sieve with meshes of 4.00 mm.

Adjust the grinding mill to obtain particles of the required dimensions, grind a small quantity of the sample and reject it. Then grind an amount of the samples slightly greater than that required for the test.

9.5.5.A. Predrying

In the case of very moist seed of Zea mays (= Maize) (above 25% moisture content) the seed is spread in a layer not deeper than 20 mm and dried at 70°C for 2-5 hours, depending on the initial water content.

In the case of other species with a moisture content exceeding 30%, samples should be dried overnignt in a warm place such as the top of a heated oven.

In other cases, the samples should be pre-dried in a constant temperature oven at 130°C for 5-10 minutes, depending on the moisture content. The partly dried material is then kept in the laboratory for two hours.

Festuca spp.

Holcus lanatus

Hordeum vulgare (all vars)

Lactuca sativa

Lathyrus spp.

Lepidium sativum

Lolium spp.

Lotus spp.

Lupinus spp.

Lycopersicon Iycopersicum

Table 9D. Tolerance levels for differences between two determinations of moisture content of tree and shrub seeds

KARL FISCHER METHOD

In recent years considerable attention has been given to the method of Karl Fischer for determining the moisture content of a wide variety of materials. The method depends upon the reaction of iodine with water in the presence of sulfur dioxide and pyridine to form hydrolic acid and sulfuric acid. Since the method is strictly stoichiometric it is, theoretically at least, one of the most accurate methods for determining moisture content. In applying the method to grain, the grain must be finely ground and the moisture extracted with anhydrous methyl alcohol. Because of certain practical difficulties in its application, the Karl Fischer method has been used only infrequently in determining moisture content in grain, although Fosnot and Haman (1945) applied the method to wheat and barley. Hart and Neustadt (1957) have adopted the method to all cereal grains and have used it to test the accuracy of official oven methods. The technical skill required and the time-consuming nature of this analytical method make its usefulness likely to remain limited.

REFERENCES

FOSNOT, R.H. and R.W. HAMAN, A Preliminary Investigation of the Application of the Karl Fischer Reagent to the Determination of Moisture in Cereals and Cereal Products., Cereal Chem., 22:41-49 (1945)

HART, J.R. and M.H. NEUSTADT, Application of the Karl Fischer Method to Grain Moisture Determination., Cereal Chem., 34:26-37 (1957) (Cited from "Storage of Cereal Grain and Their Products".)

Table 9A. Species for which grinding is obligatory

Table9B. Species for which the /ow constant temperature oven method shall be used

Table9C. Species for which high constant temperature oven method shall be used.

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