Seed Moisture Testing

 

The seed moisture content (mc) is the amount of water in the seed. It is usually expressed as a percentage on wet weight basis in any seed-testing laboratory. The seed moisture content is the most vital parameter, which influence the seed quality and storage life of the seed. Seed moisture content is closely associated with several aspects of physiological seed quality. For example, it is related to seed maturity, optimum harvest time, mechanical damage, economics of artificial      seed drying, seed longevity and insect & pathogen infestation.
 

Objective 

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

 

Definition 

The moisture content of seed sample is the loss in weight when it is dried in accordance with 1STA rules. It is expressed as a percentage of the weight of the original sample.


Principle 

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

 

Seed ageing and loss of seed germination 

Seed ageing and loss of germination cannot be stopped, but can be minimized by proper seed storage conditions. The important environmental factors influencing germination loss are relative humidity (RH) governs the seed moisture, and temperature, higher are these; the more rapid is the seed deterioration. The effects of seed moisture and temperatures are summarized succinctly in Harrington's rules, as under:
 

1. For every decrease of 1 percent in seed moisture content the life of seed is doubled. 

2. For every decrease of 5C in storage temperature the life of the seed is doubled.  

Rule 1hold's good, when the seed moisture content is between14 and 5 percent. When these two simple rules are applied, the effects are geometric.
 

Determination of seed moisture content 

As seed moisture and its management influences so many physiological seed quality parameters essential to seed quality. Seed moisture measurement appropriate to the purpose is needed in commerce and research. The optimum method for moisture testing depends upon:

 

1. Chemical composition of seed

2. Seed structure

3. Moisture content level

4. Degree of accuracy and precision required

5. Constraints of time

6. Technical expertise and cost

 

The ideal could be that is adopted to all seeds, measures moisture content from 0 to100 percent, reproducible, require less training and low in cost. It is impossible to combine all these. However, in order to measure the moisture content of seeds, methods can be broadly grouped in two categories:

 

a)  Direct method

b)  Indirect method

 

Direct method 

Under this category, the seed moisture content is measured directly by loss or gain in seed weight. These are:

1. Desiccation method 

2. Phosphorus pentaoxide method

3. Oven-drying method

4. Vacuum drying method

5. Distillation method   

6. Karl Fisher's method

7. Direct weighing balance

8. Microwave oven method

 

Indirect method 

     These are no so accurate; estimation is approximate, but convenient and quick in use. These are frequently used at seed processing plants. These measure other physical parameters like electrical conductivity or electrical resistance of the moisture present in the seed. Values are measured with the help of seed moisture meters, and these values are transformed into seed moisture content with the help of calibration charts, for each species, against standard air-oven method or basic reference method.

 

Above all Karl-Fisher's method has been considered as the most accurate and the basic reference method for standardizing other methods of seed moisture determination. The constant temperature oven drying method is the only practical method, approved by International Seed Testing Association (ISTA) and other organization to be used for routine seed moisture determination in a seed-testing laboratory.
 

Constant temperature oven drying method 

The constant temperature oven drying method is broadly grouped into two categories:

 

1)  Low Constant Temperature Oven Method 

2)   High Constant Temperature Oven Method

 

Low constant temperature oven method: This method has been recommended for seed of the species rich in oil content or volatile substances (Table I). In this method, the pre- weighed moisture bottles along with seed material are placed in an oven maintaining a temperature of 103C. Seeds are dried at this temperature for 17+-1hr.The relative humidity of the ambient air in the laboratory must be less than 70 percent when the moisture determination is carried out.

 

High constant temperature oven method: The procedure is the same as above except that the          oven is maintained at a temperature of 130-133C. The sample is dried to a period of four hours for Zea Mays, two hours for other cereals and one hour for other species (see alsoTable2).In this method there is no special requirement pertaining to the relative humidity of the ambient air in the laboratory during moisture determination.
 

Essential equipments and supplies 

1) Constant temperature precision hot-air electric oven.

2) Weighing bottles/Moisture containers.

3)  Desiccator with silica gel.

4) Analytical balance capable of weighing up to 1mg.

5) Seed grinder/An adjustable grinding mill.

6) Tong.

7) Heat resistant gloves.

8) A brush/A steel brush. 
 

Period of seed drying 

The prescribed period of seed drying shall be17-+ 1 hrs at 103C under low constant and 1 to 4 hrs at 130-133C under high constant temperatures. Maize seed be dried for 4 hrs, cereals and/or other millets for 2 hrs and the remaining species for 1 hr. Seeds rich in oil content or with volatile substances be dried for 17+- hrs under low constant temperature.. See drying period begins from the time oven returns to maintain the desired temperatures.
 

Sample size 

           The ISTA rules recommend that two replicates, each with 4 gm of seed be used for determination of seed moisture content. This seed sample weight may be modified to 0.2 to 0.5 gm per replicate, with precise weighing, for use in seed genebanks, to avoid unnecessary depletion of precious biological resources.

 

Procedure
 

1.         Seed moisture determination be carried out in duplicate on two independently 

            drawn working samples.

2.         Weigh each bottle with an accuracy of 1 mg or 0.1 mg.

3.         First weigh the empty bottle/container with its cover.

4.         Grind the seed material (if need be, in seed species listed in Table 3)), evenly

            using any  grinder/grinding   mill   that  does  not  cause heating  and/or  loss  of

            moisture content.

5.         Mix thoroughly the submitted sample, using spoon, and transfer small portions

           (4 to 5 gm) of seed samples directly into weighing bottles/containers, by even  

           distribution on bottom of the containers.

6.         After weighing, remove the cover or lid of the weighing bottles/containers.

7.         Place the   weighing   bottles/containers in          an oven,   already heated to or

            maintaining the desired temperature, for the recommended period.

8.         At the end of seed drying period, weighing bottles/containers be closed with its  

            lid / cover.

9.         Transfer the weighing bottles/containers to the desiccators having silica gel(self

            indicating -blue), to cool  down for 40 to 45 min.

10.       Weigh again the cooled weighing bottles/containers.

11.       Calculate the seed moisture content.

 

Calculation   of results

 

The moisture content as a percentage by weight (fresh weight basis) is calculated to

one decimal place, by using of the formulae: 

 

M2-M3

%age seed moisture content (mc) =                   ----------  x   100

                                                                         M2-M1

  

Where

 

M1 = Weight of the weighing bottle/container with cover in gm

M2 = Weight of the weighing bottle/container with cover and seeds before drying

M3 = Weight of the weighing bottle/container with cover and seeds after drying

{Note:   The seed moisture determination must be done in two replicates, with   precise weighing (i.e. up to three decimal places) using lightweight weighing bottles/containers.}

 

If the seed is pre-dried or dried in two steps: The seed moisture content is calculated from the results obtained in the first (pre-dried) and second stages of seed drying, using the following formula, and expressed as percentages, as under:  

 

                                                                       (S1 + S2) - (SI * S2)

%age seed moisture content (mc) =                 -------------------------

                                                                                  100

 Where

S1= is the moisture loss in the first stage, and

S2= is the moisture loss in the second stage       .

            .

Use of tolerances      

       Result is the arithmetic mean of the duplicate determination of seed moisture content,

for a given seed sample. The maximal difference of 0.2%is recommended between two replicates, for crop seed species under1STA rules. If the difference between two replicates exceeds 0.2%, the seed moisture determination in duplicate be repeated.

 

                As it is very difficult, rather impossible to, meet the replicate difference of seed

Moisture up to 0.2%in tree or shrub species, maximal limit of 0.3to 2.5%is recommended  between two replicates for seed moisture in tree or shrub species under ISTA rules.           

 

Reporting of results

 Seed moisture content be reported to the nearest 0.1% on1STA analysis certificate. 

If the seed moisture content is determined using any moisture meter, the brand name and type of the equipment be mention on the analysis certificate, under column of "other determinations" reporting of range for which the moisture meter is calibrated is the another requirement, on seed analysis certificate.          

 

Table1: Species for which the low constant temperature (103C) oven method be used    
 Allium spp                                                       Linum ustatissimum      

Arachis hypogea                                               Raphanus sativus

Brassica spp                                                    Rlcnus communis

Camelina sativa                                                Sesamum indicum

Capsicum spp                                                  Sesamum orientale

Glycine max                                                      Sinapsis spp

Gossypium spp                                                 Solanum melongene

 

 

Table 2: Species for which high constant temperature (1300 to  133C) oven method be used

 

Agrostis spp                 Citrullus lanatus                         Lolium spp                   Phaseolus spp

 

Alopecurus                   Cucumis spp                             Lotus spp                     Phelum spp

Pratensis

                                   

Anethum graveolens      Cucurbita spp                           Lupinus spp                  Pisum sativum

 

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Table3: Species for which grinding is obligatory

 

Avena spp                    Gossypium spp             Phaseolus spp               Triticum spp