SEED MOISTURE AND ITS DETERMINATION

Moisture content of seeds is one of the most important factors in the maintenance of seed quality. From the time of harvest to time of planting, seed moisture varies and if it rises above certain critical levels for any appreciable time period at any stage there is danger of undesirable stimulation of physiological processes within the seed with consequent weakening and loss of seed viability. Knowledge of moisture content therefore is needed to decide whether seeds should be dried down before storage or shipment and to determine proper conditions of storage.

Definition of Seed Moisture:-The moisture content a seed sample is the loss in weight when it is dried. It is expressed as a percentage of the weight of the original sample.

Determination of Seed Moisture: - Methods for measuring moisture content are generally classified as reference methods, routine methods and practical methods.

REFERENCE METHODS

Phosphorous pentoxide method-This method was described by leendertz (1948).In this method seeds are dried to a constant weight at 800C in a vacuum. The moisture is absorbed on phosphorous pentoxide placed in the same tube with the sample. This method is considered to be very accurate and is relatively free of the errors, namely the influence of the humidity of the air upon the sample, incomplete determination of moisture and decomposition of the sample from high temperature.
The ISTA recommends the vacuum phosphorous pentoxide method as a control in calibrating other methods suitable for routine work. The use of lower temperature and pressure, however, may not eliminate the possibility of losing non aqueous volatile matter.

Karl Fischer Method-This method was originated by Karl Fischer. It is considered to be most accurate method available for moisture determination. Determination of moisture content of seeds in this method is based upon the chemical reaction of water with Karl Fischer reagent (reaction of iodine with water in presence of SO2 and pyridine to form hydriodic acid and sulphuric acid).The water is extracted from the seed with a water solvent, methanol and titrated with the reagent. A modified stein mill is used for simultaneous grinding and extracting the grain sample. The water is extracted from the grain sample in 5 minutes at a temperature of 64.50C.Because the grain is ground and extracted quickly there is little chance of moisture loss. Because the Karl Fischer reaction is specific for the determination of water, other volatile constituents, which might be driven off by prolonged heating at high temperatures in the oven methods, are not calculated as water. The water is completely extracted because amount of water in the solvent. The only disadvantage of the method is that it requires a more skilled technician than the oven method and takes more time.
Advantage of analysis
The popularity of the Karl Fischer titration is due in large part to several practical advantages that it holds over other methods of moisture determination, including:
• High accuracy and precision
• Selectivity for water
• Small sample quantities required
• Easy sample preparation
• Short analysis duration
• Nearly unlimited measuring range (1ppm to 100%)
• Suitability for analyzing:
o Solids
o Liquids
o Gases
• Independence of presence of other volatiles
• Suitability for automation
The most important advantage of Karl Fischer titration method over conventional loss on drying (LOD) thermal methods of moisture determination is its specificity for water. Loss on drying will detect the loss of any volatile substance.