Agriculture Reference
In-Depth Information
11
Seed Moisture
Content
Testing
Few factors are more important to the quality and function of seed than moisture content. Moisture con-
tent is associated with almost every aspect of seeds and their function, including their maturity, timing of
harvest, susceptibility to mechanical injury during threshing or handling, longevity in storage, and injury
due to heat, frost, fumigation, insects, and pathogens. Thus, moisture content is perhaps the most important
factor which determines when seed is harvested, how it is handled after harvest, and how long it maintains
its quality.
Information in this chapter is largely taken from a review of measurement of seed moisture by Grabe
(1989) and the AOSA Seed Moisture Determination Handbook (2007). The ISTA Seed Moisture Handbook
and testing rules also provide useful information on this subject.
Moisture relations in Seeds
Water is a complex system and is only partially understood. The probabilities of various structural features
existing have largely been derived through statistical thermodynamical approaches. Empirical physical
properties of water such as viscosity, density, and thermodynamical properties were compiled by the United
States Bureau of Statistics in the 1920s.
The way in which water interacts with plant substances is even more complex. Water is associated
with the seed tissue in several patterns and is held at varying degrees of strength ranging from chemically
bound water and adsorbed water, to free water. Bound water is actually part of the chemical structure of
other molecules of the seed tissue and is held by hydrogen bonding (vectorized polar bonds) in a monolayer
around large starch and protein molecules, and does not exist as discrete water molecules. Adsorbed water
is held in multilayers as discrete molecules in bonding interactions with hydroxyl or amide groups, though
the arrangement and stability of this type of water is highly variable. These interactions may extend into the
surrounding liquid, forming gradient patterns of structure in a dynamic state of turnover. There may be bulk
water, but always in association with other systems, and at which point water is bound and free is dificult to
ascertain. Finally, free water is that which is held by capillary forces or is in solution within the plant (seed)
tissues (Fig. 11.1).
Grabe (1989) cited several authors in noting that bound water has different physical properties than
water by itself. He cited Shanbhag et al. (1970) in recognizing that bound water has a different freezing
point, higher boiling point, lower vapor pressure, and higher density than free water. It does not act as a
solvent for mineral salts and cannot conduct electricity.
Free water and more loosely held adsorbed water can usually be removed by the normal heat of
vaporization, while the removal of bound water requires heat in excess of this amount. The heat required to
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