Agriculture Reference
In-Depth Information
Moisture Testing Methods
It is important that accurate, rapid, inexpensive methods be available for testing seed moisture content.
Grabe (1989) stated that the ideal moisture testing methods should be useful for all seeds (species), effective
at moisture ranges from 0 to 100% fresh weight basis (fw), accurate to within 0.1%, repeatable, and require
little training to perform. Unfortunately, no single method encompasses all these features. Grabe recognized
that moisture testing methods that can measure the entire moisture range are not always quick, and those
that are most accurate are not always inexpensive. Many different methods have been developed for testing
seed moisture. These include direct as well as indirect methods (Hart and Golumbic, 1963).
Direct methods
(also known as primary methods) measure the amount of moisture directly by collect-
ing and quantifying the amount of water lost from the seed. This step is preceded by removing, expelling,
or otherwise extracting the moisture from the seed by means of drying, distillation, or use of a chemical
solvent. Direct methods are usually more time-consuming and require specialized equipment and train-
ing. Consequently, direct methods are rarely used for routine seed moisture testing. Over the years, some
direct methods have been used as reference methods against which the indirect methods are veriied and
calibrated.
Indirect
methods (or secondary) determine a seed's moisture content by measuring associated physi-
cal or chemical characteristics of seeds or the interseed environment which are related to moisture content.
They include factors such as electrical properties of the seed or the relative humidity of the interseed spaces.
They may also include such methods as near infrared spectroscopy, nuclear magnetic resonance, or micro-
wave spectroscopy.
oVEn-dryInG METHodS
Oven-drying or air-oven methods are probably considered by most analysts to be the standard against
which other methods of moisture testing are compared. This is not necessarily because they are the most
accurate, but because they require only standard equipment and supplies that most laboratories have readily
available. However, this is somewhat misleading because, although they appear deceptively simple, they
are actually subject to many variables and must be carefully controlled to ensure accurate and repeatable
results. The AOSA Seed moisture testing handbook (AOSA, 2007) lists oven drying parameters reported to
give results in agreement with direct reference methods. Some excerpts are listed in Table 11.1.
The air-oven method is conducted by drying a weighed quantity of seed in an oven at a certain tem-
perature, cooling in a desiccator, reweighing, and calculating the loss of moisture on dry weight or fresh
weight basis. The oven may be either a mechanical convection type or gravity type, but should hold tem-
perature within 1.0°C. Mechanical convection ovens are preferred because they recover the set temperature
quicker after the door is opened and closed. Either type should have a built-in thermometer to accurately
read temperatures to the nearest 0.5°C.
Seeds of some species should be ground prior to drying, while others can be dried whole and still pro-
duce reliable results. The grinder used should be adjustable to provide the prescribed particle size for each
type of seed. It should not generate undue heat and should not expose the seed to open air. After grinding,
aluminum or glass weighing dishes with tight-itting lids should be available. These should be cooled in an
air-tight desiccator that provides rapid cooling before the samples are weighed. To prevent the introduction
of errors caused by the warm container, samples should be cooled for approximately 45 minutes before
weighing. Lids to the weighing dishes should be placed on the container before removing from the oven to
prevent moisture absorption.
After drying and cooling, the sample is reweighed and the weight loss is divided by the initial weight
of the sample (minus the weight of the container) to calculate the
fresh
weight
percent moisture.
Various types of ovens may be used. The Brabender oven incorporates a balance which eliminates the
need for a separate balance, desiccator and weighing dishes.
Vacuum
ovens
dry at temperatures lower than
