Agriculture Reference
In-Depth Information
The Brown-Duval distillation method was developed in 1907 and was used for many years as the ofi-
cial method of the U.S. Department of Agriculture. It is conducted by heating a 100-gram sample of whole
seed in a lask with 150 ml nonvolatile oil to a speciied cutoff temperature (180°C for wheat) which varies
for different seeds. The water distilled off is weighed and the moisture content determined as for other dis-
tillation methods.
Distillation methods are direct (primary) methods of moisture content determination. Many of the
oven methods used for seed moisture testing are veriied against distillation methods, especially toluene
distillation.
SoLVEnT-ExTrACTIon METHodS
The use of organic solvents is superior to oven methods for extracting water from seeds. Results are not
dependent on temperature or drying time and nonaqueous volatiles are not measured as water. Water is
usually removed from the ground seed by methanol extraction and the water content of the extract is deter-
mined by Karl Fischer titration, infrared spectrophotometry or gas chromatography. However, the accuracy
of this method depends on the completeness of the water extraction rather than the accuracy of the measure-
ment of the water in the extract.
The extraction method used may vary somewhat for different seeds. Dry cereal seed may be ground
for 3 minutes at 15,000 rpm in a modiied Stein mill, followed by a 5-minute wet grind in methanol at
64.5°C. Another method is to grind dry seed at 13,000 rpm and extract the water by grinding for 3 minutes
at 63°C in methanol. A method used for maize is conducted by grinding with methanol in a household
blender for 1 minute at 12,500 rpm and 1 minute at 17,000 rpm, followed by gentle stirring for 15 minutes.
The water can be measured after irst boiling off the organic solvent at a temperature far below the boiling
point of water.
The Karl Fischer method for determining moisture content is based on titration of the methanol-water
mixture with a solution containing methanol, iodine, sulfur dioxide, and pyridine, the so-called Karl Fischer
reagent. As long as any water is present, the iodine is reduced to colorless hydrogen iodide. The titration is
terminated once the brown iodine color is observed, indicating the complete absence of water. It may also
be determined electrometrically. Because the Karl Fischer reagent is speciic for water, with no interfering
substances in the seed extracts that react with the solution, it is considered to be the most basic (direct) and
accurate moisture testing method available.
ELECTronIC MoISTurE METErS
The most practical method of moisture testing for the seed industry is ordinarily with the use of electronic
moisture meters (Fig. 11.3). They are fast, convenient, easy to use, and accurate enough for most purposes.
Consequently, they are widely used for cereals, oilseeds, edible legumes, and other free-lowing seeds.
However, they must be calibrated for each type of seed, and such calibrations may not be available for cer-
tain grass, vegetable, lower, and tree seeds.
Conductance Meters
Measurement of moisture with electronic meters is based on the relationship between electrical proper-
ties of seeds and their moisture content. Such meters measure the conductivity or capacitance of seeds.
Conductance meters measure the conductivity of seed when an electrical current is passed through them
between two electrodes. However, since bound water does not conduct electricity, conductance meters do
not measure this type of water. Therefore, conversion charts must be used which take into account bound
water in calculating the overall moisture content. Furthermore, conductance meters are accurate only for
moisture ranges between 7 and 23 percent moisture.
