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Healthy peanut plants with roots exposed. Link to photo information
The first part of the peanut-drying process is to dig up the plants, invert them, and let the sun do the work. This reduces the peanuts' moisture content from about 40 percent down to about 20 percent. Click the image for more information about it.

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Measuring Peanuts' Moisture—While Still in the Shell

By Sharon Durham
October 31, 2007

Making sure that U.S. peanuts are top-quality requires drying them enough to prevent growth of fungi that can seriously decrease their market value. Now, Agricultural Research Service (ARS) scientists have developed a way to determine moisture levels without destroying the peanuts' shells, or pods, as is currently done. ARS is the U.S. Department of Agriculture's chief scientific research agency.

Market value is directly tied to peanut quality, and one of the most important quality factors is moisture content. Peanuts must be dried, or cured, to ensure the moisture content does not exceed 10.5 percent, to ensure quality is preserved and to prevent growth of microbes naturally present in farm fields. One of these, the fungus, Aspergillus flavus, can produce a potentially dangerous mycotoxin called aflatoxin.

ARS engineers Chari Kandala and Stuart O. Nelson have pursued an alternative to opening pods for testing. Instead, they place intact peanut pods between two plates of an impedance analyzer and use radio frequency to determine the pods' moisture content.

Kandala works in the ARS National Peanut Research Laboratory in Dawson, Ga. Nelson works in the Quality and Safety Assessment Research Unit of the ARS Richard B. Russell Research Center in Athens, Ga.

U.S. producers have systems in place to reduce moisture content, but the trick is to make sure optimal levels are reached throughout an entire batch of drying pods.

In the Southeast, freshly dug peanuts—which contain up to 40 percent moisture—are allowed to dry on the vine until they reach an average moisture value of about 20 percent, when they are harvested. Soon after, they're mechanically dried in special trailers until they reach less than 10.5 percent to meet grading standards and allow for safe storage, given adequate ventilation. During drying, processors must measure the peanuts' moisture contents at regular intervals to prevent drying the peanuts too much, which increases costs and lowers peanut quality.

According to Kandala, the impedance analyzer is better than current methods at measuring pockets of moisture in the entire batch of peanuts.