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United States Department of Agriculture

Agricultural Research Service

Title: Radio Frequency Measurements for Sensing Moisture in Small Samples of Peanuts

item Kandala, Chari
item Nelson, Stuart

Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: March 1, 2003
Publication Date: July 27, 2003

Interpretive Summary: Peanuts are gaining importance as a widely consumed food item in many countries today, and peanuts and peanut products such as peanut butter and peanut oil are common items found in grocery stores. Moisture content is an important property to be measured and managed in the marketing, processing and storage of peanuts. Farmers in the U.S., when marketing peanuts, except for seed, are required to market peanuts as identity- preserved lots at the buying points, with an average moisture content not exceeding a specified value. Freshly dug peanuts usually have much higher moisture contents than the specified value and thus have to be dried before sale or storage. The moisture content value has to be periodically monitored while the peanuts are being dried so as not to over-dry or under-dry the peanuts. Over-drying the peanuts is uneconomical and may affect the quality while under drying is not permitted in marketing, storage or processing of peanuts. Moisture content values are presently checked by commercially available electronic instruments that measure the average moisture content value for bulk samples of about 200g of shelled peanuts. Some times it would be useful to measure the moisture content in smaller samples to check moisture variation in bulk samples. This would be particularly useful when small numbers of peanut kernels are packaged for consumption. An electronic method for rapidly and nondestructively measuring the moisture content of a small sample of peanuts using two radio frequencies has been explored and the results are described in this article. The measured values compared well with the standard air-oven values and this method seems to offer a basis in the development of an electronic instrument that would measure moisture content in small samples of peanuts nondestructively and rapidly.

Technical Abstract: A method developed earlier for estimating the moisture content of single peanut kernels from radio-frequency impedance measurements on a parallel-plate capacitor, holding the kernel between the plates, was used for estimating the moisture content of small samples of 6 to 8 kernels. Parallel plates with a larger surface area to accommodate the small samples were used for the measurements, and a new moisture calibration equation was developed. Capacitance, phase angle, and dissipation factor were measured for the parallel-plate system with an impedance analyzer at 1 and 5 MHz. By using these values in the moisture calibration equation, moisture contents of the small samples were predicted successfully to within 1% of their air-oven reference values for over 85% of peanut samples tested with moisture contents ranging between 6% and 20%, wet basis.

Last Modified: 4/21/2015
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