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United States Department of Agriculture

Agricultural Research Service

Title: Rf: Impedance Method to Estimate Moisture Content in Small Samples of in-Shell Peanuts

Authors
item Kandala, Chari
item Nelson, Stuart

Submitted to: IEEE Instrumentation & Measurement Society
Publication Type: Proceedings
Publication Acceptance Date: April 1, 2004
Publication Date: May 20, 2004
Citation: Kandala, C., Nelson, S.O. 2004. Rf: impedance method to estimate moisture content in small samples of in-shell peanuts. IEEE Instrumentation & Measurement Society.

Interpretive Summary: Measurement of moisture content is an important aspect in the harvesting, storage, marketing and processing of peanuts. Peanuts are dried in trailers that have perforated floor and an air plenum, till they dry to the required moisture level. At some of the drying facilities as many as 80 trailers at a time may be used for drying the in-shell peanuts. It takes considerable time and cost to achieve the reduction in the mc of the peanuts. During the drying process, it would be necessary to measure the mc value of the peanuts in each trailer at regular intervals to determine the point in time at which the desired mc level has been reached and the dryers to be turned off. Drying the peanuts further after the required mc level has been reached, may increase the drying costs and lower their quality.

Technical Abstract: Measurement of moisture content (mc) is an important aspect in the harvesting, storage, marketing and processing of peanuts, Arachis hypogaea L. Freshly harvested peanuts have a mc usually in the range of 20 to 30%. These peanuts have to be dried immediately to reduce the mc to less than 10.49% to meet the grading standards (PAC 1988). Peanuts are dried in trailers that have perforated floor and an air plenum, till they dry to the required moisture level. At some of the drying facilities as many as 80 trailers at a time may be used for drying the in-shell peanuts. It takes considerable time and cost to achieve the reduction in the mc of the peanuts. During the drying process, it would be necessary to measure the mc value of the peanuts in each trailer at regular intervals to determine the point in time at which the desired mc level has been reached and the dryers to be turned off. Drying the peanuts further after the required mc level has been reached, may increase the drying costs and lower their quality. Grain moisture measuring instruments were developed over the years since Briggs (1908) found out the existence of a linear relationship between the logarithm of the electrical resistance of wheat and its mc (Nelson, 1977). Instruments for mc determination presently available in United States are generally the RF capacitance sensing type, though some measure the electrical conductance. Hutchison and Holaday (1978) developed a moisture profile meter for peanut kernels. Single kernels were fed into a crushing roller system and the signal peaks that resulted from the dc conductance of the kernel as it passed between the crushing rollers were measured and correlated to the mc of the kernels. More recently a nondestructive method to estimate the mc of single peanut kernel by measurements on a small parallel-plate capacitor holding the kernel between the plates has been reported (Kandala and Nelson,1990). This technique, which uses the complex impedance of a parallel-plate system with the peanut kernel held between the plates at two frequencies 1 and 4.5 MHz, determined the single kernel mc within 1% of the standard oven values for 97% of the kernels measured over a moisture range between 5 and 15%. The mc measurement during the drying process of peanuts is currently done using capacitance type commercial moisture meters that measure moisture values on bulk samples of kernels. In this method a peanut sample of about 500 grams from a trailer is collected by an operator, the peanuts are shelled, cleaned, placed in the sample cup of the moisture meter and the mc is read. This is repeated for samples from all other trailers. This method is destructive and time consuming. It would be very useful if the peanut kernel moisture could be estimated from any physical measurements done on the peanut pods (in-shell peanuts) nondestructively. Further when large samples are used for measurements the average value may look acceptable but there could be some high moisture peanuts blended in the sample undetected. Pockets of high moisture peanuts can be conducive to the growth of micro-organisms which may lead to the spoilage of the peanuts. Thus it may be useful to measure the mc in smaller samples to check moisture variations in the lots instead of using bulk samples. Experiments conducted earlier on equilibrated samples indicated a good correlation between the peanut pod moisture and the peanut kernel moisture. Thus from a knowledge of the peanut pod moisture the mc of the kernels inside the pod may be estimated to an acceptable accuracy.

Last Modified: 4/17/2014
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