Peanut drying reduces the moisture content (MC) of harvested peanuts to a level at which the quality is maintained (Young et al., 1982). Drying is generally performed in two stages: 1) field curing in inverted windrows to 20–25% dry basis MC, and 2) drying in wagons or bins to about 10% dry basis MC (Baldwin et al., 1990).

Field curing is a natural process and the factors that affect it have been studied and described extensively by other researchers (Young et al., 1982). Wagon or bin drying is a process in which water is removed from farmer stock peanuts (field dried peanuts at 20–25% MC dry basis) through moisture and temperature gradients created by air flowing through the mass of peanuts. In practice, the wagon contains a deep bed of peanuts, where air is forced upwards through the peanuts. Requirements for air flow in terms of volume, temperature and relative humidity, in wagon drying, combined with the drying time (about 18–24 h), make drying in wagons an energy intensive process. A large number of studies have been conducted to determine ways to increase the energy efficiency in air drying of peanuts (Troeger, 1982; Blankenship and Chew, 1978; Rogers and Brusewitz, 1977; Chai and Young, 1995). Past studies have frequently focused on the use of high temperatures and methods of reducing the amount of running time for fans (Baker et al., 1993; Blankenship and Chew, 1979; Butts, 1996).

Little research has been conducted on the use of alternative methods for energy input, such as microwave or radio frequency energy. The microwave region of the electromagnetic spectrum has long been used in a variety of industrial applications, ranging from telecommunication to dielectric heating of foods and other materials. Due to the large number of applications available, the Federal Communication Commission (FCC) regulates the use of the frequency bands of the electromagnetic spectrum, and the two microwave frequencies reserved for dielectric heating in the United States are 915 and 2450 MHz. The mechanism of dielectric heating has been thoroughly analyzed and described in many studies (von Hippel, 1954; Rosenthal, 1992; Clark et al., 1997; Schiffmann, 1997).

The potential for ionic conduction and dipole rotation of materials is expressed by the imaginary part of the relative complex permittivity (relative dielectric loss) (Nelson, 1973; Boldor, 2003): 1 Where:

The power dissipated in dielectric heating is proportional to the frequency, dielectric properties and the electric field distribution according to the formula (Nelson, 1973; Metaxa and Meredith, 1983): 2 Where:

Microwave heating has been used in the past to reduce the MC of various fruits and vegetables such as banana (Musa acuminate), (Maskan, 2000); apples (Malus pumila) ; mushrooms (Amanita aspera); and strawberries (Aphelenchoides fragariae), (Funebo and Ohlsson, 1999; Funebo et al., 2000; Erle and Schubert, 2001); carrot (Daucus carota), (Prabhanjan et al., 1995; Sanga et al., 2000); corn (Zea mays L.), (Shivhare et al., 1991; Beke et al., 1995; 1997); potatoes (Solanum tubersum), (Bouraoui et al., 1994; Sanga et al., 2000); and broad beans (Phaseolus vulgaris), (Ptasznik et al., 1990). Blanching of endive (Chicorum endiva) and spinach (Spinacia oleracea), (Ponne et al., 1994), corn, (Boyes et al., 1997), and peanuts (Arachis hypogaea L.) (Rausch, 2002) as well as studies performed to investigate the shelf life and roast quality of microwave blanched peanuts (Katz, 2002) are also examples of utilization of microwave energy in processing of foods and agricultural commodities. Most of these studies were performed on static samples that were not characteristic of an industrial environment typified by continuous processing. With a few exceptions (Rausch, 2002; Katz, 2002), the referenced studies were based on modifications of multimode cavity home microwave ovens to allow air flow and temperature and mass measurement. An inherent problem in the use of multimode cavity ovens is non-uniform heating due to standing wave patterns created by the electric field (Rosenthal, 1992). Another drawback is that the multimode cavity home-oven frequency (2450 MHz) yields a shorter wavelength than the 915 MHz frequency found in industrial applications and therefore a shorter penetration depth (Metaxa and Meredith, 1983).

The TE₁₀ (transverse electric) traveling mode applicators for drying are widely used in the wood and paper industries (Metaxa and Meredith, 1983; Jones, 1975; Jones, 1986). The distribution of the electric field is relatively uniform at the center of any transversal section perpendicular on the main axis of a TE₁₀ waveguide (Boldor et al., 2005). A dielectric material placed at the center of the waveguide, running parallel with the electrical field, will be heated uniformly by the electric field.

The typical temperature profile and electric field distribution in continuous microwave drying of peanuts are described in another study (Boldor, 2003). Along the longitudinal axis of the waveguide, the electric field decreases exponentially as a function of distance, while the temperature profiles have three distinct regions (Boldor, 2003; Roussy and Pierce, 1995; Metaxa and Meredith, 1983). Initial heat-up occurs in the first region. The second region is the constant temperature drying region, and the third region is a region of temperature increase without any drying. For all practical purposes, any microwave drying process should stop at the end of the constant temperature drying region to maintain product quality.

This study focused on the effect of microwave power level and peanut initial MC on temperature distribution and potential for moisture removal of farmer stock peanuts (25 to 45% MC dry basis) in a continuous TE₁₀ traveling wave applicator using 915 MHz microwaves.

The general governing equation for the transport phenomena assuming negligible radiating losses during microwave drying is (Boldor et al., 2005, Metaxa and Meredith, 1983; Lu et al., 1999): 3 Where:

The coupling between heat and mass transfer phenomena described by Equation [3] makes analytical solution extremely difficult. Boldor et al. (2005) used the unique properties of the traveling wave applicators and assumptions to simplify the equation and divide the waveguide into the two regions. In the first region moisture loss and temperature gradient were considered negligible due to the volumetric heating and Equation [3] was reduced to (Boldor et al., 2005): 4 Where:

In the second region, only drying at a constant temperature occurs and Equation [3] reduces to (Boldor et al., 2005): 5 Where:

The solutions to Equations [4] and [5] and the discussion of the effect of moisture dependence on dielectric loss are presented elsewhere (Boldor et al., 2004; 2005).

Runner- and virginia-type peanuts were partially dried in windrows to MCS ranging from 25 to 45% dry basis. Samples were shipped from the USDA, ARS National Peanut Research Laboratory in Dawson, Georgia to the Department of Food Science at North Carolina State University during the months of September to November of 2002.

The microwave curing chamber (Industrial Microwave Systems, Morrisville, NC) was a traveling wave applicator composed of a conveyor belt running (v_z  =  8.4 mm/s) at the geometrical center along the axis of an aluminum waveguide. The microwaves were generated by a 5 kW microwave generator (Industrial Microwave Systems, Morrisville, NC) and transported to the curing chamber through aluminum waveguides. The curing chamber was outfitted with an electric fan and an electric heater to assist the microwave drying process. The heater was set to maintain an ambient temperature of 25 C in the chamber. The microwave generator was controlled through a data acquisition and control unit (HP34970A, Agilent, Palo Alto, CA) and a software routine written in LabView (National Instruments Corp., Austin, TX). The data acquisition unit and the software monitored and recorded the power output, reflected power and power at the exit of the microwave curing chamber through power diodes (JWF 50D-030+, JFW Industries, Inc., Indianapolis, IN).

All temperature measurements during microwave curing were performed using fiber optic probes and remote infrared temperature measurement (Mullin and Bows, 1993; Goedeken et al., 1991). The fiber optic probes (FOT-L/ 10M, Fiso Technologies, Inc., Quebec, Canada) were connected to a multi-channel fiber-optic signal conditioner (Model UMI 4, Fiso Technologies, Quebec, Canada) remotely controlled by FISOCommander software (FISO Technologies, Quebec, Canada) installed on a laptop computer (Dell Inspiron 8500, Dell Computer Corp., Round Rock, TX). The surface temperature of the peanut bed (3 cm thick) was monitored with infrared thermocouples (model OS36-T, OMEGA Engineering, Inc., Stamford, CN) placed at various distances along the waveguide. The surface temperatures were monitored and recorded using the same software routine that was used to control the generator and to record the power levels.

A Thermovision Alert N infrared camera (FLIR Systems AB, Danderyd, Sweden) was placed at the exit of the microwave curing chamber to monitor the spatial temperature distribution of the peanut bed surface. The camera was controlled by Thermovision Remote software (FLIR Systems AB, Danderyd, Sweden) installed on a laptop computer (Samsung SensPro 520, Samsung, Ridgefield Park, NJ).

Data collection was started simultaneously for all three systems (fiber optic probes, infrared thermocouples and infrared camera) in order to match the temperature measured with the fiber optic probes with the surface temperatures of the peanut bed as measured by the infrared thermocouples and the infrared camera.

Data were collected based on two experimental designs. The first reused the same samples (both runner- and virginia-type peanuts) in three consecutive passes at two power levels (1.2 and 2 kW) to simulate a three-stage microwave curing chamber.

For one sample of runner-type peanuts with an initial MC of 29%, the fiber optic probes were mounted such that both the internal temperature of the seed and the temperature at the pod surface were monitored while in the microwave curing chamber. For the other samples (one runner-type of 25% initial MC and one virginia-type of 46% initial MC), only the internal seed temperatures were monitored with fiber optic probes in the microwave curing chamber.

The second experimental design used runner-type peanuts to study the effect of six power levels (0.3, 0.6, 0.9, 1.2, 1.5, and 2 kW) and four initial MCs (33, 21, 14, and 11%) on seed temperatures, heating rates and MC. In order to get starting material of desired initial MCs, peanuts were dried with ambient air.

The power levels denoted in this paper were set points on the control panel of the software routine. Due to the non-linearity of the data acquisition and control unit, the nominal power levels (or the power output) of the generator were always smaller than the set points. The reflected power was caused by the change of impedance of the waveguide where the microwaves entered the planar applicator (Stuchly and Hamid, 1972; Griffiths, 1999). Although the impedance mismatch was minimized through the special design of the waveguide connector, the reflected power was considered lost.

The internal temperature measurements were performed on 8 to 12 peanut pods in two replicates. The fiber optic probes were placed in pods in previously drilled holes and spread to cover the whole width of the conveyor belt.

For each set of data the temperature profiles along the waveguides were determined by averaging all the measurements from the two replicates for the fiber optic probes measurements (internal temperatures) and the infrared thermocouple measurements (surface temperatures). Bulk MCs of the samples was determined using the ASAE standard (ASAE, 2000).

All temperature and MC data, with the exception of the infrared images, were processed using Microsoft Excel 97 (Microsoft Corp, Redmond, WA). The data on the spatial distribution of temperatures in the temperature images acquired with the infrared camera were processed with the Thermacam Researcher software package (FLIR Systems AB, Danderyd, Sweden).

The internal and surface temperature distribution was determined for farmer stock peanuts undergoing drying in a traveling wave microwave applicator. The experimental results confirmed previous theoretical predictions (Boldor et al., 2005) that the temperature profiles were determined only by the power level at the same MC and only by the MC at the same power level. At the same MCs, the maximum temperatures and maximum rates of temperature increase occurred in the same time at the same location in the microwave applicator at all power levels tested. The negligible effect of temperature on the dielectric properties of the farmer stock peanuts confirmed previously performed measurements (Boldor et al., 2004). At the same power levels, the maximum temperatures were the same for all MCs tested, but the rate at which that maximum was reached increased with the increasing MCs. This result shows the dependence of the dielectric loss and the attenuation constant α on the MC of the peanuts, an effect that was also previously observed (Boldor, 2003). The measured moisture losses during microwave drying were also in line with previous theoretical estimations (Boldor, 2003) and the total moisture reduction in consecutive passes at the same power level was determined.