Abstract:
The moisture measuring method and apparatus determines the moisture content of post-harvest in-shell peanuts. A crystal oscillator generates a high frequency signal that is directed through a selected sample of in-shell peanuts. Capacitance, impedance, and phase change data associated with the sample are generated at (at least) two frequencies. The data is then substituted into a semi-empirical equation to determine the moisture content of the in-shell peanuts.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for measuring the moisture content of nuts. Specifically, the invention relates to a method and apparatus for measuring the moisture content of post-harvest in-shell peanuts. 
     BACKGROUND OF THE INVENTION 
     A determination of the moisture content of peanuts is essential to peanut processing. After harvest the peanuts are dried until the moisture content is below 10.5% by weight. Drying the peanuts further adversely affects the flavor and desirability of the nut. Drying also decreases the weight of a farmer&#39;s bulk peanut product, thereby correspondingly decreasing the amount that the farmer is paid for the product. However, if peanuts are not sufficiently dried, they are susceptible to infection by the mold fungus  Aspergillus flavus  which releases the toxic substance aflatoxin. 
     To produce safe, high quality peanuts, the peanut moisture content is tested essentially continually during the drying process. Currently the most conventional method for determining peanut moisture content requires that operators shell approximately 1 kg of peanuts and load the shelled peanuts into laboratory equipment for testing. However, this process is time consuming and inconvenient and, due to the need for continuous testing, eventually wastes a significant amount of the peanut product. 
     The need exists for a relatively quick, non-destructive, and convenient means of measuring in-shell peanut moisture content. The current invention comprises a mobile method and apparatus that accurately, efficiently, and non-destructively measures the moisture content of in-shell peanuts. 
     SUMMARY OF THE INVENTION 
     The current invention is directed to a method and an apparatus for determining the moisture content of a selected sample of in-shell peanuts. At the initiation of the moisture determination process, a sample of in-shell peanuts is loaded into a sample holder. An oscillator in combination with a crystal generates a 5 volt square wave signal. The signal is buffered and directed to a filter which shapes the signal into a sine wave. 
     The signal is then split into a measurement signal, a reference signal, and a phase detection signal. The reference signal is sent through a transformer and rectifier and then measured. The phase detection signal is sent through an attenuator and into a phase detector. 
     The measurement signal is sent to a multiplexer and then through the selected sample of in-shell peanuts. The measurement signal is then directed from the multiplexer to a range resistor in combination with an operational amplifier. A first portion of the measurement signal is then rectified and measured. A second portion of the measurement signal is sent to a comparator, which outputs the signal as a square wave. The signal is then directed to a filter and converted into a sine wave. The measurement signal is then directed to the phase detector which measures a phase change between the measurement signal and the phase detection signal. 
     The moisture content of the in-shell peanut sample is calculated based on measurement of the reference signal, the measurement signal, and the phase change. Specifically, a first data set is generated at a first oscillator frequency, and a second data set is generated at a second oscillator frequency. The measurements in the first and second data sets are converted to values for impedance, capacitance, and phase change—and then the values are substituted into a semi-empirical equation to determine the moisture content of the in-shell peanuts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of the moisture measuring apparatus of the current invention. 
         FIG. 2  is a block diagram schematic of the moisture content analyzer of the current invention. 
         FIG. 3  is a circuit diagram generally associated with the crystal Y 1  and oscillator U 6 . 
         FIG. 4  is a circuit diagram generally associated with the phase detector RPD 1  (U 9 ). 
         FIG. 5  is a circuit diagram for the circuit generally associated with the comparator U 13 . 
         FIG. 6  is a circuit diagram for the circuit linking the comparator U 13  to the phase detector RPD 1  (U 9 ). 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As generally shown in the  FIG. 1  schematic, the current invention comprises a system for measuring the moisture content of peanuts P while the peanuts are still in their shell (i.e. “in-shell peanuts”). The system includes a moisture content analyzer CI, a sample holder H, and a computer L. In the preferred embodiment, data derived from the analyzer CI is processed and displayed on a computer L, preferably on a portable laptop computer screen. The laptop L may also control the functions of the analyzer CI during the measurement process. 
     As shown in  FIG. 1 , an upper portion of the sample holder H comprises a hollow acrylic cylinder  10  fitted with a parallel-plate electrode assembly  20 ,  22 . In the preferred embodiment, the cylinder  10  is 190 mm long with an internal diameter of 50 mm and a wall thickness of 7 mm. The electrode assembly comprises two parallel rectangular aluminum plates  20 ,  22  that are 140 mm long and 50 mm wide. The plates  20 ,  22  are positioned inside the cylinder  10  approximately 25 mm from the ends of the cylinder  10 . The distance between the plates  20 ,  22  is about 42 mm. 
     The cylinder  10  rests on top of a rectangular acrylic box  16 . The box  16  includes a drawer  18  that slides in and out of an opening in the front side of the box  16 . The box  16  is constructed so that when the drawer  18  is closed, the sample peanuts P are supported inside the cylinder  10 . However, when the drawer  18  is pulled forward, the cylinder  10  aligns with a hole (not shown) in the top of the box  16  so that the sample peanuts P fall through the cylinder  10  and into a rear portion of the drawer  18 . 
     In operation, the moisture-measuring process is initiated by closing the drawer  18  and filling the cylinder  10  with in-shell peanuts P. The parallel plates  20 ,  22  are then energized and measurements are obtained. After the measurements are complete, the drawer  18  is moved to the open position so that all peanuts P fall out of the cylinder  10 , through the hole in the top of the box  16 , and into the drawer  18 . The drawer  18  is then removed from the box  16  and the peanuts P in the drawer  18  are deposited back into a source container. The drawer  18  then slides back into the box  16  so that the cylinder  10  can once again be filled with sample peanuts P and the measurement process can be repeated. 
     One aspect of the current invention comprises an improved moisture content analyzer CI. The analyzer CI automatically measures selected properties of the peanut sample at three frequencies: 1, 5, and 9 MHz.  FIG. 2  shows a block diagram associated with the 1 MHz signal, however the circuitry associated with the 5 and 9 MHz signals is essentially similar.  FIGS. 3-6  are circuit diagrams that show the moisture content analyzer CI in greater detail. 
     As best shown in  FIGS. 2 and 3 , an original signal is generated by a crystal Y 1  in combination with an oscillator U 6 . In the preferred embodiment, the oscillator U 6  comprises an ICM 7209 chip. The oscillator U 6  generates a 1.0 MHz square wave which has a 5V amplitude. As discussed supra, similar circuits are used to generate 5 and 9 MHz signals. 
     As shown in  FIG. 3 , the original signal is then buffered by operational amplifier U 7 . The signal then flows to operational amplifier U 8  which filters and shapes the signal into a sine wave of the same amplitude (i.e. 5V). In the preferred embodiment the U 7  and U 8  operational amplifiers comprise AD841 amplifiers, which are well-known in the art. 
     As shown in  FIGS. 2 and 3 , a measurement portion of the original signal is then directed into a multiplexer at S 1 . As best shown in  FIG. 4 , another portion of the original signal is sent through an operational amplifier U 10  (also an AD841). The original signal is then split into a reference signal and a phase detection signal. The reference signal is directed to an amplitude-measuring system comprising a transformer T 1  and a rectifier D 1 . This reference signal is measured and designated as ER 1 . The phase detection signal is directed through an attenuator R 10  and is fed into a phase detector RPD 1  (U 9 ). 
     As best shown in the lower right portion of  FIG. 3 , the measurement signal at S 1  enters a multiplexer at pin  13  and exits at pin  12 . The measurement signal is then directed through an electrode plate  20  and into the sample holder H. After passing through the peanuts P in sample holder H, the measurement signal is received by an electrode plate  22  and directed back into the multiplexer at pin  4  and emerges at pin  8 . At S 5  the measurement signal is directed away from the multiplexer and into a circuit associated with a comparator U 13 , as shown in greater detail in  FIG. 5   
     As best shown in  FIG. 5 , the measurement signal at S 5  is directed into an operational amplifier U 11  (also an AD841) and through a variable range resistor R 12 . The signal is then buffered by an operational amplifier U 12  (also an AD841). A first portion of the measurement signal is sent through the transformer T 2  and then rectified and measured as EM 1 . The magnitude of the impedance of the in-shell peanut sample at 1 MHz is calculated as |Z 1 |=R 12  (ER 1 /EM 1 ). 
     As best shown in  FIG. 5 , a second portion of the measurement signal is sent to a comparator U 13 , which outputs the signal as a square wave. In the preferred embodiment, the comparator U 13  comprises an AD9866. 
     As shown in  FIG. 6 , the measurement signal is then directed to a filter U 14  (an AD841) and converted into a sine wave. The measurement signal is then buffered by U 15  (also an AD841) and directed to the phase detector RPD 1  (U 9 ). As described supra (and shown in  FIG. 4 ), the phase detector RPD 1  (U 9 ) also receives the phase detection signal. The phase detector RPD 1  output voltage is designated as PH 1 . The output voltage PH 1  is proportional to the phase angle θ 1 . 
     After Z 1 , and θ 1  have been measured and calculated for the 1 MHz signal, the real and imaginary parts of the impedance R and X are calculated as R=|Z| Cos θ and X=|Z| Sin θ. The 1 Mhz value of capacitance C 1  of the peanuts P in the sample holder H is given as: 
     
       
         
           
             
               C 
               ⁢ 
               
                   
               
               ⁢ 
               1 
             
             = 
             
               1 
               
                 2 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 π 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 fX 
               
             
           
         
       
     
     Once capacitance C 1 , phase angle θ 1 , and impedance Z 1  have been calculated for the 1 MHz signal, the computer L (see  FIG. 1 ) then switches the multiplexer to allow a 5 MHz signal to pass through the sample holder H. The signals are processed through a circuit similar to the 1 MHz circuit but with the range resistor R 12  set at a different value. The impedance magnitude Z 2  and the phase angle θ 2 , and capacitance C 2  are determined for the 5 MHz frequency as was done for 1 MHz signal. Similar measurements are made with a 9 MHz signal and impedance Z 3 , and phase angle θ 3 , and capacitance C 3  are determined at this frequency. 
     Moisture Content Determination 
     The moisture content of a given in-shell peanut sample is determined by calculating phase angle θ, capacitance C, and impedance Z as described supra, at (at least) two frequencies and then substituting the calculated values into a previously-derived moisture content algorithm. The moisture content algorithm was originally derived by the inventors by identifying eight “moisture calibration groups” of in-shell peanut samples. The moisture groups included in-shell peanuts with known moisture levels varying from 6% to 25%. Each of the eight calibration groups was subdivided into 30 sets of samples so that there was a total of 240 samples Each sample was placed in the sample holder, and C, θ and Z values of each sample were obtained using the CI analyzer (as described supra) at each of at least two frequencies. 
     The moisture content of each sample was then determined by the standard air-oven method. Multi-linear regression analysis (MLR) was applied to the measurements for each of the 240 samples, with the moisture content of each sample as a Y variable, and the difference in the C, θ and Z values (at the two frequencies) as the corresponding X variables. 
     Based on these measurements, the moisture content (MC) regression calibration equation thus derived by the inventors is:
 
 MC=A   0   +A   1 ( C 1 −C 2)+ A   2 (θ1−θ2)+ A   3 ( Z 1 −Z 2)+ A   4 ( C 1 −C 2) 2   +A   5 (θ1−θ2) 2   +A   6 ( Z 1 −Z 2) 2  
 
     Where A 0  to A 6  are calibration constants derived through MLR analysis using peanut samples with a known moisture contents. 
     In summary, as briefly described supra, the moisture content (MC) of a selected in-shell peanut sample is determined by determining the values of C, θ and Z using the moisture content analyzer C 1  at (at least) two frequencies (for example 1 and 5 MHz), and then substituting these values into the calibration equation described supra along with the previously-determined reference values of the calibration constants A 0  to A 6 . 
     The inventors have verified the performance of the calibration equation described supra based on Standard Error of Calibration and coefficient of determination (R 2 ). 
     For the foregoing reasons, it is clear that the invention provides an innovative peanut moisture measuring method and apparatus. The invention may be modified in multiple ways and applied in various technological applications. The current invention may be modified and customized as required by a specific operation or application, and the individual components may be modified and defined, as required, to achieve the desired result. 
     Although the materials of construction are not described, they may include a variety of compositions consistent with the function of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.