INTRODUCTION
Thailand is one of the agricultural exporter countries, for example;
fruits, vegetables, fish, rubber etc. Fruits such as orange, pineapple
etc are the important exported fruits of Thailand. A number of the nondestructive
and destructive methods was been used to inspection. For instance, mango
and durian inspection was studied by Krairiksh et al. (2004) using
inverse scattering technique and Kongrattanapraset et al. (2001)
using force vibration and ultrasonic, respectively. In addition, Nelson
and Bartaly (2002) measured the permittivity of a homogenized macaroni,
cheese, wheat apple juice.
In this research, we study the behavior of dielectric constant, dielectric
loss factor and loss tangent of four juices  apple, orange, lemon and
pineapple which depend upon frequency and age. Firstly, the parameters
of material are reviewed and followed by measurement. Results are shown
in topic 3 and finally concluded.
DIELECTRIC PARAMETERS
In this topic, we briefly review the permittivity of materials. This
parameter indicates the electromagnetic energy stored in a material.
Generally, the permittivity is a complex number whose real and imaginary
parts depend upon frequency as equation Balanis (1989):
where
and are
the real and imaginary parts of complex permittivity, respectively.
Divided Eq. 1 by permittivity of free space ,
it sis known as the relative complex permittivity,,:
where the real part, έ_{r }(f), is the dielectric constant
and the imaginary part, ε_{r}″(f), is the dielectric
loss factor.
The ε_{r}″(f) indicates the loss and conductivity
of material as below equation:
where σ is the conductivity and f is the operating frequency of
electromagnetic wave.
The definition of loss tangent, which is another important parameter
of dielectric, is the ratio of imaginary part to real part of complex
permittivity given by;
where δ is the loss tangent of material.
MEASUREMENT
The electrical equipments to measure the complex permittivities of fruits
are Network Analyzer E5061A, coaxial cable RG58A/U and dielectric probe.
The regular calibration procedure is performed under opencircuit, shortcircuit
and distilled water at temperature of 25°C references. Dielectric
constants and dielectric loss factors are calculated by Agilent Technologies
85070 Dielectric Probe Kit Software version E1.00. In this paper, four
samples of fruits; apple, orange, lemon and pineapple, are studied on
a frequency range from 20 kHz to 1.5 GHz.
RESULTS
The variation of permittivity properties of four fruits on the first
day was performed at 20 MHz, 790 MHz and 1.5 GHz as shown in Table
1 and Fig. 1.
The trend of loss tangent is likewise and the loss tangent on date 2
is the lowest. At the low frequency, the loss tangents are high and then
drastically decrease on high frequency range. The dielectric constant
and dielectric loss factor parameters on the first date are shown in Fig.
2.
From Fig. 2, At the low frequency, the dielectric constant,
dielectric loss factor and loss tangent of lemon and apple are the highest
and lowest, respectively. After that all of parameters decrease as the
frequency increases.
Loss tangent of apple and orange at 20 MHz, 286, 790 and 1.5 GHz, shown
in Fig. 3, is the same pattern. The oscillation of loss
tangent occurs at 20 MHz and then it is stable at 286, 790 MHz and 1.5
GHz.
Table 1: 
Permittivities of four juices at various frequency 


Fig. 1: 
Loss tangent of fruits vs frequency for (a) apple (b)
orange (c) lemon and (d) pineapple 

Fig. 2: 
Dielectric parameters of sample fruits vs frequency
on the first date in terms of (a) dielectric constant (b) dielectric
loss factor and (c) loss tangent 

Fig. 3: 
Loss tangent of sample fruits vs date for (a) apple
and (b) orange 
CONCLUSIONS
It is seen from this study that loss tangent, dielectric constant and
dielectric loss factor are obviously shown at low frequency. The oscillation
of these parameters on each date is thought to be due to the uncompleted
fermentation. However, these parameters tend to decrease as the age increases.
ACKNOWLEDGMENTS
The authors wish to thanks the EMC laboratory at the department of Electrical
Engineering, Khon Kaen University, Khon Kaen, Thailand who has supported
all the test equipments.