(585x) Dielectric Properties Of Simulated Green Coconut WATER From 500 To 3,000 Mhz At Temperatures Between 10 and 80 ºc

Introduction

Green coconut water is a known
refreshing beverage and is distinguished for its content of minerals, vitamins
and sugars. It can be consumed naturally from the fruit or processed in order
to extend its shelf life. Nowadays, conventional thermal treatments such as
pasteurization and sterilization are used to preserve this kind of product
(Zhu, et al., 2012). However, high temperature is required to inactivate the
target enzymes peroxidase and polyphenol oxidase, resulting in degradation of color, taste and
nutritional value (Matsui et al., 2008; Zhu, et al., 2012).

Liquid foods can also be heated by
electromagnetic waves, such as microwaves (Decareau,
1985; Ryynänen, 1995). Microwave heating is supported
on the ability of a dielectric material to absorb electromagnetic energy and
convert part of it to heat (Datta et al., 2005;
Nelson and Datta, 2001). This method offers
advantages in pasteurization and sterilization processes when compared to
conventional heating methods. For instance, this treatment allow more rapid
heating and shorter processing time as a consequence of the molecular level
interactions of the food and the alternating electric field (Decareau, 1985; Venkatesh and Raghavan, 2004). Microwave heating has also disadvantages;
the main limitation of this technology is the non-uniform temperature
distribution during the process as a result of the limited power penetration of
electromagnetic waves in liquid water (Giese, 1992; Thostenson
and Chou, 1999).

Knowledge about dielectric
properties of liquid foods is required in order to design thermal treatments
for preservation using microwave energy (Giese, 1992). Dielectric properties
characterize the interaction of electromagnetic waves with the media (Datta et al., 2005; Ryynänen,
1995) and provide information regarding heating rate and penetration depth.

Dielectric properties of materials
are defined in terms of relative permittivity e' and dielectric loss factor e?. The relative permittivity
represents the material energy storage capability in response to an applied
electric field and the dielectric loss factor refers to energy dissipation as
heat (Datta et al., 2005; Sosa-Morales et al., 2010).
The power penetration depth of electromagnetic waves in materials is defined as
the depth in which the power is reduced to 1/e (e = 2.7183) (Ryynänen, 1995; Venkatesh and Raghavan, 2004). These properties are affected for many
factors; the main are composition, temperature and the electric field
oscillation frequency (Içier and Baysal,
2004).

In this work, dielectric properties
of simulated green coconut water were studied at frequencies from 500 to 3,000
MHz and temperatures between 10 and 80 ºC in view of continuous thermal
processing of green coconut water.

Materials and methods

Simulated green coconut water was
obtained dissolving fructose, glucose, sucrose, calcium chloride, magnesium
chloride, monobasic potassium phosphate, sodium sulfate and potassium sulfate
in distillated water to mimic the average composition of this product. The
samples were cold stored and temperature was adjusted from 10 to 80 °C using an
oil bath TC-550 (Brookfield, USA).

The dielectric properties
measurements were carried out using the open-ended coaxial line probe technique
with a Dielectric Probe Kit 85070E connected to an E5061B Network Analyzer
(Agilent Technologies, Malaysia). An Electronic Calibration Module 85093C
(Agilent Technologies, Malaysia) was used for improving the measurement
quality. Before the measurements, the network analyzer was allowed to warm up
for at least 90 min. The system was calibrated using the equipment standard
configuration: air, short-block and deionized water.
The measurements were performed for field frequencies between 500 and 3,000 MHz
at temperature intervals of 10 ºC from 10 to 80 ºC. In this work, the
dielectric properties were analyzed at 915 and 2,450 MHz which are used
industrially and household, respectively. The measurements were carried out in
triplicates at each temperature and the standard deviations were calculated. Results were correlated with temperature and the
penetration depths were calculated using proper equations for electromagnetic
wave penetration (Datta et al., 2005; Ryynänen, 1995; Zhu et al., 2012).

Results and discussion

Figure 1 shows the relative
permittivity and the loss factor of the simulated green coconut water at
temperatures from 10 to 80 °C as a function of the frequency. It can be seen
that these dielectric properties significantly depend on both temperature and
frequency. For all testing temperatures, the relative permittivity e' decreased with increasing
frequency between 500 and 3,000 MHz, which can be justified by the decreased
ability of the water molecule dipole in accompanying the oscillating electric
field at increasing frequency. In addition, this property also decreased with
temperature increase at any given frequency, because the thermal agitation
disturbs the dipole alignment with the electric field.

Figure 1: Dielectric propeties
of simulated green coconut water as affected by frequency and temperature

The dielectric loss factor e? showed in interesting dependence
with temperature and frequency, as can be seen in Figure 1. The drop on the
loss factor for frequencies between 500 and 1,000 MHz is associated with the
ionic conduction in the solution. This mechanism does not contribute with the
polarization of the media and only generate energy loss due to heating. For
increasing field frequency, there is less ionic movement and, therefore, a
decrease in the loss factor.

For higher frequencies, it can be
seen In Figure 1 that there is an increase in the loss factor, which as
associated with the dielectric relaxation of the water molecule. The dipole of
the water molecule is unable to follow very rapid field reversals, generating
heat and increasing the loss factor. The relaxation frequency of water at 20 ºC
is 17,004 MHz (peak of the relaxation curve) and this value increases with the
temperature, in accordance to the trend of the curves in Figure 1 (lower
temperatures show a stronger increase in the loss factor). It is known that for
frequencies beyond the relaxation frequency, the water dipole gradually losses
the ability to follow the oscillating field, reducing both the relative
permittivity and the dielectric loss factor (Datta et
al., 2005).

The relative permittivity and the
dielectric loss factor of the simulated green coconut water were successfully
correlated with temperature for the commercial frequencies of 915 and 2,450
MHz. The coefficients of the linear correlation for the relative permittivity
are a0 = 8.392E+01, a1 = ?2.648E-01 for 915 MHz (R2 = 0.9975) and a0 = 8.099E+01,
a1 = ?2.335E-01 for 2,450 MHz (R2 = 0.9986) and of the polynomials adjusted for
dielectric loss factor are a0 = 1.340E+01, a1 = 4.058E?02, a2 = 1.118E?03 for
915 MHz (R2 = 0.9956) and a0 = +2.037E+01, a1 = ?2.873E?01, a2 = 2.326E?03 for
2,450 MHz (R2 = 0.9873). The power penetration depth decreased with the
temperature increment at lower frequencies. At frequencies above 2,450 MHz,
penetration depth increased with temperature increment from 10 to 50 ºC and
decreased at temperatures between 60 and 80 ºC.

Conclusions

Dielectric properties of simulated
green coconut showed a temperature and frequency dependent behavior, which was
characterized. The relative permittivity decreased with temperature increasing
from 10 to 80 ºC at any given frequencies in the studied range. The loss factor
and penetration depth are generally lower at high frequencies from 500 to 3,000
MHz. At temperatures from 50 to 80 ºC and frequencies between 2,000 and 3,000
MHz the loss factor has the lowest values.

Acknowledgements

The authors
would like to acknowledge financial support from FAPESP
2012/04073-0 (The State of São Paulo Research Foundation) and to CNPq (National Council for Scientific and Technological
Development).

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