DIELECTRIC PROPERTIES OF SIMULATED GREEN COCONUT WATER FROM 500 TO 3,000 MHZ AT TEMPERATURES FROM 10 TO 80 ºC. Abstract.

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1 DIELECTRIC PROPERTIES OF SIMULATED GREEN COCONUT WATER FROM 500 TO 3,000 MHZ AT TEMPERATURES FROM 10 TO 80 ºC Arlet P. Franco; Carmen C. Tadini; Jorge A. W. Gut University of São Paulo/NAPAN - Food and Nutrition Research Center, São Paulo-SP, Brazil. Abstract Dielectric properties can characterize the interaction of the electromagnetic energy with a material. These properties are relevant to radio frequency and microwave heating of food. They are mainly influenced by temperature, composition and frequency. The dielectric properties of simulated green coconut water were evaluated using an open-ended coaxial line probe technique at temperature intervals of 10 C from 10 to 80 C. The properties were correlated with temperature for the commercial frequencies used for microwave heating (915 and 2,450 MHz). The temperature increase lowered the relative permittivity ( ) because the thermal agitation disturbs the polarization of the water molecule. Meanwhile, dielectric loss factor ( ) increased with temperature at 915 MHz because of the improved ionic conduction, and it decreased with temperature at 2,450 MHz due to the effect of the dielectric relaxation of the water molecule. 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 in the fruit or processed in order to extend its shelf life by inactivation of enzymes and microorganisms (Matsui et al., 2008). Nowadays, conventional thermal treatments such as pasteurization and sterilization with the addition of preservatives 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 (Decareau, 1985; Ryynänen, 1995), such as microwaves. Heating is supported on the ability of a dielectric material to absorb electromagnetic energy and convert part of it to thermal energy (Datta et al., 2005; Nelson and Datta, 2001). The food material heating is the main result of two mechanisms: ionic conduction and dipolar rotation. This heating 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 rapid 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 food products with high unbound water content (Giese, 1992; Thostenson and Chou, 1999). Knowledge about dielectric properties of liquid foods is required in order to design thermal treatments using microwave energy (Giese, 1992). Dielectric properties characterize the interaction of electromagnetic waves with the material (Datta et al., 2005; Ryynänen, 1995) and provide information regarding heating and penetration depth. Dielectric properties of materials are defined in terms of the relative complex permittivity, where the real part ( ) is known as relative electric permittivity and the imaginary part ( ) is the dielectric loss factor. The relative permittivity represents the material energy storage capability in response to an applied electric field and the dielectric loss factor refers to energy 1

2 dissipation as heat (Datta et al., 2005; Sosa-Morales et al., 2010). These properties are affected by many factors. The main factors are material composition, temperature and the electric field oscillation frequency (Içier and Baysal, 2004; Ryynänen, 1995). Dielectric properties of several liquid food products have been studied over different frequency ranges and different temperatures, including apple, pear, orange, grape and pineapple juices (Zhu et al., 2012), honey (Guo et al., 2011), milk, dairy products and soy beverages (Coronel et al., 2008), grape juice (Garcia et al., 2001), grapes and sugar solutions (Tulasidas et al., 1994), milk (Kudra et al., 1992); but there are no references about green coconut water dielectric properties. Green coconut water is a complex product, its composition changes according to the maturity stage of the fruit; therefore simulated green coconut water was prepared with the average composition of fruits harvested with 7 month of development (Matsui et al., 2007). 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 this product. Materials and methods Samples 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 according to Matsui et al., (2007). The samples were cold stored and temperature was adjusted from 10 to 80 C using a thermostatic oil bath TC-550 (Brookfield, USA). Dielectric properties measurements The dielectric properties measurements were carried out using the open-ended coaxial line probe technique with a Dielectric Probe Kit 85070E (performance configuration) connected to an E5061B Network Analyzer (Agilent Technologies, Malaysia). An Electronic Calibration Module 85093C (Agilent Technologies, Malaysia) was used for improving the signal quality. Before the measurements, the network analyzer was warmed 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. The network analyzer calculates the relative permittivity and loss factor values through the reflection coefficient measurement of the transmitted signal from the tip of the probe in contact with the sample. In this work, the dielectric properties were analyzed at 915 and 2,450 MHz which are the frequencies used in industry and household, respectively. After each measurement the probe was cleaned with distilled water and dried with a soft paper. The dielectric properties measurements were carried out in independent triplicates at each temperature, and the results were correlated with temperature. Determination of power penetration depth The power penetration depth of electromagnetic waves in materials is defined as the depth in which the power is reduced to e -1 (e = ) (Ryynänen, 1995; Venkatesh and Raghavan, 2004). Penetration depth of microwaves in a food material was obtained using the relative permittivity and loss factor values (Ryynänen, 1995), and it was calculated according the following equation: 2

3 [ ( ) ] Where p is the penetration depth (m) and 0 is the wavelength in the free space (m). The penetration depth of microwaves into simulated green coconut water was calculated at 915 and 2,450 MHz, at temperature intervals of 10 C from 10 to 80 C. 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 ( ) 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. In case of the dielectric loss factor ( ), it showed in interesting dependence with temperature and frequency, as can be seen in Figure 1b. Increasing temperature from 10 to 80 C resulted in loss factor increment between 500 and 1,000 MHz. This may be as a result of the predominant ionic dispersion at low frequencies of microwaves. Ionic conductivity generally increases with temperature as a consequence of reduced viscosity and increased mobility of the ions (Tang et al., 2002). Datta et al. (2005) referred to this fact as thermal runaway, which is the material ability to absorb increasing quantities of microwave energy as its temperature increases. At microwave frequencies, the dielectric loss factor is composed of two components: dipole loss and ionic loss, which has different behaviors depending on frequency and temperature conditions. Usually dipole loss decreases with temperature, while the ionic loss increases with temperature (Al- Holy et al., 2005). Increasing field frequency from 500 to 1,000 MHz showed the dielectric loss factor drop for all testes temperatures, which 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 1b that there was an increase in the loss factor for temperatures between 10 and 50 C, which as associated with the dielectric relaxation of the water molecule. The dipole of the water molecule is unable to follow 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 curve) and this value increases with the temperature, in accordance to the trend of the curves in Figure 1b (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). 3

4 Relative permittivity ' C 20 C 30 C 40 C 50 C 60 C 70 C 80 C Frequency, MHz (a) Loss factor '' C 20 C 30 C 40 C 50 C 60 C 70 C 80 C Frequency, MHz (b) Figure 1. Relative permittivity ( ) (a) and dielectric loss factor ( ) (b) of simulated green coconut water at temperatures between 10 and 80 C and field frequencies from 500 to 3,000 MHz. The relative permittivity and the dielectric loss factor values of the simulated green coconut water were successfully correlated with temperature for the commercial frequencies of 915 and 2,450 MHz (Figure 2). The coefficients of the linear fit for the relative permittivity are: a 0 = , a 1 = for 915 MHz (R 2 = 0.997) and a 0 = , a 1 = for 2,450 MHz (R 2 = 0.999). The coefficients of the polynomials adjusted for dielectric loss factor are: a 0 = , a 1 = 4

5 , a 2 = for 915 MHz (R 2 = 0.997) and a 0 = , a 1 = , a 2 = for 2,450 MHz (R 2 = 0.999) Relative permittivity ' , Temperature, C (a) Loss factor '' , Temperature, C (b) Figure 2. Relative permittivity ( ) (a) and loss factor ( ) (b) of simulated green coconut water at frequencies of 915 and 2,450 MHz and temperatures from 10 to 80 C. Figure 3 shows the penetration depth values, calculated for temperatures from 10 to 80 C, at frequencies of 915 and 2,450 MHz. The penetration depth exhibit a decreasing behavior with the temperature increment at frequency of 915 MHz, the penetration depth drop from 32.9 at 10 C to 17.6 mm at 80 C. At 2,450 MHz, penetration depth slightly increased with temperature increment from 10 to 60 ºC and decreased at temperatures between 60 and 80 ºC. 5

6 Penetration depth, mm , Temperature, C Figure 3. Microwave penetration depth into simulated green coconut water (mm) at temperatures between 10 and 80 C and field frequencies of 915 and 2,450 MHz. As can be seen in Figure 3, the frequency of 915 MHz showed higher penetration depth values than 2,450 MHz. Actually, 915 MHz is the frequency used by the industry because food products of several sizes are processed and greater penetration depths are required for large products. Penetration depth changes at frequency of 2,450 MHz with increasing temperature reflects the dielectric loss factor behavior, which can increase or decrease depending of the sample temperature, as a consequence of different mechanisms of dielectric response of a salt solution to an applied electric field. The penetration depth values of the simulated green coconut water were successfully correlated with temperature for the commercial frequencies of 915 and 2,450 MHz (Figure 3). The coefficients of the linear fit for 915 MHz are a 0 = , a 1 = (R² = 0.994) and of the polynomial adjusted for 2,450 MHz are a 0 = 7.666, a 1 = , a 2 = (R² = 0.998). Conclusions Dielectric properties of simulated green coconut showed a temperature and frequency dependent behavior. The relative permittivity decreased with temperature increasing from 10 to 80 ºC at any given frequency. The loss factor and penetration depth were 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 showed the lowest values, while for temperatures from 10 to 30 C the lowest value was found at frequencies around 1,000 MHz. The penetration depth decreased linearly with temperature at the frequency of 915 MHz and increased with temperatures between 10 and 60 C at the frequency of 2,450 MHz. Acknowledgements The authors would like to acknowledge financial support from São Paulo Research Foundation (FAPESP) under grant #2012/ and from CNPq (National Council for Scientific and Technological Development). 6

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