Broadband dielectric properties of honey: effects of temperature

Abstract

Dielectric properties of jujube honey were investigated at 298–358 K by broadband dielectric measurements. Four relaxation processes were observed and analyzed, which are caused by long range correlation of density fluctuation, cooperative motions of molecules, rotational polarization of bound water and collective reorientation of free water, respectively. The results of temperature dependence of dielectric parameters show that with increasing temperature, the interaction among the molecules e.g. water, fructose and glucose molecules etc. weaken, and the honey gradually forms a complete sugar solution. At a given temperature, the penetration depth at 27 MHz is much greater than that at 915 MHz and 2.45 GHz. And based on the calculated penetration depth, dielectric heating at 27 MHz seems to has more advantages for large volume of materials.

Introduction

Honey is a supersaturated solution of sugars, wherein the fraction of monosaccharides (e.g. fructose and glucose) and disaccharides is about 75% and 10–15%, respectively (da Silva et al. 2016). Such high amount of sugars presenting in honey make it easy to crystallize spontaneously at room temperature (Lazaridou et al. 2004). Crystallization would lead to an increase in water activity, which allows the growth of osmophilic yeasts and affects its stability (Gleiter et al. 2006). Thus, thermal treatment is often used to dissolve sugar crystals, destroy yeasts and improve stability (Escriche et al. 2009). Nevertheless, traditional thermal treatments owing to slow heat conduction and long heating time are easily to damage the quality and nutritional value of honey, and cause the formation of harmful substances e.g. 5-hydroxymethylfurfural (HMF) (Turhan et al. 2008; Fennema 1996; Tosi et al. 2002; Guo et al. 2011). To overcome these disadvantages, some new heating methods may be needed in honey industry.

Dielectric heating, including radio-frequency(RF) and microwave (MW) heating, with fast volumetric heating can overcome these disadvantages, and its fundamental principle is direct transfer electromagnetic energy into materials (Wang et al. 2007). And the key factors influencing dielectric heating effect are the dielectric properties of food materials which depend on the frequency, temperature, water content and ionic conductivity of material. The influences of temperature on dielectric properties are controlled by the free and bound water contents in food materials (Calay et al. 2010). Several reports show that the dielectric properties of honey mostly depend on the frequency and water at room temperature (Ahmed et al. 2007; Guo et al. 2010).

The information on dielectric properties of honey is the key to developing successful pasteurization processes based on RF and MW energy (Wang et al. 2003). Hence, the purpose of this work is to investigate the effects of temperature on the dielectric properties of jujube honey from 40 Hz to 110 MHz and 500 MHz to 40 GHz.

Materials and methods

Honey

The jujube honey was obtained from Yifeng Tang Ecological Bee Garden Co., Ltd, Xi’an, Shaanxi, China. The samples were measured in fresh state and without crystallization. The initial water content in jujube honey is 18% supplied by company.

Broadband dielectric measurements

The radio frequency dielectric measurements were performed by Agilent 4294A precision impedance analyzer (Agilent Technologies) from 40 Hz to 110 MHz. The cell for dielectric measurements consists of concentrically cylindrical platinum electrodes. All the experimental data were corrected for errors arising from stray capacitance (C_r), cell constant (C_l), and residual inductance (L_r) by Schwan method (Schwan 1963). They are determined with three standard substances (pure water, ethanol and air) and KCl solution of varying concentrations. Then the corrected data of capacitance C_s and conductance G_s at each frequency were converted to permittivity and conductivity by equations ε = C_s/C_l and κ = G_sε₀/C_l (ε₀ is the permittivity of vacuum).

The microwave dielectric measurements were carried out in the frequency range from 500 MHz to 40 GHz by Agilent E8363C PNAseries network analyzer (Agilent Technologies, made in America) equipped with an Agilent 85070E open-ended coaxial probe (Agilent Technologies, made in America). The permittivity ε and dielectric loss ε″ of jujube honey at different frequency were obtain by immersing the probe into the samples. The error of the analyzer is calibrated before measuring the dielectric properties of honey by measuring three standards: deionized water, air, and a short circuit.

The dielectric data of jujube honey was measured at the temperatures of 298, 308, 318, 328, 338, 348, 358 K. And the temperatures of honey were controlled by a circulating thermostatted water jacket.

Dielectric analysis

The complex permittivity of honey is expressed as

$${\epsilon }^{\ast }(\omega )=\epsilon (\omega )-j{\epsilon }^{″}(\omega )=\epsilon (\omega )-j\frac{\kappa }{\omega {\epsilon }_0}$$ 1

where $\epsilon$ and ${\epsilon }^{″}$ are permittivity and dielectric loss respectively, ω is angular frequency and j = (− 1)^1/2, and κ is conductivity. The electrode polarization (EP) was observed in the dielectric spectra of honey. To eliminate the EP effect, we fit raw data with Cole–Cole equation (Eq. 2) containing the EP term $A{\omega }^{-m}$ (A and m are adjustable parameters)

$${\epsilon }^{\ast }(\omega )={\epsilon }_h+\sum _g\frac{\mathrm{\Delta }{\epsilon }_g}{1+{(j\omega {\tau }_g)}^{{\beta }_g}}+A{\omega }^{-m}$$ 2

ε_h is high-frequency limit of permittivity, Δε_g and τ_g (= 1/(2πf_0g), f_0g is characteristic relaxation frequency) represent dielectric increment and relaxation time of the gth relaxation, respectively; the parameters β_g characterize the shape of the curve, which generally satisfy 0 < β_g < 1.

Penetration depth

Penetration depth (D_P) is the distance from the surface of the material at which the electromagnetic power is reduced to 1/e (e = 2.7182) of its surface value (Hippel and Robert 1954). It is the measure of how deep the electromagnetic power can penetrate into a material. It plays a crucial part in evaluating heating uniformity and designing dielectric heating equipment. It is a function of frequency and the dielectric properties of the material, and can be calculated by Eq. 3.

$$D_p=\frac{c_0}{2\pi f\sqrt{2\epsilon \left(\sqrt{1+{\left({\epsilon }^{″}/\epsilon \right)}^2}-1\right)}}$$ 3

where c₀ (= 3.0 × 10⁸ m/s) is the velocity of light in free space and f is the frequency of electromagnetic power.

Results and discussion

Conductivity

Figure 1 shows the frequency dependence of conductivity at different temperatures. The conductivity, which is mainly dominated by concentration and mobility of free ions, increases with increasing temperature. And the concentration of free ions in honey is almost constant with increase in temperature, thus, the effects of temperature on conductivity of honey are mainly controlled by mobility rather than concentration of free ions. The mobility of free ions is inversely proportional to the viscosity of honey which decrease with increase in temperature (Gómezdíaz et al. 2009), so, the conductivity of honey decreases with increasing temperature.

Fig. 1.

Frequency dependence of conductivity of jujube honey at different temperatures

Relaxation mechanism and temperature dependence of dielectric parameters

Figure 2a represents the frequency dependence of permittivity of jujube honey at different temperatures. As temperature increases, the electrode polarization phenomena at low-frequency is shift to higher frequency. As frequency increases, two relaxation processes are observed, which are readily differentiated from each other at low temperature, and gradually overlap with increasing temperature. The jujube honey is mainly composed of monosaccharide molecules such as fructose and glucose etc., and fructose is the carbohydrate in greatest proportion (da Silva et al. 2016). And based on the previous reports (Kaminski et al. 2008; Singh et al. 2011; Hwang et al. 2013), the low and high frequency relaxation may be the slow mode relaxation (Fischer clusters) and structure α relaxation process, respectively. The slow mode relaxation is due to long range correlation of density fluctuation affected by the content of glucose (Sidebottom 2007). And the structure α relaxation is related to the cooperative motions of the molecules (Yamamoto et al. 2015).

Fig. 2.

a Frequency dependence of permittivity for jujube honey at different temperatures. b Temperature dependences of dielectric parameters for jujube honey. Black lines and symbols represent dielectric increment Δε. Blue lines and symbols represent relaxation time τ

To describe the two relaxation processes in detail, the dielectric parameters were obtained by fitting the permittivity spectra (Fig. 2a) with Cole–Cole equation (Eq. 2). Figure 2b represents the frequency dependence of dielectric increment Δε and relaxation time τ of jujube honey. As temperature increases, the Δε_s and τ_s of slow mode relaxation decrease, meaning the number and size of Fischer clusters formed in honey reduce. Because according to reports, the clusters may form in monosaccharide molecules by hydrogen bonds which maybe weaken or even break with increasing temperature (Kaminski et al. 2010). The Δε_α of structure α relaxation significantly decreases, which is in line with the previous reports (Shinyashiki et al. 2008; Yamamoto et al. 2015). This seems to support the view that the re-orientational motion of water molecules becomes less cooperative with saccharide molecules with increasing temperature. And this may indicate that the honey gradually forms a complete sugar solution. It is interesting to observe that the τ_α increases then decrease with increasing temperature, which is not consistent with previous reports (Shinyashiki et al. 2008; Yamamoto et al. 2015). Nevertheless, the cause of this phenomenon needs to further study.

Microwave dielectric spectrum

Figure 3 shows the influences of temperature on the dielectric loss of jujube honey at the frequency range from 200 MHz to 40 GHz. In the measured frequency region, an obvious relaxation process was observed in the range of 1–5 GHz which is much lower than 20 GHz of pure water at 293 K. It may be the contributions from bound water and free water (Yang et al. 2018). Because the characteristic frequency of bound water is much lower than that of free water, and the water presenting in pure honey is mostly in bound form. Although other relaxation mechanisms also present in honey, the relaxations caused by structural relaxation of saccharide molecule (Moran et al. 2000) or dipolar fluctuation of the exocyclic hydroxylmethyl groups (Noel et al. 1992, 1996) are present far below 500 MHz (Shiraga et al. 2015). The characteristic frequency of this relaxation is shift to higher frequency with increasing temperature, which corresponds to previously reported by Guo et al. (2011). Maybe because as temperature increases, the hydrogen bonds between water molecules and saccharide molecules weaken, and the re-orientation motions of water molecules would become easier.

Fig. 3.

Frequency dependence of dielectric loss for honey at 298–358 K

To further explain the effects of temperature on microwave dielectric properties of jujube honey, microwave dielectric spectrum was well fitted by Eq. 2. As the representative example, the frequency dependence of dielectric loss of jujube honey at the temperatures 298 K, 308 K and 328 K was showed in Fig. 4. Two sub-relaxations were observed in the dielectric spectrum. The low-frequency relaxation results from rotational polarization of the water adsorbed on saccharides, and the high-frequency relaxation is caused by collective reorientation of free water. Both relaxation processes are shift to higher frequency with increase in temperature. This may be because with increasing temperature, the hydrogen bonds between saccharide molecules and water molecules weaken, and rotational polarization of the bound water molecules become easier. At the same time, the motion of free water molecules become faster, and the collective reorientation of the free water also become easier.

Fig. 4.

Frequency dependence of dielectric loss of jujube honey at temperatures 298 K, 308 K and 328 K. (Symbol—experimental data, solid line—overall relaxation obtained by fitting, dash dot—free water, dash—bound water)

Penetration depth

Figure 5 shows the influences of temperature on penetration depth of jujube honey calculated by Eq. 3 at 27 MHz, 915 MHz and 2.45 GHz. There are some differences in the tendency of penetration depth with temperature at different frequencies. As temperature increases, the penetration depth increases at 27 MHz, however, at 915 MHz and 2.45 GHz, it fluctuates and has a maximum value at 328 K. This indicates that there is an optimal temperature for jujube honey by dielectric heating within the measured temperature range. It also shows that penetration depth decreases with increasing frequency at a given temperature, and the penetration depth at 27 MHz is much greater than that at 915 MHz and 2.45 GHz. Meaning the penetrating power of electromagnetic waves of the former is better than that of the latter. Because to effective pasteurization with dielectric heating, the thickness of food material should not exceed 2 or 3 times the penetration depth (Schiffmann 1995). Furthermore, according to the calculated penetration depth, dielectric heating at 27 MHz seems to has more advantages for large volume of materials. Nevertheless, besides the frequency and dielectric properties of food material, the distributions and magnitudes of electromagnetic waves inside food is also affected by the shapes, sizes etc., and make it difficult to estimate the efficiency of dielectric heating. Thus, to successfully apply dielectric heating to food industry, more researches on dielectric properties of various foods under different conditions is needed.

Fig. 5.

Effects of temperature on penetration depth Dp of jujube honey at 27 MHz, 915 MHz and 2.45 GHz

Conclusion

In this work, the broadband dielectric properties of jujube honey were explored at temperatures from 298 to 358 K. Four relaxation processes were observed and analyzed, and corresponding relaxation mechanisms were suggested. From the low to high frequency, the relaxations resulted from long range correlation of density fluctuation, cooperative motions of molecules, rotational polarization of bound water and collective reorientation of free water respectively. The temperature dependence of dielectric increment and relaxation time of these relaxations were also analyzed and discussed. The results implied that as temperature increases, the number and size of Fischer clusters formed in honey decrease, re-orientational motion of water molecules becomes less cooperative with saccharide molecules, and hydrogen bond interactions between water and saccharide molecules are weakened. Besides, the honey may gradually form a complete sugar solution with increasing temperature.

The penetration depth of jujube honey at 27 MHz, 915 MHz and 2.45 GHz were also calculated at the temperatures from 298 to 358 K. The affects of temperature on penetration depth at 27 MHz is more than that at 915 MHz and 2.45 GHz. And based on the calculated penetration depth, dielectric heating at 27 MHz seems to has more advantages for large volume of materials.

Compliance with ethical standards

Conflict of interest

Tha authors declare that there is no conflict of interests.