Dielectric properties of kiwifruit associated with a combined radio frequency vacuum and osmotic drying

Highlights

• Impedance analyzer was used to measure dielectric properties of kiwifruit.

• Effects of AD and OD on RF-vacuum drying characteristics were determined.

• A combination drying with RF-vacuum and osmotic dehydration was proposed.

Abstract

To develop a novel combination drying method combining radio frequency (RF) with vacuum and osmotic dehydration (OD), information on dielectric properties is essential so as to understand the interaction between the electromagnetic field and the matrix undergoing drying. In this paper, the dielectric properties of kiwifruit samples were determined across the frequency range of 10 and 3000 MHz using an open-ended coaxial-line probe and impedance analyzer at moisture contents ranging from 19.8% to 79.6% on a wet basis (w.b.) and temperatures between 20 and 80 °C. The effect of pre-treatments including OD and hot air drying (AD) on the dielectric properties, physicochemical properties and RF-vacuum drying characteristics of kiwifruit were also studied. The results showed that both the dielectric constant and loss factor decreased with decreasing moisture content and increasing frequency. At high moisture contents (above 65% w.b.), the dielectric constant decreased slightly with increasing temperature, whereas at lower moisture contents (below 50% w.b.), the dielectric constant increased sharply with increasing temperature. In addition, the OD resulted in less kiwifruit quality deterioration than AD in terms of titratable acidity, ascorbic acid, soluble solids and color. Although dielectric permittivities of OD treated kiwifruits were slightly lower than those of AD treated samples, the effect of the OD treatment on RF energy coupling was negligible compared to the AD. Therefore, OD may provide an alternative way for AD as a pre-treatment before RF drying for kiwifruits with high-quality characteristics.

Introduction

Recently, kiwifruit (Actinidia deliciosa) has received considerable attentions as a source of phytochemicals, such as phenolics, flavonoids and chlorophyll, increased consumption of which can have substantial health benefits for consumers (Hwang et al., 2017). The global production of kiwifruit was around 4.27 million metric tons (Mt) in 2016, with the majority of this production originating in China (2.39 Mt), Italy (0.52 Mt), New Zealand (0.43 Mt) and Chile (0.23 Mt) (FAOSTAT, 2018). However, fresh kiwifruit is susceptible to microbial spoilage and softening due to its high moisture content (i.e. over 80% on a wet basis (w.b.)) and deteriorates quickly over a very short period of time if improperly handled (Concha-Meyer et al., 2016). In addition, a seasonal availability often limits the consumption of fresh kiwifruits at certain times of year. Therefore, proper postharvest handling and shelf life extension to overcome the seasonal variation in supply is important to maximize the yield of high-quality kiwifruit products.

Drying is a unit operation that can be used to stabilize and preserve fresh fruits under ambient storage conditions by reducing their water activity (Zhang et al., 2017). In addition to its role in preservation, the associated reduction in bulk volume and weight of the resultant products lowers handling, packaging and transportation costs (Mujumdar and Law, 2010). Conventional drying methods, including AD (Maskan, 2001a, Maskan, 2001b), vacuum drying (VD) (Orikasa et al., 2014), OD (Castro-Giraldez et al., 2011) and freeze drying (FD) (Hwang et al., 2017) used for fruit drying often take a considerable amount of time, varying from several hours to over three weeks. Kiwifruit drying at an industrial scale is most commonly performed by AD, but this is characterized by low energy efficiency and long drying times (Maskan, 2001a). Water diffusivity may be accelerated when higher temperatures are applied during AD, but longer treatment times still remain under elevated temperatures, leading to a greater level of undesirable deterioration of product quality attributes including taste, color and nutritional value (Zhang et al., 2006). Currently, increasing demands for high-quality dehydrated fruits and vegetables have prompted research into dielectric heating based on microwave (MW) and radio frequency (RF) energy (Zhang et al., 2017, Zhou and Wang, 2018). MW and RF drying methods, often referred to as fourth generation drying technologies (Vega-Mercado et al., 2001; Wang et al., 2013), which may provide opportunities to reduce drying times and improve product quality and thus hold potential for practical postharvest applications for kiwifruit drying.

RF drying has attracted increasing attention due to its many advantages over MW heating including greater heating uniformity, deeper penetration depths and more stable product temperature control (Wang et al., 2014, Zhou et al., 2018). Pilot-scale RF systems have demonstrated practical applications in pest control (Gao et al., 2010; Wang et al., 2007a, Wang et al., 2007b), pasteurization/sterilization (Li et al., 2017a, Kou et al., 2018) and dehydration (Wang et al., 2014, Zhou et al., 2018, Zhou and Wang, 2018). However, dielectric drying based on RF energy alone has several disadvantages, such as high energy consumption and uneven heating (Zhou et al., 2018, Zhou and Wang, 2018). A combination of drying technologies is often applied to develop fast and energy-saving processes for effectively combining advantages and possible synergies among drying methods and minimize the limitations of individual drying technologies applied alone (Calin-Sanchez et al., 2014, Huang et al., 2012). RF drying can be combined with conventional drying methods, such as VD, OD and AD (Zhou et al., 2018). However, information on dielectric properties of the food matrix undergoing dehydration is critically important in order to develop effective postharvest dehydration methods based on RF combined with conventional drying.

The interaction between dielectric materials and RF energy is governed by the relative complex permittivity ԑ^∗ (ԑ^∗ = ԑ′-jԑ″). The real part of the relative complex permittivity (ԑ′, dielectric constant) describes the ability of a material to store energy. The imaginary part (ԑ″, loss factor) is associated with energy dissipation or the ability to transform from dielectric to thermal energy. Dielectric properties have been reported for various agricultural products and foods over different frequencies, temperatures and moisture contents for disinfesting (Sosa-Morales et al., 2009, Wang et al., 2003, Wang et al., 2005), pasteurization (Li et al., 2017b, Zhu et al., 2012) and drying (Guo and Zhu, 2014, Zhang et al., 2016). In addition, several studies are available on the dielectric properties of fruits across MW frequencies at different moisture contents and temperatures (Feng et al., 2002, Sipahioglu and Barringer, 2003, Wang et al., 2011). Furthermore, OD involving the immersion of food into hypertonic solutions is often used as a pre-treatment before RF and MW drying, resulting in water removal and simultaneous solute diffusion (Castro-Giraldez et al., 2011). The effects of OD on dielectric properties of fruits and vegetables, such as potato (De los Reyes et al., 2007), carrot or strawberries (Changrue et al., 2008), and mangosteen (Therdthai and Visalrakkij, 2012), were also determined in the literature over the last decade. These studies focused on improving dielectric properties of materials with additional OD treatments. Up to now, however, the effects of frequency, temperature, moisture content and different pre-treatments including AD and OD on the dielectric properties of kiwifruits have not been reported.

The objectives of this research were to fill these gaps in knowledge by: (1) measuring the dielectric properties of kiwifruits across a frequency range of 10–3000 MHz at four temperatures (20, 40, 60 and 80 °C) and five moisture content levels (20%, 35%, 50%, 65% and 80% w.b.) relevant to their dehydration, (2) determining the regression equations for describing the influence of moisture content, temperature and frequency (27, 40, 915 and 2450 MHz) (i.e. the most commonly used for dielectric drying applications), (3) studying the effect of pre-treatments (i.e. AD and OD), on the dielectric and physicochemical properties of kiwifruit, and (4) determining the effect of AD and OD on the RF-vacuum drying characteristics of kiwifruit slices.