Research PaperInfluence of mode stirrer and air renewal on controlled microwave drying of sliced zucchini
Introduction
The use of microwaves (MWs) could be successful to enhance heat transfer rates realised in conventional hot air drying process. In fact, in MW-assisted drying heat is not transferred from the surface but it is generated in the bulk of the material by absorption of electromagnetic (EM) energy from the MW field. Volumetric heating develops internal pressure gradients that drive water flow from the interior to the surface of the material thus realising fast drying. On the other hand, during the MW drying process, highly non-uniform temperature fields can be established inside the MW oven because of the non-uniformity of the EM field. Energy density changes both in space and in time are emphasised by the strong temperature-dependent MW absorption and by the decrease in volume of the material, as moisture is lost during the process. Therefore, quality loss for the final product is often observed, and a controlled MW power delivery is required to avoid or limit over-heating. The latter is not an easy task to realise because of well-known difficulties in measuring and controlling process temperatures. As a result, a wide and generalised expansion of MW industrial applications for drying has been slow.
In response to such critical issues, several techniques have been proposed to improve temperature uniformity in fruits and vegetables during MW drying process (Clary et al., 2005, Cui et al., 2005, Püschner, 2005), e.g. rotating supports (Calín-Sánchez et al., 2014, Geedipalli et al., 2007, Sutar and Prasad, 2007), as well as rotating stirrers (Zhenfeng, Raghavan, & Orsat, 2010). In this way, it is possible to shorten drying times and improve product quality, while achieving high nutritional and sensory quality (Zhang, Tang, Mujumdar, & Wang, 2006).
In this framework, the present study aims to show how different operational devices could be effective to improve the uniformity of the MW drying treatment, which obviously affects the final product quality and process speed. In particular, turning table, air renewal and stirrer mode were used and their effects evaluated individually or in combination during the drying of zucchini slices at 65 °C. The MW plant used was able to adapt the delivered MW power to keep the zucchini slices temperature at a fixed level while drying.
Section snippets
Microwave prototype
Drying experiments were carried out using a lab scale MW plant (Fig. 1), which houses a magnetron with a nominal power output of 2 kW operating at a frequency of 2.45 GHz; the generated MWs were fed to a metallic cubic chamber (1 m3) equipped with a fan placed on the bottom of the cavity for air renewal. A stirrer was placed inside the oven to improve heating uniformity. Since stirrer performance can be improved by increasing size and asymmetry (Wu & Chang, 1989), a fan with three blades 19 cm
Drying curves
The MR histories showed different trends depending on different configurations (Fig. 3). The corresponding normalised drying rates (NDR) were calculated from the kinetics curves after performing a polynomial regression of the experimental data to smooth out short-term fluctuations and finally evaluating the analytical derivatives:where the denominator represents the maximum drying rate obtained for time t0.
The rising rate typical of the initial drying period is
Conclusions
MW drying of zucchini slices under controlled temperature conditions was investigated. An IR thermography assisted system has proven to be an effective tool enabling on–off temperature control by detecting the instantaneous maximum temperature among the samples under test. Significant differences in terms of quality of the final product and drying times were found between the configurations under test. The time required to complete the drying process was about 166 min for the MW basic
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