Elsevier

Biosystems Engineering

Volume 158, June 2017, Pages 95-101
Biosystems Engineering

Research Paper
Influence of mode stirrer and air renewal on controlled microwave drying of sliced zucchini

https://doi.org/10.1016/j.biosystemseng.2017.03.012Get rights and content

Highlights

  • Controlled microwave drying of zucchini slices using infrared thermography is studied.

  • Mode stirrer and/or air renewal were evaluated on drying time and quality of zucchini.

  • Stirrer and air renewal reduced the drying time of zucchini slices by about half.

  • The samples treated by stirrer and air renewal showed less variation in colour.

An on-line temperature-controlled microwave (MW) system, based on infrared thermography, was developed for drying zucchini slices. Moreover, the effects of different configurations, including turntable rotation, mode stirrer and air renewal, alone or in combination, on drying kinetics and quality of dried product were evaluated. The combination of stirrer and air renewal reduced the drying time of zucchini slices by about 48% with respect to the basic configuration and by 22% with respect to air renewal only. The lowest oscillation of temperatures about the fixed level (65 °C) was found when operating with the stirrer mode, because of the improved uniformity of the MW distribution. The samples treated by stirrer and air renewal configurations showed less variation in colour: the decrease of brightness was about 19% with respect to fresh samples and the value of global variation of colour, ΔE, was 18. It is important to note that air renewal in MW processing is an almost zero energy-cost option.

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:NDR=tmw(t)tmw(t)|t=t0where 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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