Microwave pretreatment followed by associated microwave-hot air drying of Gundelia tournefortii L. : drying kinetics, en
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ORIGINAL
Microwave pretreatment followed by associated microwave-hot air drying of Gundelia tournefortii L.: drying kinetics, energy consumption and quality characteristics Safoora Karimi 1 & Narges Layeghinia 1 & Habib Abbasi 1 Received: 4 February 2020 / Accepted: 18 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Gundelia tournefortii L. is a well-known medicinal plant. Here, drying characteristics of it were investigated using combined microwave-hot air (CMWHA) drying method which consists of microwave pretreatment (specific powers of 450, 600 and 750 W), called primary shock, followed by associated microwave-hot air drying (180, 300 and 450 W assisted with 60, 70, and 80 °C inlet air temperature). Drying kinetics of the proposed method was modeled. Thus, experimental moisture ratio data was fitted to 10 thin layer models. It was found that Midilli et al. model had the best fit to show kinetics behavior. According to the results, drying rate was increased by increasing microwave power at a certain temperature. The maximum drying rate was obtained in the combined condition of 450 W and 80 °C with a primary shock of 750 W. Effective moisture diffusivity (Deff) ranged between 1.21 × 10−6 and 6.19 × 10−6 (m2/s); however it was 8.74 × 10−7 for hot air drying. Moreover, activation energy was in the range of 12.86 to 15.82 (kJ/mol). Comparison between energy consumptions showed that the lowest value was attributed to combined conditions of 180 W, 80 °C and a primary shock of 600 W which lead to 69% energy saving compared to hot air method. Quality of dried products was evaluated by measurement of colour parameters (L*, a*, and b*), total colour difference (ΔE), browning index (BI), Chroma, Hue angle, rehydration, and ratio of shrinkage. Generally, it was found that CMWHA method is the best choice to dry Gundelia tournefortii L. in terms of time, mass transfer rate, energy, and quality. Keywords Colour . Diffusivity . Drying kinetics . Mass transfer . Rehydration . Shrinkage
Nomenclature AAD Average Absolute Deviation. a Dimensionless drying constants. a* Colour parameter. b Dimensionless drying constants. b* Colour parameter. BI Browning index. Deff Effective moisture diffusivity (m2/s). D0 Pre-exponential constant of Arrhenius equation (m2/s), Ea Activation energy (kJ/mol). k Drying constant (1/min). L Half-thickness of slices (m). L* Colour parameter. * Habib Abbasi [email protected] 1
Department of Chemical Engineering, Jundi-Shapur University of Technology, Dezful 64616-18674, Iran
MR Mt Mt + dt Me M0 MRexp,i MRpre,i MRm n N R2 RMSE R S T t V
Dimensionless moisture ratio. Moisture content (gr water per gr dry matter) at time t. Moisture content (gr water per gr dry matter) at time t + dt. Equilibrium moisture content (gr water per gr dry matter). Initial moisture content (gr water per gr dry matter). Experimental moisture ratio for any test i. Predicted moisture ratio for any test i. Mean value of moisture ratios. Dimensionless drying constant. Number of experimental data. C
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