Effect of drying methods on drying kinetics, energy features, thermophysical and microstructural properties of Stevia rebaudiana leaves

BACKGROUND: Stevia leaves were subjected to convective hot-air, infrared and vacuum drying at 40, 60 and 80 °C, followed by an assessment of thermophysical properties and microstructure, along with drying kinetics modelling and evaluation of energy features for all drying operations. RESULTS: Effective moisture diffusivity (D_eff) showed dependency on temperature with values ranging from 1.08 × 10⁻¹² to 7.43 × 10⁻¹² m² s⁻¹ for convective drying, from 0.71 × 10⁻¹² to 6.60 × 10⁻¹² m² s⁻¹ for infrared drying, and from 1.29 × 10⁻¹² to 5.39 × 10⁻¹² m² s⁻¹ for vacuum drying. The thermal properties of the dried Stevia leaves under different drying conditions showed values of density, specific heat, thermal diffusivity, thermal conductivity and thermal effusivity ranging from 95.6 to 116.2 kg m⁻³, 3050 to 3900 J kg⁻¹ K⁻¹, 4.28 × 10⁻⁷ to 5.60 × 10⁻⁷ m² s⁻¹, 0.16 to 0.23 W m⁻¹ K⁻¹ and 244 to 305 W s^0.5 m⁻² K⁻¹, respectively. As for microstructure, convective hot-air drying showed better preserved leaf characteristics, compared to infrared- and vacuum-drying, whereby scanning electron microscopy (SEM) image analysis also revealed noticeable differences at higher temperatures. Statistical analysis showed that the Midilli–Kuçuk model fitted best the experimental data of drying curves (0.961 < r² < 0.999, 0.000064 < SSE < 0.005359, and 0.000074 < χ² < 0.006278). Comparison of the drying methods with respect to energy features showed that convective drying at 80 °C led to lowest specific energy consumption (61.86 kW h kg⁻¹) with highest efficiency (8.5%). CONCLUSION: The results of this study contribute to a better understanding of the drying behaviour and showed that thermophysical properties of dried Stevia leaves and energy features are affected by drying methods.