Effects of infrared assisted refractance window drying on physicochemical and quality attributes and thermodynamic analysis of thyme (Thymus vulgaris L.)

In this research, the process of drying thyme with Infrared- assisted Refractance Window (IR-RW) method was investigated as a new technology. The effect of water temperature (WT) and weight of samples (SW) on kinetics, energy, thermodynamic properties, qualitative characteristics, bioactive and essential oil were determined and optimized. Based on the response surface method (RSM-CCD), WT (70-90 °C) and SW (60-120 g) to achieve the best drying time, energy consumption (SEC), color (ΔE), bioactive properties and essential oil efficiency (EO) were optimized. The results showed that Midilli’s model evaluates the stages of drying of the thin layer of thyme more appropriately than other models. Also, the effective diffusivity (Deff) and Activation Energy (Ea), enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) were obtained between 8.67×10-9- 3.02×10-8 m2/s, 20.71- 31.17 kJ/mol, 17.69-28.31 kJ/mol, -0.1490- -0.118 kJ/mol K and 67.69-73.04 kJ/mol, respectively. Increasing the WT raises the DR, brightness, AA, Deff and ΔH and decreases the drying time, SEC, ΔH, ΔG, CO2, Chroma, a*, b* and ΔE. The highest value of total phenol content (TPC), rehydration ratio (RR), EO was recorded at the WT of 80 °C and the SW was 90 g. The optimal values of the process variables are 85.02 °C for water temperature (WT) and 86.10 g for sample weight (SW). In addition to these values, the optimal values for drying time, SEC, ΔE, AA, TPC, TFC and OE were 91.003 minutes, 8.37 kWh/kg, 1.42, 71.68%, 37.93 mg GAE/ 100 g d.m, 35.11 mg QE/100 g d.m and 2.04%, respectively, at the desirability value of 0.807. Thus, these findings provide valuable insights for producers regarding the drying characteristics and properties of thyme.