Thin-layer convective air drying of lemon verbena (lippia citriodora) leaves: Experimental kinetics, modeling, effective moisture diffusivity, activation energy, color change, rehydration ratio and essential oil

Lemon verbena leave is a flavoring food additive and is also a source of valuable compounds such as essential oils, flavonoids and phenolic acids. However, like many other aromatic plants, lemon verbena leave is perishable due to its high moisture content. The aim of this work was to study the effect of air temperature (45, 55, and 65°C) on the quality attributes of lemon verbena leaves during hot-air drying (HAD). The drying kinetics was also modeled. The results showed that higher drying temperature led to a significant decrease (p˂0.05) in the rehydration ratio due to a change in the structural features of the dried leaves. The essential oil content of dried samples was also significantly different (p˂0.05) from that of the fresh leaves due to high loss of volatile components and ranged from 0.42 to 0.85. Moreover, a significant increase in the value of effective moisture diffusivity (Deff) and color change was observed when the samples were dried at 65°C compared to 45°C. The value of Deff varied from 1.140×10-10 to 2.280×10-9 m2/s and the activation energy was found to be 31.04 kJ/mol. The greatest R2 (≥0.999) and the lowest RMSE and SSE were obtained for the model proposed in this research. In total, the findings of this study contribute to the lack of information about observed variation in the value of Deff, rehydration ratio, essential oil, and color change of lemon verbena leaves at different drying temperatures as well as about the value of Ea during convective drying.