Measurement and estimation of aromatic plant dielectric properties. Application to low moisture rosemary

► This manuscript is a new contribution to develop continuous extraction processes based on solvent free microwave extraction of essential oils, commonly used as flavouring agents. These processes are strongly dependent on the absorption of microwaves by the treated material, and therefore it is necessary to know the dielectric properties of the aromatic plants that contain the essential oil of interest, and its evolution during the drying process. ► Manuscript presents for the first time a systematic study about the calculation of the dielectric properties of aromatic plants from their main component. ► The predicted results are validated with experimental values obtained from partly dried rosemary. ► Rosemary dielectric properties experimental data. Essential oils are important in the flavour and cosmetic industries. Recently, microwave energy has been applied in different extraction processes to obtain essential oils with quality and yield similar to those obtained using steam distillation, but using less energy and time. The dielectric properties of any given material are of critical importance when seeking to understand its response to applied electromagnetic waves. Nevertheless, data related to aromatic plants are scarce. This paper addresses the measurement and estimation of dielectric properties of herbal matrices as a part of the study of the fundamental phenomenology behind the extraction of essential oils with microwaves. A cavity perturbation system was used to measure the properties of the plant matrix, and dielectric mixture equations were used later to try to reproduce the results obtained. The dielectric properties of partly dried rosemary, rosemary's essential oil and oven-dry plant material were measured at temperatures between 20 °C and 160 °C, and atmospheric pressure, at frequencies of 2450 MHz and 910 MHz. The results show a strong influence of water content on the dielectric properties. A new predictive approach that uses the results from measurements of dielectric properties from the different constituents to obtain effective mixture dielectric properties is proposed. The predicted values of the dielectric constants were better than those of the loss factors.