Abstract
Permittivity of materials is of utmost importance for microwave applicators’ design and to predict high-frequency dielectric heating of materials. In the case of aromatic plant biomass, however, there are few data that help researchers design microwave applicators for microwave-assisted extraction. In this work, the permittivity of cypress and rockrose biomass samples were measured versus temperature, density, and moisture content. A resonant technique based on a coaxial bi-reentrant microwave cavity was employed to obtain the complex permittivity of biomass samples as a function of those magnitudes around the 2.45 GHz ISM frequency. The obtained measurements show that large variations for permittivity values can be found with moisture content and density changes for both cypress and rockrose biomass. Temperature also has effects in a lesser degree, although it has an important influence on the cypress biomass loss factor. Polynomial expressions fitting the experimental data were provided in order to facilitate the estimation of intermediate values, which were not explicitly arranged in this work. As a general trend, the permittivity of cypress and rockrose biomass increases with increasing values of moisture content and density, whereas the biomass loss factor increases when temperature rises.
Highlights
Chemical products contained in plants have been used in traditional medicine, pharmaceutic and perfumery products, and as a source of antioxidant treatments
Resonant procedures seem to be the best ones to suit the conditions of heterogeneity, temperature, density, and moisture content during the permittivity measurement of cypress and rockrose biomass
The obtained obtainedmeasurements measurementsfor forcypress cypress biomass permittivity shown in this subsection
Summary
Chemical products contained in plants have been used in traditional medicine, pharmaceutic and perfumery products, and as a source of antioxidant treatments. Essential oils (EOs) contained in aromatic plants may, be valuable products provided that they are economically extracted. EO and methanol extracted from Mediterranean cypress (Cupressus sempervirens L.) possess antimicrobial and antibiofilm properties, and can be used as natural preservative ingredients in food and/or pharmaceuticals [1]. Rockrose (Cistus ladanifer) is widely spread in the Mediterranean area, and may be a good source of natural antioxidants since extracts from this plant show methanolic, phenolic, and flavonoid contents as described in [2]. Microwave-assisted extraction (MAE) of EOs and other valuable chemical compounds from aromatic herbs, and its variants, has been actively investigated during the last decades since it promises substantial cost and time savings and provides greater yields than conventional procedures. In [3,4], for instance, MAE was applied to basil, garden mint, thyme, and other species, such as ajowan, cumin, and star anise, without using any solvent to collect those essential oils
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