Kamaruddin M., Yusof M., Ngadi N., Zakaria Z., Arsad A., Kidam K., 2017, Dielectric Properties for Extraction of Orthosiphon Stamineus (Java Tea) Leaves, Chemical Engineering Transactions, 56, 1771-1776.
A dielectric properties study was performed at ISM frequencies and a range of temperatures (25 – 45 °C) on the extraction of Orthosiphon Stamineus (Java Tea) leaves system in order to relate their dielectric properties to microwave heating mechanisms and design of microwave applicator quantitatively. The main results concluded that the heating mechanism of the extraction mixture in an electromagnetic field was controlled by the dielectric properties of solvent (water), where the solvent was the major component (> 90 % v/v) as well as the component with highest dissipation factor (tan d). The penetration depths of extraction mixture at ambient temperature (25 °C) are 3.8 cm, 3.2 cm and 1.4 cm at ISM frequencies of 0.433 GHz, 0.915 GHz and2.45 GHz. These tiny penetration depths limit the potential to achieve the successful scale up of a microwave- assisted extraction of Orthosiphon Stamineus leaves in batch mode at ISM frequencies. This will lead to inhomogeneous bulk temperature distribution within the extraction mixture and irreproducible extraction yield without sufficient stirring and stirrer compatible with microwave system. A fast heating rate based on a high value of tan d of the extraction mixture revealed that the microwave heating technique has a great potential in reducing the processing time especially at heating up stage compared to conventional thermal heating technique in extraction of Orthosiphon Stamineus leaves. The dielectric properties of extraction mixture are worth to be considered to certify the consistency and reproducibility of the microwave-assisted extraction at large scale production.
