Abstract:
Compared with conventional thermal method, microwave heating has the advantages of being direct, volumetric, rapid and selective. In spite of its demonstrated advantages, current research on the processing of carrageenan has focused on laboratory-scale, exploratory efforts. In order to realise the potential of microwave processing of carrageenan, work is needed to integrate microwave processing system design with robust process control system development. Microwave processing of carrageenan has been investigated as a reasonable alternative to conventional method. In this project, work was performed in two sections. The first focus of this research was to develop a precisely controlled, rapid and uniform microwave circuit with on-line monitoring features. The second focus was to investigate the rapid processing of carrageenan to reveal microwave heating mechanisms, thermal or non-thermal. The research efforts included the assembly of experimental components, development and implementation of uniform heating methods, installation of process controller, and development of an on-line monitoring technique. Special microwave effects observed in microwave heating were further studied to improve understanding in microwave/carrageenan interactions and in microwave heating mechanisms (thermal or non-thermal). Microwave processing of carrageenan in a single-mode resonant cavity using fixed frequency technology, was automated to advance the current state of technology by bridging the gap between device and process. Automatic hardware and software for controlling the non-linear and complex electromagnetic interactions during extraction were utilised. Efficient coupling, uniform and controlled heating were achieved through tuning and power control. The integrated microwave processing system utilised in this study comprises a monomode waveguide fitted with power and temperature controls, together with a continuous-flow reactor operating at atmospheric pressure. The circuit has been tested by processing of iota carrageenan from Eucheuma denticulatum and kappa carrageenan from Kappaphycus alvarezii in aqueous organic solvents; even without purification the extraction products were found to have virtually identical FTIR and l3C - and 'H- NMR spectra to reference samples of kappa and iota carrageenan, respectively. This indicates that the carrageenans can be extracted in high purity, without the need for the purification procedures used in conjunction with conventional extraction of carrageenans. The principal advantages of the microwave system are substantial reduction of extraction time and low consumption of organic solvents. In addition, results revealed that the microwave assisted reaction can enhance the yield of the polysaccharide. Establishing temperature effects is very important for a quantitative description for both heating methods. Temperature profiles in thermal and microwave fields were determined experimentally. Thermal gradients may be inherently different for the two heating methods. In the microwave process, heat energy rapidly penetrates the material and the temperature rises much faster than conventional processing. Radial temperatures during microwave heating were quite uniform and increasing slightly toward the centre. Conventional samples tend to be cooler in the middle whereas microwave samples tend to be warmer at the centre. Microwave also offers the advantage of a more linear temperature increase. The phenomenon of reduced processing time, increased and better quality yield requires further study to elucidate the issue of "microwave effect".