[Thesis] Enhanced extraction of natural substances using microwave energy

Process intensification is proposed using microwave and ultrasound energy in order to improve organic material liberation from natural cells (wastewater sludge, microalgae and grape marc). Enhanced process kinetics are achieved after a short pre-treatment stage up to the boiling temperature, followed by the conventional procedure. The novel technologies are compared to conventional thermal treatment in order to study possible non-thermal effects of the pre-treatments.Up-scaling concepts are evaluated in terms of absorbed energy balance, interaction of material and the electric field (dielectric properties) and thermal degradation of valuable compounds.

MEng. Katalin Solyom will defend her Doctoral Thesis on Monday Valladolid 16/12/2013 at 11.30h at Salon de Grados, Escuela Ingenierías Industriales, Valladolid University.

What is the limiting step in extraction from naturals?

In industrial processes, where natural raw materials are involved, the substance of interest may have intracellular localization. In order to facilitate the accessibility of the product in question, the natural cells have first to be damaged. This primary stage of the processes is often recognised as the rate limiting step, causing slow process kinetics, and therefore leading to long operation times, low recovery efficiencies or high operation costs.
Microwave irradiation can improve cell wall rupture of high moisture materials. Due to the rapid heating up and evaporation of intracellular water, the pressure gradient may lead to cell wall damage.

How microwave technology can help?
The microwave technology is widely used for analytical purposes due to the above-mentioned features, achieving faster and more economical procedures. However, the electric energy used to transfer microwaves into any sample could lead to high operation costs in industrial scale applications. For this reason, an effective and short microwave pre-treatment on the natural material may be convenient prior to the conventional processing. This additional stage would not replace the existing process, but may modify the cell structure in order to facilitate the conventional method in continuation, thus intensifying the conventional process kinetics.

The aim of this thesis -> facilitate intracellular material liberation
The aim of this Doctoral Thesis was to study the effect of microwave pre-treatments on processes with microorganisms (wastewater sludge and microalgae) and plant cells (grape marc) to facilitate intracellular material liberation and thereby enhance the subsequent conventional process kinetics.

Our objetives in the study were:
In order to attain the aforementioned goal, the following partial objectives have been raised:
»    Study the effects of microwave pre-treatments in the presence of solvent and in solvent free systems. Microwave pre-treatment was able to liberate up to 70% of the total phenol content of grape marc, and achieve a 4-fold faster process kinetics.

»    Assess the application of ultrasound assisted extraction as a competing novel process to microwaves. However a cooled ultrasound pre-treatment at 45°C enhanced the process kinetics, the thermal and ultrasound treatment at 70°C achieved two times faster kinetics and extraction yield.»    Evaluate three concepts for up-scaling, which are necessary to study in laboratory experiments:
1.    Determination of the energy density absorbed on laboratory scale.
The dissipated thermal energy can be measured as the contribution of the sensible heat, used for the heating up of the system; the latent heat from the evaporated solvent amount; and finally from the heat loss through the reactor surface.


2.    Dielectric properties: interaction between microwaves and raw material for further process modelling.
In the case of natural raw materials with high moisture content the amount of water in the system has stronger influence on the dielectric properties than the used temperature. The applied cavity perturbation method was proved to give reliable results for a complex sample matrix, as the grape marc.3.    Study of the thermal degradation of the substances of interest in different experimental environments.

How it compares with other treatments, like hydrothermal treatment?
In the comparison of thermal treatments of grape marc and its filtered extract the sensibility of valuable compounds, such as the anthocyanins, were higher with one order of magnitude in the presence of the solid grape marc.
As a conclusion of the applied pre-treatments it was found that the temperature is the most important factor in both techniques. Applying short treatment times and high temperatures, which can be best obtained using microwave energy, excessive degradation of the sensible compounds can be avoided. It was also observed that the cell structure and solvent play an important role in the achievement of the improved process kinetics.

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