Development of a microwave pretreatment for the extraction of active compounds from natural byproducts

Resumen

During the last decades, industry has paid an increasing attention to intensification techniques due to its potential to save time, space, energy and material agents, like solvents. In particular, the extraction of active compounds from natural products has thrived because of these techniques. Microwave technology is one of those intensification methods whose advantages have been widely proved, although mainly just at laboratory scale. Thus, the aim of this thesis is to develop a microwave intensification process feasible to be implemented at industrial scale. For this purpose, an innovative approach to facilitate its scale up was proposed: in this work, microwaves have been used as a pretreatment prior to the conventional extraction. The main feature of this step was its short duration. Not only was this brief radiation time beneficial for the stability of the active compounds (thermal degradation), but it also allowed the use of a compact microwave oven, feasible at a larger scale to uniformly irradiate the material. Three abundant agricultural wastes like grape pomace, olive pomace and saffron flowers have been used to assess the convenience of the proposed microwave pretreatment for the extraction of polyphenols. The thesis is divided into three sections. In the first one, the valorisation of grape pomace has been developed. In the second part, an analysis of the possible interaction of microwave on the solid-liquid interface has been studied. And finally, the knowledge developed has been proved with other agricultural wastes (olive pomace and saffron flowers) to check if the pretreatment was suitable for other products too. The assessment of the microwave pretreatment was performed regarding a compromise between yield enhancement (an indication of the raw material exhaustion grade) and product richness (crucial parameter that determines the proportion of polyphenols in the final dry product). The results obtained led to the conclusion that the microwave pretreatment proposed, developed and implemented at pilot-plant scale in this thesis is a suitable alternative for the industrial intensification of extraction processes. Its feasibility has been proved with three different agricultural wastes, obtaining, in all the cases, promising outcomes. Additionally, this work has been combined with a study of the material-microwaves interaction with the aim of getting a deeper understanding of the effect of microwaves in natural products extraction processes.