An Overview of Fungal Applications in the Valorization of Lignocellulosic Agricultural By-Products: The Case of Two-Phase Olive Mill Wastes

DOR (dry mill olive residue) is an agricultural by-product generated during two-phase olive oil extraction. It is a microtoxic and phytotoxic phenolic-rich lignocellulosic residue and is produced in high amounts in Mediterranean countries. Several techniques have been proposed for its valorization. Agaricomycetes are one of the most efficient lignin-modifying enzyme (LME) producers on Earth (e.g. laccases, peroxidases or the novel peroxygenases). The secretion of these enzymes varies according to the specific strain and also depends on other factors such as the presence of certain elicitors in the medium, like lignocellulosic materials or phenols, which may be crucial to stimulate LME secretion. Due to the enzymatic capability of fungi, numerous studies have been performed in order to valorize the DOR, such as its recent use as a medium for enzyme production or organic soil amendment. An improved valorization of these by-products and a better understanding of fungal enzyme production could lead to the development of biorefineries that utilize various components in biomass and their intermediates, thus maximizing the value derived from biomass feedstock. In this chapter, we summarize some studies performed on this topic (mainly based on the usage of white-rot fungi in the transformation of DOR), and we discuss possible trends, challenges and future prospects for the use of fungi in an environmentally sustainable scenario.