Recent Advances in Microbial Bioconversion as an Approach to Boost Hydroxytyrosol Recovery from Olive Mill Wastewater

Olive mill wastewater (OMWW) is a highly complex matrix derived from olive oil extraction, containing phenolic compounds, lipids, minerals, and organic acids. Hydroxytyrosol (HT), an outstanding antioxidant and health-promoting phenolic compound, has garnered significant interest as a natural preservative and functional ingredient. Enzymatic hydrolysis, utilizing purified enzymes to cleave glycosidic or ester bonds, and microbial bioconversion, employing whole microorganisms with their intrinsic enzymes and metabolic pathways, are effective biotechnological strategies for fostering the release of HT from its conjugated forms. These approaches offer great potential for the sustainable recovery of HT from OMWW, contributing to the valorization of this environmentally impactful agro-industrial by-product. Processed OMWW can lead to clean-label HT-enriched foods and beverages, capitalizing on by-product valorization and improving food safety and quality. In this review, the most important aspects of the chemistry, technology, and microbiology of OMWW were explored in depth. Recent trends and findings in terms of both enzymatic and microbial bioconversion processes are critically discussed, including spontaneous and driven fermentation, using selected microbial strains. These approaches are presented as economically viable options for obtaining HT-enriched OMWW for applications in the food and nutraceutical sectors. The selected topics aim to provide the reader with a solid background while inspiring and facilitating future research and innovation.