The Use of Coffee Residues as Sustainable Cultivation Substrates in Microbial Biotechnology: Up-to-Date Review and...

The growing volume of agro-industrial and food-processing residues has intensified interest in their use as low-cost substrates for microbial bioprocessing. Coffee-derived waste streams, including spent coffee grounds (SCGs), wastewater, pulp, husk, and silverskin, represent abundant but still underutilized biomass resources. This narrative review evaluates their potential as liquid or solid substrates or as components of cultivation media for selected microbial systems, including microalgae, bioremediation- and bioprocess-related bacteria, edible fungi such as Pleurotus spp., and yeasts in the genera Pichia, Kluyveromyces, Saccharomyces, and Yarrowia. The review compares the suitability of individual coffee residues based on substrate composition, pretreatment requirements, inhibitory compounds, process limitations, and reported outputs. Coffee-derived residues can reduce substrate costs, support waste valorization, and partially replace conventional nutrients in microbial processes. However, their broader application is limited by compositional variability, conditioning or hydrolysis requirements, difficulties in process standardization, and downstream processing costs. Current evidence most strongly supports fungal cultivation on SCG-containing substrates, bacterial treatment of caffeine-rich wastewaters, yeast fermentation of hydrolyzed residues, and microalgal use of conditioned liquid streams. The review identifies key research gaps and outlines realistic directions for developing coffee-based microbial bioprocesses within a circular bioeconomy framework.