New way of valorization of raw coffee silverskin: Biohydrogen and acetate production by dark fermentation without exogenous inoculum

Coffee silverskin, a roasting by-product, was tested for hydrogen production by dark fermentation for the first time. The indigenous microflora converted organic matter into hydrogen without biomass pretreatment, reaching hydrogen yields of up to 24.1 mL H2/g COD (chemical oxygen demand). Hydrogen yield was maintained at high biomass load (minimal dilution) during semi -batch bioreactor operation. Acetate was the main metabolite produced, while butyrate, ethanol and propionate were produced in lower amounts. PCR amplicon sequencing of the 16S rRNA gene showed that hydrogen -producing bacteria affiliated to Enterobacteriaceae (27.5%) and Clostridium (5.1%) were selected from the indigenous microflora. In agreement with observed metabolites, acetate -producing Enterococcus (54.1%) was the main bacterial phylotype identified. This study opens a new approach for valorization of coffee silverskin through hydrogen production.