Coffee silverskin as a renewable resource to produce butanol and isopropanol

Coffee is the second largest traded commodity after oil and large amounts of by-products are generated in the coffee industry every year. In particular, coffee silverskin (CS) and spent coffee grounds (SCG) are the main coffee industry residues. CS is about 4.2% (w/w) of coffee beans and the valorisation of this waste through the biorefinery approach may boost the circular economy development. In the present contribution, CS was pretreated with one of the mainly investigated biomass pretreatment reported in literature: alkaline hydrolysis in NaOH solutions. After enzymatic hydrolysis of the pretreated CS, the obtained sugars were used as carbon source to produce butanol and isopropanol by Clostridium beijerinckii DSM 6423. Moreover, fermentation tests were also carried out with synthetic media to investigate the effects of the alkaline pretreatment on the fermentation process. When pretreated CS was used as feedstock, the largest yields were 0.16 and 0.31 gsolvent/gsugars isopropanol and butanol, respectively. The reported results foster further studies regarding the reuse of CS for solvents production through fermentation processes.