Investigation of Enzymatic Hydrolysis of Coffee Silverskin Aimed at the Production of Butanol and Succinic Acid by Fermentative Processes

Coffee silverskin (CSS) is an agro-food waste obtained in large amount from the roasting phase of green coffee beans. Its potential use as biorefinery feedstock together with other agro-food wastes has been suggested, even though few experimental studies proved the production of bio-commodities at lab-scale from lignocellulosic sugars from CSS. The present work mainly aims at (i) the maximization of the sugar release by CSS pretreatment and hydrolysis; (ii) the fermentation of the CSS hydrolysate in order to produce butanol and succinic acid as bio-commodities having industrial interest. Alkali-pretreated CSS was enzymatically hydrolyzed in glass bottles on a rotary shaker. Biomass loading (5-12.5%w v(-1)) and cellulase concentration (1-80 FPU gcellulose(-1)) were the operating conditions investigated to maximize the sugar release. Biomass loading at 10%w v(-1) gave the best results in terms of sugar concentration and sugar yield. Curve fitting of experimental data was performed: increasing of hydrolysis rate and sugar concentration was observed increasing cellulase concentration. Biomass loading at 10%w v(-1) and enzyme loading at 80 FPU g(cellulose)(-1) were applied to produce the hydrolysate to use as the medium for fermentation tests: the sugars from CSS were the only carbon source. The batch fermentations by Clostridium acetobutylicum DSM 792 and Actinobacillus succinogenes DSM 22257 were characterized in terms of metabolite and sugar conversion. Butanol (2.2gL(-1)) and succinic acid (20.8gL(-1)) were produced, respectively. The results highlight the possibility of using CSS to produce fermentable sugars, solvents, and biochemicals of industrial interest.