Combined pretreatments of coffee silverskin to enhance fermentable sugar yield

A huge amount of lignocellulosic agro-food processing wastes (AFWs) is produced by agricultural practices and food industries. Coffee silverskin (CSS) is an AFW produced during the coffee bean-roasting process. Pretreatment methods are required to promote the enzymatic hydrolysis of AFWs, including CSS, aimed at their use as feedstock in a sugar-based biorefinery. A combined pretreatment, based on ultrasound and mild alkaline solution, has been optimized for CSS. The effects of sonication time, biomass loading, NaOH concentration and alkaline pretreatment residence time were investigated according to the response surface methodology. The maximum sugar yield (Y-S=0.6 g g (-1)(sugars in pretreated CSS)) was obtained after enzymatic hydrolysis of CSS pretreated with 5-min sonication at 11% w v(-1) biomass loading, and 75-min autoclave in 5% w v(-1) NaOH. Fermentation inhibitors in the pretreatment solvent were absent or present at concentrations not affecting the growth of Clostridium sp. relevant for biofuel production. The phenolic content was 25 mg(GAE graw_CSS)(-1).