Ultrasound-assisted dilute acid pretreatment of coffee silverskin for biorefinery applications

Coffee silverskin (CSS) is an agro-food processing waste (AFW) largely produced in roasting factories at almost constant rate over the year. The CSS can be used as a source of fermentable sugars within a biorefinery approach for its exploitation. Pretreatment process and enzymatic hydrolysis are required to efficiently recovery monomeric fermentable sugars from AFWs such as CSS. Current research is focused on the selection of novel processes for biomass pretreatments able to provide effective lignin removal, to minimize the production of by-products that may inhibit the fermentation of the produced sugars, and to be cost saving with respect to conventional pretreatments. The aim of the present study was to investigate the effect of the ultrasound (US) assisted dilute acid pretreatment on the effective separation of lignin and polysaccharides of CSS. The effects of two operating conditions - biomass loading and sulphuric acid concentration - were studied to assess their effects on the sugar yields after enzymatic hydrolysis. The production of typical species that may inhibit the fermentation of the hydrolysate was monitored. Inhibitors like furfural, 5-hydroxymethyl furfural (HMF), ferulic and p-coumaric acid were present at concentrations lower than 1 mg/L after enzymatic hydrolysis of the pretreated CSS. As a result, the hydrolysate of US dilute acid pretreated CSS may be used for fermentation without detoxification step