The cascade biorefinery approach for the valorization of the spent coffee grounds

Six million tons of Coffee Spent Grounds (SCG) are produced every year all around the world. Their physical and chemical characterization, rich in high value molecules and organic compounds, make SCG ideal for the recovery of bioactive molecules and bioenergy production according to the “cascade biorefinery approach”. This work investigates the effects of the implementation of sequential processes for the recovery of the coffee oil, rich in valuable molecules, and the productions of bioethanol and biogas from SCG. The use of a 50:50 (v/v) ethanol-iso-propanol mixture allowed a coffee oil recovery of about 16% w/w. In particular, the extracted coffee oil was rich in tocopherols (about 15 mg/100 g SCG). On the other hand, n-hexane showed a lower performance in terms of coffee oil extraction (10% w/w) but allowed for the extraction of different molecules: linoleic and palmitic acids (38% and 34% w/w of the coffee oil, respectively) and of Cafestol (383 mg/100g SCG) and Kahweol (194 mg/100g SCG). The extracted SCG underwent to an acid-enzymatic hydrolysis process followed by a solid/liquid separation. The liquid fraction was then used for the bioethanol production which reached the final concentration of 50 g/L, while the solid fraction was used for biogas production by Anaerobic Digestion. The final methane yield resulted in a production of about 250 NLCH4/kgVS. A complete train of operations, composing a biorefinery approach, can be therefore adopted to fully valorise this particular organic waste.