Spent coffee grounds as a valuable source of phenolic compounds and bioenergy

Spent coffee grounds collected from coffee bars (SCG-1) or recovered from coffee capsules (SCG-2) were investigated as a potential source of phenolic compounds and energy. Preliminary characterization of these materials provided a total phenolic content of 17.75 mg GAE/g for SCG-1 and 21.56 mg GAE/g for SCG-2. A solvent-extraction procedure using aqueous ethanol as the solvent and operating under mild temperature conditions was developed and tested. A two-level factorial design was used to study the effects of temperature (T = 30–50 °C), extraction time (E = 60–120 min), liquid-to-solid ratio (R = 20–40 mL/g) and ethanol concentration in the aqueous mixture (C = 30–70 vol%) on the recovery of phenolic compounds. Under the best conditions, over 90% of the phenolic compounds contained in the starting waste materials were recovered. T, R and C were the most influential factors and all of them had a positive effect on the extraction efficiency.The calorific values of the two coffee wastes were 23.72 MJ/kg (SCG-1) and 24.07 MJ/kg (SCG-2). They were only marginally affected by the extraction procedure, which supports the possibility of integrating the recovery of phenolic compounds with the use of the resulting solid residue to produce pellets or other agglomerates for heating purposes. A case study application aimed at evaluating the potential valorization of the spent coffee produced in the Province of Rome is also presented.