Evaluation of spent coffee grounds as a source of phenolic antioxidants

Spent coffee grounds (SPG), i.e. the solid residue remaining after the production of the beverage, are generated in large amounts by coffee consumption. SPG have no commercial value and are currently disposed of as conventional solid waste or, to a very limited extent, sent to compost facilities. However, the presence in this material of high levels of phenolic compounds with significant antioxidant activity suggests the possibility of using SCG as an inexpensive source of natural antioxidants. In this study we have investigated the feasibility of obtaining a phenolic-rich extract from SPG by an environmentally friendly procedure based on the use of ethanol–water mixtures as the solvent. Total phenols were determined by the Folin-Ciocalteu method and expressed as gallic acid equivalents (GAE), while antioxidant activity was assessed by the DPPH assay. After extraction, the solvent was separated from the solid and vacuum-evaporated to obtain a crude extract which was submitted to analytical determinations. The total amount of phenolics in the waste was in the range of 2000-2500 mgGAE/100 g (dw). The highest extraction yields were achieved by the mixture ethanol-water 60:40 (v/v), which was used to evaluate the influence of temperature (T), extraction time (t) and liquid-to-solid ratio (L/S) on the recovery of phenolics. Under optimal extraction conditions (T = 50-60 °C, t = 30-45 min and L/S = 50-60 mL/g) up to 1800-2000 mg of phenolics per 100 g of dry SCG were obtained. All the extracts were found to possess a high radical scavenging activity.