Influence of Extraction Conditions on the Recovery of Phenolic Antioxidants from Spent Coffee Grounds

The extraction of phenolic compounds from Spent Coffee Grounds (SCG), a waste produced in large amounts worldwide, was investigated. SCG were rich in fiber and protein and had a total phenolic content of 18.75±0.43 mg GAE per gram dry weight. Extraction experiments were carried out in batch mode, using ethanol-water mixtures as solvent. A two-level factorial design was used to study the effects of temperature (T = 20-60°C), extraction time (E = 30-80 min) and liquid-to-solid ratio (R = 10-50 mL g^-1) on the recovery of phenolic compounds. Under the best conditions (T = 60°C, E = 180 min, R = 50 mL g^-1), over 90% of the phenolics present in SCG were recovered. T and R were the most influential factors and both had a positive effect on the extraction efficiency. The antioxidant activity of the extracts, determined by the DPPH method, was highly correlated with the amount of phenolics in each sample, suggesting that these compounds are the main responsible for the antioxidant properties of the extracts. Overall, the results of this study provide clear evidence of the potential of SCG as a source of natural antioxidants and give useful directions on how to improve their recovery by proper selection of extraction conditions.