Antioxidative phenolics obtained from spent coffee grounds (Coffea arabica L.) by subcritical water extraction

Abstract

Response surface methodology (RSM) was employed to optimize subcritical water extraction (SWE) of spent coffee grounds (SCG) with emphasis on the yield of total phenolics and radical scavenging capacities on 2,2-azinobis-(3-ethylbenzothiazolin-6-sulfonic acid) (ABTS+) and 1,1-diphenyl-2-picrylhydrazyl (DPPH+). The analysis involved three variables: temperature (160–180°C), time (35–55min) and solid-to-liquid ratio (14.1–26.3g/L). The optimal conditions for total phenolic compounds (86.23mg GAE/g), antioxidant activity in terms of ABTS+ (81.38 mmol TE/100g) and DPPH+ (42.13 mmol TE/100g) scavenging activities were predicted to be in ranges of 160-180°C/37.9–55.0min/14.1g/L. The verified TPC (88.34mg GAE/g), antioxidant activity on ABTS+ (88.65 mmol TE/100g) and DPPH+ (38.28 mmol TE/100g) based on superimposing contour plots (179°C, 36min, and 14.1g/L) were in good agreement with the predicted data obtained under optimal conditions. 3-O-caffeoylquinic acid, 4-O-caffeoylquinic acid and 5-O-caffeoylquinic acid were found to be the major contributor to the antioxidant activities of SCG extract.