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

- Subcritical water was first used to recover phenolics from spent coffee grounds (SCG).
- The maximum antioxidant activity of SCG extract was optimized by response surface methodology (RSM).
- The predicted values of phenolics yield and the antioxidant activity were well fitted to the experimental data.
- Chlorogenic acids were the main compounds contributing to the antioxidant activity of SCG extract.

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 (35-55 min) and solid-to-liquid ratio (14.1-26.3 g/L). The optimal conditions for total phenolic compounds (86.23 mg GAE/g), antioxidant activity in terms of ABTS+ (81.38 mmol TE/100 g) and DPPH+ (42.13 mmol TE/100 g) scavenging activities were predicted to be in ranges of 160-180 °C/37.9-55.0 min/14.1 g/L. The verified TPC (88.34 mg GAE/g), antioxidant activity on ABTS+ (88.65 mmol TE/100 g) and DPPH+ (38.28 mmol TE/100 g) based on superimposing contour plots (179 °C, 36 min, and 14.1 g/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.

The spectra on the left of the image could be better explained and is not clear what the difference of these two chromatograms is. A suggestion is to express somehow what they indicate, like which peaks are interesting to observe and what is the result of that (the presence of the CQAs that are antioxidant compounds, for example).If possible, indicate with arrows the desired region on the response surfaces (for antioxidant activity high temperatures with low ratios, and for total phenolics temperatures around 175 °C, since time appears little important in this case)211