Coffee silverskin: chemical characterization and extracts evaluation

Coffee silverskin (CS) is the main by-product of coffee roasting. It is a thin tegument that covers the coffee seeds; during roasting coffee beans expand and this fine layer is detached [1]. In the last years, CS has gained more and more attention and interest since, although it accounts for only a minimal berry fraction, it contains interesting macromolecules and secondary metabolites which are responsible of different biological activities [2]. Indeed, in view of more sustainable economy, several authors have studied and proposed different approaches to take advantages of coffee silverskin and its phytochemical profile [3]. In this context the great majority of studies have reported the content of nutrients and bioactive compounds such as caffeine, chlorogenic acids, melanoidins [4] but to the best of our knowledge there is a dearth of investigations on minor compounds content such as flavonoids, iridoids, alkaloids (different from caffeine) and xanthones. Moreover, the aroma fraction of this by-product has not yet been studied even if the characterization of CS odorants could be fascinating for food and food flavour industries. Hence, the first aim of this work was to develop an efficient and fast method for the quantification of 30 bioactive compounds including alkaloids, phenolic acids, flavonoids, secoiridoids and xanthones by using HPLC-MS/MS system. For this purpose, different extraction procedures were tested, and the best ones were selected and applied for extract preparation. The CS extracts were evaluated for their antioxidant and antibacterial activities. The second aim of this work regarded the characterization of volatile fraction of CS employing two different instruments, i.e., GC-O/FID and GC×GC-MS system. Among different tested procedure four ultrasound-assisted extractions which employed different solvents have been selected considering the extraction yield, the effectiveness of compounds extraction and the potentials nutraceutical applications of the extracts. Caffeine was the most abundant bioactive compounds (1.03.6% of dry weight of extract) followed by chlorogenic acids (sum of three chlorogenic acids, 3115.65444.0 µg g-1) in all extracts. All seven unconjugated phenolic acids were detected and vanillic, ferulic and caffeic acid were the most abundant. Rutin (1.68.7 μg g-1), quercetin (1.52.5 μg g-1), kaempferol (0.81.7 μg g-1) and quercitrin (0.20.5 μg g-1) were found in all the CS extracts while targeted secoiridoids have not been detected. The methanolic and aqueous extracts possessed the best and the worst radical scavenging activities (RSA) by DPPH assay while methanolic together with EtOH:H2O (70:30, v/v) extract showed a protective activity against peroxide exposure as they significantly increased cell viability in respect to H2O2 treated cells. All silverskin extracts did not inhibit the growth of anyone of the bacterial species included in this study while data indicated that silverskin extracts obtained by water-based solvents might deserve a deeper future investigation on biofilm-related activities [5]. The studies on the CS volatile fraction demonstrated that this co-product contains an interesting aromatic portion with high similarity to coffee beans since almost all odorants found in CS have commonly reported in coffee. Moreover, some odor-active compounds, including two important actors of coffee flavour, were found with similar intensity in both matrices, e.g., 4-hydroxy-2,5-dimethylfuran-3(2H)-one and 2-methoxy-4-vinylphenol [6]. In conclusion, the low cost of CS and the biological activities of the obtained extracts, which are attributed to their phytochemical compositions, could suggest a possible application of CS as ingredients in food and pharmaceutical formulations. References [1] Bessada S.M.F.; Alves R.C.; Oliveira M.B.P.P. Cosmetics, 2018,5, 5. [2] Iriondo-DeHond A.; Aparicio García N.; Fernandez-Gomez B.; Guisantes-Batan E.; Velázquez Escobar F.; Blanch G.P.; San Andres M.I.; Sanchez-Fortun S.; del Castillo M.D. Innov Food Sci. Emerg. Technol, 2019,51, 194–204. [3] Narita Y.; Inouye K. Food Res. Int, 2014,61, 16–22. [4] Janissen B.; Huynh T. Resour. Conserv. Recycl, 2018,128, 110–117. [5] Nzekoue F. K., Angeloni S., Navarini L., Angeloni C., Freschi M., Hrelia S., Vitali A. L., Sagratini G., Vittori S., Caprioli G. Food Res Int, 2020,133 109128. [6] Angeloni S., Scortichini S., Fiorini D., Sagratini G., Vittori S., Neiens S. D., Steinhaus M., Zheljazkov D. V., Maggi F., Caprioli, G. Molecules, 2020,25, 2993.