Coffee silver skin as a source of polyphenols: high resolution mass spectrometric profiling of components and antioxidant activity

Many beneficial effects of coffee can be ascribed to chlorogenic acids (CGAs) which are phenolic compounds formed by the esterification of cinnamic acids, such as caffeic acid, ferulic and p-coumaric acids with (-)quinic acids. Green coffee beans (GCB) can contain 10% of dry weight CGAs at most, and coffee is the major source of CGAs in human diet. [1] CGAs are highly bioavailable in humans [2] and different biological effects of CGAs have been reported, partially explaining the health effects of coffee. Moreover, Uto-Kondo et al. [3] reported that the potential cardioprotective effect of coffee might be due to the antiatherogenic property of CGAs, which are able to enhance the HDL-mediated cholesterol efflux from the macrophages. Hence, coffee phenolic acids have different biological effects and represent an important class of bioactive compounds for promoting or maintaining healthy conditions. In this regard, different standardized CGAs extracts from GCB are commercially available. While the CGAs profile in GCB is well known, only few and contradictory information is available on the qualitative and quantitative composition of CGAs in coffee silver-skin (SS), the tegument of coffee beans that constitutes a by-product of the roasting procedure. Aim of this study was to evaluate whether the residues generated in the industrial process of coffee production representing environmental problems, such as SS, coffee silver-skin from decaffeinated green coffee beans (SSd) or spent coffee (SC), could be used as a suitable industrial source for CGAs extraction. In more details, the phenolic fraction has been extracted by conventional methods and its amount determined by using both colorimetric and HPLC-UV approaches. Moreover, a full detailed identification of the phenol constituents of these by-products was carried out by LC-ESI-MS using a high-resolution Orbitrap mass analyzer and the results compared to those obtained from GCB analysis. In addition, we evaluated the antioxidant activity of all these extracts. The results show that the CGAs content (calculated as % relative to the starting material) in silver-skin (both SS and SSd) do not significantly differ from GCB, while SC contains a significantly lower amount (10% in respect to GBC). The quali-quantitative profile of SS and SSd, superimposable to that of GBC, indicate that CGAs are mainly present in free form, unlike SC, where phenolic acids were found to be in bound form (i.e. as melanoidins). Moreover, caffeoyl-quinic acids were confirmed to be the most abundant species among CGAs, followed by feruloyl-quinic acids. In addition, since the antioxidant activity of SS extracts is similar to that of GBC, we can conclude that decaffeinated SS, containing a negligible amount of caffeine, might represent a valuable industrial source of CGAs alternative to GCB, and their extracts of great interest for nutraceutical usage.