Is coffee silverskin actually a suitable source of either food supplements or additives? UPLC-PDA-ESI-TOF/MS metabolic profiling of aqueous extracts: natural antioxidants vs phytotoxins

In the last decades coffee has been widely recognized as an important source of antioxidants and radical scavengers, with increasing evidence supporting its health-promoting potential. Many of these benefits are ascribed to chlorogenic acids (CGAs) contained in coffee beans. A growing interest in the valorization of coffee wastes like spent coffee grounds and by-products like Silverskin (SS) is justified by their richness in polyphenols. As a consequence of the increasing consumption of Green Coffee (GC) as antioxidant-rich dietary supplement, concern has been expressed about its use as food additive or supplement. In fact, atractyligenin and its derivatives, compounds structurally related to the phytotoxin atractyloside, have been recently extracted from raw coffee and their phytotoxic activity has been evaluated. A deeper knowledge about the presence of such compounds also in SS, is supported by the recent interest in the use of SS as a source of food additives or dietary supplements. In this contribution, the UPLC-PDA-ESI-TOF/MS metabolic profiles of SS aqueous extracts of different coffee varieties have been determined and compared to that of GC. Atractyligenin, carboxyatractyligenin and five glucosyde derivatives were identified. In contrast to GC, where only atractyligenin glucopyranosyl derivatives were detected, in all SS also the aglycones were identified and their content was higher in Arabica than in Robusta. Compared to GC, GPCA reaches higher concentration in Arabica and comparable concentration in Robusta SS. Interestingly, the content of CGA in aqueous extracts of SS is reversed compared to atractyligenin derivatives. Moreover, an atractyligenin derivative not previously detected in GC, has been detected in GC. Concluding, the use of SS as food or dietary supplements needs careful evaluation and further and adequate controls on SS raw material are necessary in order to assess its suitability as functional ingredient for food or nutraceutical products.