Microwave extraction of polyphenols from the waste biomass coffee silverskin and its valorization as a source of antioxidants

Coffee is one of the largest consumed drinks all over the world, anyway generating different wastes, including coffee silverskin, which originates from the roasting process. Up to now, these waste residues have no specific use. Remarkably, in recent years, coffee silverskin is attracting great attention as a promising source of bioactive compounds, including valuable polyphenols, such as chlorogenic acids and caffeine. However, the current conventional extraction procedures require long extraction times and high amounts of solvents, making these procedures significantly energy-demanding, expensive and practically disadvantageous. Now, microwave-assisted extraction is getting increasing interest, resulting eco-friendly, energy efficient and time-saving. On this basis, the aim of the present research is to propose the optimization of the solid-liquid extraction of valuable antioxidant polyphenols from coffee silverskin under microwave irradiation, investigating the effect of the main reaction parameters, also developing a statistical modeling. Maximum yield in total polyphenols of about 45 mgGAE/g as mg of gallic acid for g of starting biomass was experimentally obtained. This represents a promising achievement, fully justifying the coffee silverskin valorization in the circular economy perspective. Moreover, the most promising extracts were evaluated for the corresponding antioxidant activity using the DPPH assay, which yielded interesting results. Finally, the esterification between chlorogenic acid in the extracts and ethanol or butanol was achieved, working in the presence of commercial and/or biomass-derived heterogeneous acid catalysts, to obtain the corresponding esters that could be advantageously exploited as antioxidants in cosmetic and nutraceutical formulations.