Silverskin: from waste to a renewable source of bioactive compounds

Silverskin is the thin layer covering the coffee beans. During the roasting coffee process, it is removed becoming a by-product. The large production (150.000 t/year on global scale, of which 7.500 t/years in Italy) of this waste calls for its re-functionalization in order to reduce the impact of industrial by-products in the environment. Currently the only use of coffee silverskin (CS) is as renewable energy source or as fertilizer (1,2), no other utilization has been developed for CS so far. Despite this, the chemical composition of CS indicates the presence of interesting bioactive molecules (i.e. lipids, chlorogenics acids, caffeine) that confer to this by-product antioxidant and prebiotic properties. (3)This evidence suggests its re-use as well in other industrial fields such as nutraceutical, cosmetic and pharmaceutical as aimed by CirCO project. In this hypothesis of revalorization, it cannot be excluded the potential use of CS lipid matter in cosmetic formulations or as additive for the technology of paper processing (4). For the isolation of CS oil, supercritical CO2 was selected as more sustainable separation technique, alternative to classical solvent extraction processes.(5) We observed that an appropriate modulation of process parameters (T, P) influence the oil rate and the chemical composition opening the way for targeted extraction. In details the application of mild conditions (35\ub0C, 100/200psi) led to majority extraction of palmitic (C16:0) and linoleic (C18:2) acids with an overall yield of 1.9% (goil/100gCS). The increase of pressure (100 bar\uf0e0300 bar) and temperature (35\ub0C\uf0e060\ub0C) promoted the extraction of arachidic acid (C20:0), behenic acid (C22:0) and lignoceric acid (C24:0) with a calculated oil yield of 3.3% (goil/100gCS). The best identified conditions were employed for the industrial scale-up giving results comparable to laboratory scale. For a possible commercial process application, it is essential to test the applicability of appropriate models for SFE of lipids from oil-containing substrates.(5) For this purpose a mathematical model is constructed based on the experimental data collected, employed apparatus and chemical-physical characteristic of biomass in tightly collaboration with prof. Manenti at Politecnico of Milan