Multi-Application Potential of Lipids Extracted from Coffee Silverskin Using Supercritical CO₂ Technique

Coffee production generates wastes in each step of the value chain: only 1-5% of the coffee cherry remains in the final cup. However, reconsidering coffee production in a circular prospective, new value could be given to process outputs. Coffee silverskin (CS) is the thin fibrous layer covering green coffee beans and the sole by-products of coffee roasting. CS is a flammable material and its management poses safety and disposal issues. Nevertheless, the chemical composition of CS makes this waste a valuable resource of high-value compounds. In particular, the peculiar composition of its lipids makes their valorisation possible in several fields. In a sustainability and circular economy prospective, this work investigated the extraction of lipids from CS using supercritical CO2 (sc-CO2), as part of the rECOBIOpack project. CS deriving from a mixture of Arabica coffee beans was supplied by illycaffè S.p.A (Trieste, Italy). The sc-CO2 extraction of lipids from CS was optimized at lab-scale by Design of Experiment (DoE) investigating the influence of pressure (100-500 bar) and temperature (40-80 °C) on the extraction yield and on the composition of the lipidic extract in term of fatty acids (FAs). The optimal extraction conditions (400 bar and 60°C) were then scaled up using an industrial sc-CO2 apparatus at ILSA S.p.A. (Arzignano, Italy). The DoE results showed an extraction yield between 0.19-2.92%; this difference was mainly affected by pressure. Regarding the characterization of FAs, the amount of saturated FAs was higher compared to unsaturated FAs. The main FAs were palmitic and linoleic acids; however, the composition of free FAs resulted in a higher content of long chain FAs compared to glycerides. The optimal extraction condition in term of yield was then employed at industrial scale. Unlike laboratory results, the extract was characterized by the presence of a liquid and a solid phase. Compared to lab-scale, a slightly higher yield (3.4%) of the solid lipidic extract was obtained. Concerning the liquid fraction, it was mainly composed by water and characterized by the presence caffeine. The results obtained using sc-CO2 were comparable with those obtained by conventional extraction using n-hexane, a toxic and fossil-based organic solvent, confirming the advantages in the use of a more sustainable methodology. The feasibility of scaling up sc-CO2 extraction of lipids from CS has been presented in this study. The distinctive composition of CS lipidic extract makes promising its use in different industrial fields, promoting biobased and more sustainable solutions for future applications.