The world's population will reach approximately 9.7 billion by 2050, thus leading to increasing pressure for food (UNDESA, 2019). The food chain needs to become more sustainable, since food waste accounts for 8% of global anthropogenic greenhouse gas emissions, 20% of freshwater consumption and 30% of global agricultural land use, making it a serious economic, environmental and nutritional problem (Duenas & Garciá-Estévez, 2020; United Nations Development Programme, 2020). The United Nations has called for intensified research on green technologies for reducing waste and converting it into useful resources to make new products (Duenas & Garciá-Estévez, 2020; United Nations Development Programme, 2020).

Coffee beans are one of the most important food commodities worldwide, with an estimated production of 10.5 million tonnes for the 2020/21 harvest (ICO, 2021). Several by-products are generated during the processing of coffee fruits to obtain the commodity (endosperm). Pulp is the main by-product obtained during the semi-humid coffee fruit processing method, while the husk is the main by-product obtained from the dry method (da Silva et al., 2022). The pulp and husk combined correspond to approximately 42% of total fresh coffee weigh (Janissen & Huynh, 2018). Other by-products or wastes are generated inside a coffee ground factory, such as defective coffee beans which are rejected, roasted silverskin, and spent coffee generated in the production of “instant-coffee” (dos Santos et al., 2021). Spent coffee is also generated inside homes (often referred to as “spent filter coffee”). Overall, it is estimated that about 65% of roasted coffee beans will be converted into spent coffee (Janissen & Huynh, 2018). It is estimated that 15–20 million tonnes of coffee by-products or wastes were generated worldwide in the 2020/21 coffee harvesting season alone.

Coffee pulp and husk are commonly used to feed ruminants, despite containing tannins and caffeine which are toxic to these animals (Hoseini, Cocco, Casucci, Cardelli, & Corti, 2021). Another common practice is the incorporation of coffee by-products into the soil to act as a fertilizer. However, caffeine has been considered an emerging contaminant which exhibits toxic effects in various aquatic organism (Hoseini et al., 2021). For this reason, the decaffeination of coffee by-products before such applications is needed to mitigate this problem (da Silva et al., 2022; Hoseini et al., 2021). The same by-products; however, are a potential source of a broad range of bioactive metabolites, and they could be explored in a biorefinery approach (da Silva et al., 2022). Caffeine is an example of a valuable polar compound that exhibits properties beneficial to human health. For these reasons it is incorporated in a variety of food, cosmetic and pharmaceutical products (dos Santos et al., 2021). Another example of a polar, bioactive metabolite which is widespread in coffee-related by-products is 5-O-caffeoylquinic acid (chlorogenic acid); this molecule has been shown to have hepatoprotective, neuroprotective, anti-inflammatory, and antibacterial properties, thereby gaining increasing industrial attention (Lu, Tian, Cui, Liu, & Ma, 2020). Coffee also contains non-polar, bioactive compounds such as phytosterols and fatty acids like campesterol and palmitic acid(Nzekoue et al., 2020; Tsegay, Abshiro, Chandravanshi, & Ele, 2020). Therefore, coffee-related by-products could serve as alternative raw-materials for the production of different standardized extracts for the food, cosmetic and pharmaceutical industries. To obtain an extract with a desired metabolite composition, solvent selection is critical.

Deep Eutectic Solvents (DES) have been recognized as green extraction media and studies have detailed a myriad of hydrophilic DES, which have been shown to be efficient for the extraction of high to medium polarity compounds (Mišan et al., 2019). Several parameters can be considered in the design of DES for use in extraction processes. Among them, viscosity and polarity stand out as viscosity directly impacts the mass transfer rate (the lower the viscosity, the higher the mass transfer) and polarity aids the understanding of intermolecular interactions between the compound to be extracted and the solvent, influencing the absolute solubility (Vilková, Płotka-Wasylka, & Andruch, 2020). Co-solvents are sometimes used to modulate viscosity and polarity of DES, particularly through the use of water (Dai, Witkamp, Verpoorte, & Choi, 2015). So far, eleven DES have been employed to extract bioactive compounds from three types of coffee by-products. Choline chloride and/or glycerol based-DES, with and without added water, were used to extract biomolecules, including caffeine and chlorogenic acid, from coffee pulp and spent coffee (both industrial and home-made) (López-Linares, García-Cubero, Coca, & Lucas, 2021; Loukri, Sarafera, Goula, Gardikis, & Mourtzinos, 2022; Ruesgas-Ramón et al., 2020). To the best of our knowledge, the extraction of coffee by-products by hydrophobic DES has not been reported despite their potential to replace hazardous hydrophobic solvents for the extraction of lipophilic bioactive metabolites (Van Osch et al., 2019).

This study aimed to determine the performance of both hydrophilic and hydrophobic green solvents for the extraction of coffee chain by-products or wastes. For this, (i) eleven types of by-products from the coffee chain were chromatographically profiled and four of these were selected for further investigation; (ii) Seventeen different alternative green solvents, including hydrophilic and hydrophobic DES, were tested as extraction solvents; (iii) Solvent selectivities were compared with reference solvents and methods; (iv) The polarity of prepared DES was characterized; (v) Polar solvents were tested for their efficiency to extract caffeine and chlorogenic acid from the different by-products; (vi) Less polar solvents were tested for their ability to extract other compounds from the investigated matrices, and (vii) the greenness of the extractions was assessed using two complementary, multi-parameter metrics.