Subcritical Water and Supercritical Carbone Dioxide: Efficient and Selective Eco-Compatible Solvents for Coffee and Coffee By-products Valorization

This review aims to establish the state of the art of the existing literature on the valorization of coffee and coffee by-products (i.e. green coffee, coffee husk, pulp, silverskin and spent coffee grounds) through the use of pressurized fluids as solvents, including subcritical water (SCW) and supercritical carbon dioxide (SC-CO2). The first part reviews the exploitation, the composition, the properties and the ways of valorization of coffee beans and their by-products, highlighting their high potential as raw material. The second part is dedicated to subcritical H2O and supercritical CO2 applied to coffee and coffee by-products valorization, discussing the involved mechanisms, the parameters’ influence and the superiority of subcritical H2O and supercritical CO2 in comparison to other solvents and techniques. Indeed, subcritical H2O is considered as a super solvent, catalyst and reagent at the same time, being the most efficient technology for carbohydrate and polyphenol recovery as well as biocrude-oil production. Supercritical CO2 is considered as a chameleon solvent, with unlimited tunability, able to selectively extract high value molecules but also compete with organic solvents in amount and quality of the produced extract. In addition, subcritical H2O and supercritical CO2 are complementary solvents, targeting together all the types of molecules in coffee and coffee by-products, leading to the development of a Green Solvent Bio-Refinery (GreSBiR) with sequential uses of subcritical H2O and supercritical CO2.