Subcritical water and supercritical carbon dioxide: efficient and selective eco-compatible solvents for coffee and coffee by-products valorization

Abstract

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.