Water as green extraction solvent: Principles and reasons for its use

Water can be considered, in many cases, the greenest solvent. In the last years, the number of publications which used water as a green extraction solvent has notably increased. One of the main reasons that makes water an interesting solvent are the physicochemical changes of water from ambient to near-critical conditions under which subcritical water extraction (SWE) (or pressurized hot water extraction (PHWE)) can be performed. This review presents a critical overview about the greenness of water as extraction solvent. The changes of its physicochemical properties from ambient to near-critical conditions will be briefly described, and some of the most relevant applications of SWE published from 2015 to 2017 will be discussed. Finally, some basic ideas about different methodologies that can be employed to evaluate the greenness of an extraction process will be provided.