Green extraction of secondary metabolites from plants: Obstacles, current status, and trends

There is a constant search for new extraction methods for secondary metabolites from plants to improve efficiency and reduce the process cost and waste. Green extraction techniques such as microwave-assisted extraction, ultrasonic-assisted extraction, and shock wave-assisted extraction are an alternative that minimizes energy intake and reduces extraction time. Additionally, using green solvents reduces waste pollution to the environment by eliminating organic solvents. Using green extraction methods along with green solvents such as natural deep eutectic solvents, deep eutectic solvents, CO 2 in supercritical conditions, water, and ethanol challenge the efficiency of traditional extraction methods and organic solvents. It is well known that plants are a rich source of diverse secondary metabolites that can be used in pharmacology and biomedicine for health purposes. On the downside, in the extraction process, toxic adducts can be frequently obtained because of the use of elevated temperature and organic solvents. The use of novel green extraction methods enhances extraction yield by cell disruption omitting heat and preventing structural damage to the secondary metabolites. Green extraction pursues that using safer and less hazardous reagents, cleaner extraction methods, and the obtention of safer products can develop environmentally friendly chemistry that can be a healthy alternative for biological applications. The objective of this review was to lay out different extraction methods used to obtain secondary metabolites from plants, comparing the efficiency of green extraction methods using green solvents in contrast with traditional extraction methods using organic solvents. In this review, we outline the state-of-the-art the last five years of the green extraction methods used for the obtention of secondary metabolites from plants. Positively, an overview of the green extraction methods will present the researcher with a wide range of cleaner techniques that can replace traditional extraction methods of secondary metabolites obtained from plants. • Novel assisted extraction methods with cavitation effect are effective for secondary metabolite obtention. • Coupling green extraction methods is efficient for metabolite extraction. • NADES and DES are powerful green solvents that are viable to for secondary metabolite extraction from plants. • More often than not, green solvents can exceedingly outperform organic solvents in yield of extraction. • Green extraction coupled with green solvents is a viable option for secondary metabolite extraction.