With the principles of circular economy, sustainable development and the concept of Green Chemistry in mind, many efforts have been made to reduce the environmental impact of extraction technologies to retrieve active compounds from natural resources.
The extraction process should avoid the co-elution of unwanted substances with the target biomolecule, minimizing the solvent waste and reducing the energy consumption while increasing the extraction efficiency. Many different techniques have been developed using alternative eluents such as water or agro-solvents. Pressurized liquids extraction (PLE), supercritical fluids extraction (SFE) or gas-expanded liquids (GXLs) extractions are the most widely employed methods. [1] These techniques offer the possibility of using green solvents in closed loop sytstems, enhancing the extraction selectivity by finely tuning temperature and pressure during the process, minimizing bioaccumulation of by-products. Compressed fluids technologies organized in multi-units operation platforms performing selective extractions and separations can largely contribute in solving the problem of solvent waste while reducing the post processing operations.
Ultrasound-assisted extraction (UAE) technology together with microwave-assisted extraction (MAE) are methods capable of improving the overall process efficiency and they are widely applied in sample preparation and paired with conventional extraction methods.
The solvent selection is a critical step in the sustainable development of a green extraction process. Hansen solubility parameters (HSP) have been successfully used to evaluate the best alternative solvents replacing toxic and hazardous petrochemical ones. [2] Many data regarding solvent performances, extraction yields and target molecule solubilities in various mediums have been collected, but more studies and new computational improvements are needed to persist in the continuos attempt of green extraction process optimization, reduce the number of experiments and the overall environmental impact.
Bio-based solvents such as carbon dioxide (CO2), ethanol, terpenes, vegetable oils and organic ethers
among others are examples of emerging green solvents together with ionic liquids (ILs) and natural deep eutectic solvents (NADES). As example of the differences within conventional fractionation techniques to purify a natural product and the advancements in the process optimization made by SFE using supercritical CO2 (sc-CO2) as solvent, we report the example of propolis purification. This complex mixture of plants exudates collected by bees is traditionally purified through steam and vacuum distillation of ethanolic propolis extracts to fractionate the contained valuable phenolic compounds with a wide variety of therapeutic effects. Nevertheless traditional methods involve the use of high distillation temperatures and large solvent employement. [3] Propolis purification performed through sc-CO2 extraction can improve the flavonoid extraction selectivity thus the antioxidant power of propolis extract, mainly employing ethanol as co-solvent. The extraction yields are correlated to the solvent density and the relatively low phenolic yields of SFE extraction compared to traditional methods can be increased by repeating the extraction process on the same propolis sample, while the purification steps can be reduced or avoided completely. [4]
The compounds extracted from natural sources have many potential applications as antioxidants, preservatives, antimicrobials, aroma, flavors and active pharmaceutical ingredients. For this reason the design and implementation of green approaches for phytochemicals extraction and processing is currently the main goal of researchers and natural products industries.
References:
[1] Herrero M. et al. Green extraction processes, biorefineries and sustainability: recovery of high added-value products from natural sources, The Journal of Supercritical Fluids (2018); https://doi.org/10.1016/j.supflu.2017.12.002 [Journal Impact factor = 4.577] [Times cited = 78 ]
[2] A.d.P. Sánchez-Camargo, et al. Hansen solubility parameters for selection of green extraction solvents, Trends in Analytical Chemistry, (2019); https://doi.org/10.1016/j.trac.2019.05.046 [Journal Impact factor = 9.801] [Times cited = 47]
[3] Paviani L. C. et al. Supercritical carbon dioxide selectivity to fractionate phenoliccompounds from dry ethanolic extract of propolis, Journal of food process engineering (2010); 33(1), 15–27. doi:10.1111/j.1745-4530.2008.00256.x [Journal Impact factor = 2.98] [Times cited = 51]
[4] Reis J.H.O. et al. Supercritical Extraction of Red Propolis: Operational Conditions and Chemical Characterization. Molecules (2020); 25(20):4816; doi:10.3390/molecules25204816
[Journal Impact factor = 4.411] [Times cited = 5]
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