Extending the design space in solvent extraction - from supercritical fluids to pressurized liquids using carbon dioxide, ethanol, ethyl lactate, and water in a wide range of proportions
Abstract
Plants and food byproducts contain high-value bioactive compounds like carotenoids, tocopherols, flavonoids, and other phenolic compounds that can potentially be extracted prior to usage of the lower-ranked biomass, all in line with a circular bioeconomy. Extraction is typically performed by leaching using an organic solvent, or alternatively with more advanced techniques such as supercritical fluid extraction (SFE), pressurized liquid extraction (PLE), microwave assisted extraction (MAE) or ultrasound assisted extraction (UAE).
With the exception of SFE, mass transfer is enhanced and solvent consumption is minimised by the use of elevated temperature, which has a negative impact on the extraction yield and structural preservation for thermally labile bioactive compounds. Hence, in this study we aim to explore the use of CO2 in extraction solvent mixtures as a viscosity-lowering entrainer.
Furthermore, we aim to disregard the instrumental boundaries between SFE and PLE by extending the design space for extraction method development, using solvent mixtures of pressurized carbon dioxide, water, and either ethanol or ethyl lactate, covering a uniquely large area of the solvent's relative permittivity. Such a strategy will inherently enable extraction method optimisation for target compounds differing a lot in polarity, here demonstrated for beta-carotene, alpha-tocopherol, and quercetin in sea buckthorn berry pomace.
Our results show that by the use of experimental design, optimum extraction conditions were found for each of the three compounds: supercritical CO2 with 10 vol% of ethanol at 300 bar and 80 degrees C for beta-carotene, liquid CO2/ethanol/water (50/44/6, v/v/v) at 210 bar and 60 degrees C for alpha-tocopherol, and liquid CO2/ethyl lactate/water (33/54/13, v/v/v) at 200 bar and 62 degrees C for quercetin. Extraction rates were fast in all three cases, giving complete extractions within less than 20 min at the highest flow rate explored, 3 mL min(-1).
The developed extraction methods gave significantly higher extraction yields than published optimised extraction methods based on PLE. Moreover, the optimized methods for the less polar compounds, beta-carotene, and alpha-tocopherol, enabled a significantly lower consumption of organic solvents (30-52%), which makes them greener in comparison with the more conventional PLE extraction methods.