A comparative study of supercritical fluid extraction and accelerated solvent extraction of lipophilic compounds from lignocellulosic biomass

Abstract

Lipophilic compounds are non-structural, heterogeneous compounds rich in terpenes, sterols, fatty acids, hydrocarbons, and glycerides. They have found widespread uses in different industries, such as the pharmaceutical, medical, cosmetic and nutraceutical sectors. They are typically extracted from wood using traditional techniques such as solvent extraction hydro- and steam- distillation. However, these techniques have several drawbacks such as long extraction times, high energy consumption, extensive solvent use and degradation of thermosensitive compounds, which are highly volatile. In this study, supercritical fluid extraction (SFE) and accelerated solvent extraction (ASE) were evaluated to extract lipophilic compounds from lignocellulosic biomass such as pinewood sawdust and Cannabis Sativa L. Their advantages of using low amounts of solvent, short extraction times and high selectivity allow them to be used as an alternative extraction technique to traditional methods. Moreover, SFE uses carbon dioxide, which is safe, cheap and readily available, and it does not alter the structure of the compounds. In contrast, ASE uses elevated temperatures and high pressures to prevent the evaporation of highly volatile compounds. In order to solve challenges from both an economic and an environmental perspective, the interaction of process conditions on lipophilic compounds extraction efficiency was modelled and optimized using Response Surface Methodology (RSM) and BoxBehnken design (BBD). The extraction variables optimized for pinewood sawdust compounds were, SFE: co-solvent (ethanol) flow rate (1-2 ml/min), carbon dioxide (CO2) flow rate (1-3 ml/min), Temperature (40-60 °C) and pressure (200-300 bar), and for ASE: static time (10-15 mins), static cycle (1-3) and temperature (80-160 °C). The process parameters were optimized, and the experimental data was modelled using RSM for statistical analysis of the BBD extraction process. The experimental data's quadratic polynomial models gave a coefficient of determination (R2 ) of 0.87 and 0.80 for ASE and SFE, respectively. The optimum conditions of ASE were temperature (160 °C), static time (12.5 mins), and static cycle (1), which resulted in a maximum yield of 4.2%. The optimum SFE conditions were temperature (50 °C), pressure (300 bar), CO2 flow rate (3.2 ml/min), and a 2 ml/min co-solvent (ethanol) flow rate that yielded 2.5% lipophilic compounds. The extraction efficiency of pinewood sawdust lipophilic compounds with ASE was higher compared to the SFE. Although ASE uses high temperatures that may degrade thermolabile compounds, the short extraction times may work in their favor since the extracts are not exposed to high temperatures for long periods. SFE uses low temperatures and long extraction times compared to ASE. Several properties affect the extraction efficiency, such as volatility, dissolving power, solubility, and fluid density of the extracting solvent. The extraction efficiency of lipophilic compounds by SFE may be affected by the supercritical fluid's solubility and differences in densities at different pressures. In ASE, the high yields were influenced by the high polarity of the solvent mixture and temperature with a short extraction time. The extraction variables optimized using RSM for Cannabis Sativa L. for SFE were pressure (200-300 bar), co-solvent (ethanol) flow rate (1-2 ml/min) and CO2 flow rate (1-2 ml/min). The R2 was determined to be 0.9108. The optimum conditions were 300 bar pressure, 1 ml/min co-solvent (ethanol) flowrate, and 2 ml/min CO2 flowrate, which gave a maximum yield of 88%. The high efficiency observed was brought by the increase in the flow rate of CO2 at high pressures, which reduces the mass transfer resistance, while the cosolvent enhanced the solvating power of CO2. The ASE had a high extraction efficiency for the pinewood sawdust lipophilic compounds. However, the method's selectivity was very low according to the results obtained by pyrolysis gas chromatography-mass spectrometry (Py-GC/MS). The thermosensitive compounds, such as terpenes, decreased from 2.01% to 1.69% upon the addition of Tetramethylammonium hydroxide (TMAH). The initial concentration of terpenes was 7.21% in pinewood sawdust by SFE. Upon the addition of TMAH, the concentration of terpenes of the pinewood sawdust decreased to undetectable levels. The initial concentration of the terpenes of Cannabis Sativa L. was 14.29% and decreased in the presence of TMAH to 0.39%. The Fourier Transform Infrared Spectroscopy (FTIR) confirmed the presence of lipophilic compounds functional groups and a fingerprint region of lipophilic compounds of pinewood sawdust and Cannabis Sativa L. Thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) showed high thermal stability (250 – 400 ℃). This research demonstrated the ability of SFE to extract lipophilic compounds from pinewood sawdust Cannabis Sativa L.