Supercritical Fluid for Extraction and Isolation of Natural Compounds

Abstract

Supercritical fluid (SCF) extraction has emerged as an effective and efficient method for separating important phytoconstituents. The extraction process is simple and environmentally friendly, generating minimal to no waste. This procedure offers various advantages over traditional extraction techniques. This chapter discusses the procedure, advantages, and different types of phytoconstituents isolated using supercritical fluids, with a preference for natural products.

1 Introduction

Supercritical fluid (SCF) extraction is an analytical method to separate the analyte from the sample matrix using supercritical fluids as solvents (Hedrick et al. 1992). This technique is rapid, inexpensive, sustainable, and simple to execute, compared to the traditional Soxhlet extraction, where solvent costs are usually high, requiring several hours accompanied by an additional concentration step that aids pollution (Sapkale et al. 2010).

SCF extraction was initiated along with supercritical fluid chromatography in the late twentieth century for isolating forensically relevant compounds (Khaw et al. 2017). It later gained popularity when supercritical toluene was used mainly in the petroleum industry with many commercial interests.

Over the last few years, SCF extraction has gained recognition for its many established advantages, particularly supercritical carbon dioxide, because of its easy-to-use properties (2017). CO₂ has a near ambient critical temperature of 31 °C, allowing many biological materials and natural products to be processed around 35 °C without denaturation. It is being extensively used in decaffeination and power generation processes and is widely used to extract natural products, leaving no toxic residues behind (Khaw et al. 2017).

The advantage of supercritical CO₂ (ScCO₂) is that its extraction properties can be precisely varied with just minute changes in temperature and pressure. The properties can also be modified using solvents like ethanol (Camel 2001). Other than CO₂, various solvents are used to extract bioactive components from plants, namely, propane, DME, SF2, and ethanol (Bizaj et al. 2021).

2 Methodology/Mechanism

A supercritical fluid is a substance whose thermodynamic properties are higher than the critical temperature and pressure of the source compound. The maximum temperature, beyond which the gaseous state of a substance cannot be liquified, irrespective of the amount of pressure applied, is called the critical temperature of the substance. Critical pressure is the minimum pressure required to condense a gaseous substance to a liquid at its critical temperature (Alekseev et al. 2020). For carbon dioxide, the critical temperature is 304.2 K and 73.0 atm.

In the supercritical region, a homogenous fluid materializes, which has unique physiochemical properties. In this region, the surface tension of the supercritical fluid is equal to zero, the dissolving and swelling capacity increases, and the viscosity decreases (Alekseev et al. 2020). The physiochemical properties can be modulated by changing the parameters of the supercritical state. The density of supercritical fluids changes with variations in pressure and temperature; a slight increase in pressure can cause a drastic increase in the density of the supercritical fluid, which in turn causes an increase in the solubility of the supercritical solvent. Once extraction is complete, solvent recovery is relatively simple due to the volatility of the supercritical fluid leaving behind the extracted analyte (Pourmortazavi et al. 2014). Such manipulations of the physiochemical properties make SCFs an excellent solvent for extraction due to their high selectivity, solubility, and extraction efficiency (Yousefi et al. 2019).

The setup (Fig. 1) for supercritical fluid extraction involves a pump, a pressurized compartment, and a collecting vessel. The solvent is commonly stored in a tank connected to a pressurized pump. The commonly used solvent is CO₂, pumped into the system as a liquid below 5 °C and at around 50 bars of pressure. The fluid is cooled to remain a liquid but heated to critical condition after pressurization. The pressure must be maintained in the extraction cell, and heating should be provided to counteract the cooling caused by the adiabatic expansion of the CO₂. Raw material from which the natural product is extracted is placed in the extraction cell, where pressure and temperature are controlled. The raw material is also pre-treated to modulate the moisture content and particle size for optimal extraction. The supercritical fluid is allowed to enter the pressurized extraction cell, where the natural products to be extracted dissolve in the supercritical fluid based on its solubility, which in turn is dictated by its density and pressure. Once the extraction is completed, the fluid with the dissolved natural product is passed through a chamber with lesser pressure, reducing the fluid’s dissolving power, and the natural product gets precipitated out. The depressurization of the supercritical CO₂ causes the fluid to become a gas and can be collected separately for further use (Sapkale et al. 2010).

Fig. 1

Setup of ScCO₂ extraction

Table 1 lists the chemicals/phytochemicals extracted from the species. Some of the chemicals listed in this table are generic in name, as the literature does not specify the individual compound. The table is arranged in alphabetical order of the species name.

Table 1 List of chemicals isolated from different species using ScCO₂