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New advances in supercritical fluid extraction of natural bioactive compounds from natural plant materials

New advances in supercritical fluid extraction of natural bioactive compounds from natural plant materials
In this review, recent advances in green technologies for the extraction of natural bioactive components from plant sources are discussed. Bioactive compounds of plant origin are defined as natural compounds present in small amounts in plants. Researchers have shown interest in extracting bioactive compounds because of their benefits to human health and because they are environmentally friendly and generally considered safe. Various new extraction methods and traditional extraction methods have been developed, but, so far, no unique method has been proposed as a benchmark for the extraction of natural bioactive compounds from plants. The selectivity and productivity of traditional and modern extraction techniques usually depend on the selection of key input parameters, knowledge of the nature of the plant sample, structure of the bioactive compound, good scientific skills. The aim of this work is to discuss the recent advances in supercritical fluid extraction techniques, especially supercritical carbon dioxide, and the fundamentals of extraction of bioactive compounds from natural plant materials such as herbs, spices, aromatic and medicinal plants.

Keywords: bioactive compounds, supercritical extraction, supercritical fluids, co-solvent, essential oils, medicinal plants
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I. Introduction
Extraction is one of the methods used to isolate components from plant-based materials. Currently, many researchers are using various extraction methods at laboratory, pilot and commercial scale to extract different target compounds present in plants [ 1 ]. Bioactive compounds are naturally occurring secondary metabolites extracted from various plant parts such as leaves, stems, roots, seeds, flowers, and fruits by using several extraction procedures [ 2, 3 ]. The demand for these compounds has increased because they are considered natural and safe for use in numerous industries such as cosmetics, food, feed, agriculture, and pharmaceuticals [ 3 , 4]]. Bioactive compounds have been found to have a wide range of health-promoting properties in humans and animals, such as antibacterial, antimicrobial, anti-inflammatory, anti-aging and anticancer effects [ 2 , 4 , 5 , 6 ].

Bioactive compounds – essential oils, carotenoids, fatty acids, phenolic acids, flavonoids – are usually extracted by hydrodistillation, solvent extraction, soxhlet extraction, pressing and hydrodistillation, but they have some limitations, such as being too time-consuming, using too much organic solvent, loss of some volatile compounds, degradation of heat-insensitive compounds, may leave toxic solvent residues in the extract, low yield and low extraction efficiency [ 3 ]. Therefore, in recent years, green chemical methods for extraction purposes have been developed to reduce energy and solvent consumption, reduce processing time, and replace conventional solvents with environmentally friendly alternatives.

Modern methods for the extraction and isolation of bioactive compounds from plant-based materials are receiving increasing attention in the field of research and development. For example, ultrasound-assisted extraction (UAE), pressurized liquid extraction (PLE), supercritical fluid extraction (SFE http://www.careddi.com/brc/66.htm) and microwave-assisted extraction (MAE) [ 7 , 8 , 9 , 10 ] are currently available and environmentally sustainable techniques. According to Chemat et al. [ 11 ], more environmentally friendly technologies are defined as ”extraction methods based on the detection and development of extraction processes that will reduce energy consumption, enable the use of solvent substitutes, renewable natural products, and ensure safe and high quality extracts/products” [11 ].

Ultrasound-assisted extraction is based on acoustic waves, frequency and stimulation of cell wall fragmentation and discharge of cell contents [amplitude of mechanical techniques8, 10, 12 ]. This process reduces extraction time, is suitable for heat-insensitive and unstable compounds, improves extraction rates, and reduces solvent and energy consumption as it affects transfer quality, temperature, and solvent/sample ratio [ 8 , 10 ]. Pressurized liquid extraction is a solid-liquid technique that involves the application of high pressure and temperature to raise the boiling point of the solvent and allow its rapid penetration into the sample matrix [ 7]].PLE helps to reduce the solvent usage, shorten the extraction time and increase the extraction rate. However, it is not recommended for heat-sensitive compounds due to the high extraction temperature [ 7 , 10 ]. Supercritical fluid extraction is an advanced technique for the extraction of bioactive compounds using supercritical fluids as solvents. It has received more attention than conventional methods due to its significant advantages such as higher selectivity, diffusivity and ecological properties [ 13 ].

Among the new technologies mentioned, we discuss in this review supercritical fluid extraction (SFE), which is one of the best techniques for the removal of natural chemical components (e.g. flavonoids, essential oils, seed oils, carotenoids and fatty acids) from natural plant materials, and the fact that it represents a sustainable alternative to conventional extraction systems [ 10 , 13 ]. Researchers have shown the considerable benefits of SFE relative to conventional techniques3, 9, 10 ]. Carbon dioxide (CO 2) is an excellent solvent that has received special attention in SFE because it is chemically inert, economical, readily available, separable from extracts, non-toxic, and an approved food-grade solvent [ 7 , 9 ]. Supercritical carbon dioxide (SC-CO 2 ) is a nonpolar solvent commonly used in SFE due to its gas-like and liquid-like properties, low critical temperature and pressure, and selectivity and potential for the extraction of heat-sensitive compounds . In addition, low polar compounds and small molecules are easily soluble in SC-CO 2 , but large molecules and polar compounds are extracted by adding a co-solvent to improve the extraction rate, which can be ethanol, methanol or water. Special attention should be paid to temperature and pressure during SFE, as any change can affect the whole process [ 3 , 13 ].

The purpose of this paper is to upgrade the review of the statement of SFE by discussing the properties and principles of SC-CO 2, its application in the extraction of natural bioactive compounds from plant-based materials and its current use in different industries.

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2. properties of supercritical fluids as a novel extraction technique
2.1. Background of Supercritical Fluid Extraction
SFE is a novel extraction technology, a more environmentally friendly method to produce indigenous substances suitable for various industries from sustainable sources such as herbs, spices, aromatic plants and medicinal plants. This advanced technology includes the separation/removal of target bioactive compounds by supercritical fluids.

The supercritical phase was first discovered in 1822 by Baron Charles Cagniard de la Tour, who noticed changes in the behavior of solvents at specific values of pressure and temperature [ 14 , 15 ]. The term ”critical point” was coined by Thomas Andrews in 1869 as a result of his experiments on the effect of temperature and pressure on partially liquefied carbonic acid sealed glass tubes. He described it as the end point of the phase equilibrium curve reached at critical temperature (Tc) and critical pressure (Pc) when the presence of the two phases disappears [ 10 , 14 ]. A few years later, Hannay and Hogarth discovered the application of the SFE method and the technique using the basis of CO 2 in the supercritical state developed in 1960 [ 16 ]. The first practical application of supercritical fluids was started in Germany for the decaffeination of raw coffee beans; a few years later, a method was developed in Australia for the extraction of oil from hops using liquid CO 2 [ 17 ]. By the 1980s, industrial applications of both technologies had been developed and effectively adopted in different countries [ 16 ]. Currently, various products are being produced using this technology and are accepted worldwide.

2.2. Concepts and principles of supercritical fluid extraction
Basically, a simple SFE process consists of extraction and separation as the basic steps [ 17 ]. In the extraction process, either solid or liquid samples can be used depending on the system setup, but solid samples are used more often compared to liquid samples. For solid samples, the column is filled with a pretreated (dried and ground) sample and a pressurized supercritical solvent flows through the column and dissolves the extractable compounds in the solid matrix. The dissolved compounds are transported out by diffusion into the mixture of extractables and solvent by decompression, temperature elevation, or both [separation of separators17, 18, 19 ].

Many companies now produce SFE devices at different scales depending on their intended use, for example for laboratory, pilot studies or industrial use. The simplest systems consist mainly of a cooler for cooling the solvent gas, a solvent pump to push the fluid through the system, an extraction column to hold the sample to be extracted, a separator to collect the extract, a heat exchanger process material to regulate the temperature, an oven to keep the extraction column above the critical temperature of the extract fluid, and a back pressure regulator to keep the pressure in the system above the critical pressure of the fluid [ 20 , 21].SFE can be implemented in two different modes – dynamic and static – which can be used individually or in combination during the extraction process. In the dynamic mode, a supercritical fluid flows steadily through the extraction column containing the sample, while in the static mode, the sample absorbs the supercritical fluid and no fluid flows out of the extraction column during the process [ 22 ].

In most cases, co-extraction of unwanted compounds occurs during the extraction process, which may lead to poor quality of the extract. Thanks to advanced technology, this problem can be solved by performing SFE with a graded separation process, thus improving the selectivity of SFE [ 23 ]. This concept can be performed in multiple steps during the separation process by connecting some separators in series and adjusting the processing conditions, such as pressure and temperature, according to the equilibrium solubility of the target compound [ 24 ].

During the SFE process, different variables such as extraction temperature, pressure, type, percentage of co-solvent and sample volume must be optimized to improve the extraction rate of the target compound. In addition, the solubility and mass transfer resistance of the feedstock are related to these variables [ 25, 26, 27 ]. The solubility of extractable compounds in SFE must be maximized, as it is one of the main factors affecting the extraction effectiveness and extraction quality [ 13]]. The solubility of solutes in SFE has been reported to depend on temperature and pressure, which have an effect on the density of the fluid.SFE is a convenient technique because it can be tuned to the solvent capacity or selectivity of supercritical fluids (SCF) and can be directly connected to gas chromatography (GC) or supercritical fluid chromatography for analysis [ 27]. Various studies have shown that SFE has many advantages, including short processing times, suitability for extraction of volatile and heat-resistant compounds, higher productivity in terms of increased yields, reduced solvent use, and environmental protection through the use of safe solvents. It plays a crucial role in different fields such as food, pharmaceuticals, agriculture and cosmetics [ 18]]. As discussed, today SFE is not only used for laboratory research purposes, but its applications have been commercially developed on an industrial scale, for example, for the production of natural food ingredients (hops, aromas, flavors, colorants, vitamin-rich extracts, specific lipids), nutritional products, from food and drug, and for the removal of pesticides [ 27, 28]. Supercritical fluids are used with different properties, namely density, viscosity and diffusivity, which can be changed to improve their transport properties.

2.3. Properties of supercritical fluids
Supercritical fluids are chemical solvents that can be compressed above their critical point and are often considered environmentally friendly and are commonly used in extraction processes because they provide excellent results due to their unique properties [ 29 ].SCFs are used as substitutes for organic solvents in laboratory processes and in various industries such as food, pharmaceuticals, agriculture and cosmetics. As shown in Table 1, many compounds have been considered as SCFs, for example, hydrocarbons (pentane, butane, hexane), aromatics (benzene, toluene), alcohols (methanol, isopropanol, n-butanol) and some gases (carbon dioxide, ethylene, propane) [ 18 ]. Among the aforementioned SCFs, CO 2 is undoubtedly the most commonly used solvent because it has many different advantages [ 25 , 29 ].